MARGINS Theoretical Institute on the2 MARGINS Theoretical Institute on the Seismogenic Zone Experiment (SEIZE) Snowbird, Utah, 16-21 March, 2003 MARGINS web site:

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MARGINS Theoretical Institute on theMARGINS Theoretical Institute on theMARGINS Theoretical Institute on theMARGINS Theoretical Institute on theMARGINS Theoretical Institute on the

Seismogenic Zone Experiment (SEIZE)Seismogenic Zone Experiment (SEIZE)Seismogenic Zone Experiment (SEIZE)Seismogenic Zone Experiment (SEIZE)Seismogenic Zone Experiment (SEIZE)Snowbird, Utah, 16-21 March, 2003Snowbird, Utah, 16-21 March, 2003Snowbird, Utah, 16-21 March, 2003Snowbird, Utah, 16-21 March, 2003Snowbird, Utah, 16-21 March, 2003

MARGINS web site: http://www.ldeo.columbia.edu/margins

Contents

An Author Index is located in the back of this volume

Evolution of physical properties of the Nankai Trough plate-boundary thrust from the trench into theseismogenic zone inferred from 3-D seismic images ................................................................................... 3

Comparison of earthquake focal mechanisms and source processes between Nicoya and Osa Peninsulas,Costa Rica ..................................................................................................................................................... 4

Issues and Opportunities in Seismic Reflection Imaging of the Sub-Continental Seismogenic Zone: Results ofReflection Modeling ..................................................................................................................................... 5

Coseismic, postseismic and interseismic deformation along the Kamchatka subduction zone .......................... 6

Measuring seafloor deformation across the Nazca/South America Plate convergence offshore Peru: Imagingthe up-dip limit .............................................................................................................................................. 7

The evolution of the forearc Sandino basin: off Nicaragua sector of the Pacific convergent system (CentralAmerica). ...................................................................................................................................................... 8

Hydrodynamic Response of Subduction Zones to Seismic Activity: A Preliminary Study for the CostaRica Margin.................................................................................................................................................. 9

Comparisons between seismicity and fluid flow behavior along the western Costa Rica margin .................... 10

Seismic Attenuation in the Subduction Zone of Costa Rica.............................................................................. 11

The Seismogenic Zone Along the Alaska-Aleutian Trench .............................................................................. 12

Along strike variability in the seismogenic zone and thermal models of subduction below the Nicoya Penin-sula, Costa Rica ........................................................................................................................................... 13

Secular, Transient and Periodic Crustal Movements in Japanese Subduction Zones, and Dynamics UnderlyingThem ........................................................................................................................................................... 14

Images of Seamount Subduction Beneath Nankai Margin ................................................................................ 15

What We Know about Subduction Thrust Faults .............................................................................................. 16

Variations in Basement Topography and Sediment Thickness on the Philippine Sea Plate Subducting Alongthe Nankai Trough....................................................................................................................................... 18

Deformation in granular aggregates: Implications for strength of porous rocks and shear within fault zones .... 19

Comparative study on exhumed seismogenic faults and modern Nankai seismogenic megathrust ................. 20

Very focused expulsion of pore fluid along the western Nankai accretionary complex detected by closely-spaced heat flow measurements .................................................................................................................. 21

Continental deformation in the Central Andes controlled by changing subduction parameters and geologicinheritance ................................................................................................................................................... 22

Partitioning of Seismogenic Strain in the Offshore Costa Rica Forearc ........................................................... 23

Seismogenic Strain at the Costa Rica Convergent Margin ................................................................................ 24

Possible Forearc Sliver at the Lesser Antilles ................................................................................................... 25

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Fault Friction and the Transition From Seismic to Aseismic Faulting .............................................................. 26

Characteristics of the Nicaragua convergent margin and their possible influence on seismicity and tsunamigeneration .................................................................................................................................................... 27

Relationship of Fluids and Deformation at Seismogenic Depths: Structural Study of the Rodeo Cove ThrustZone, Marin Headlands, California ............................................................................................................ 28

Evolution of a ductile shear zone in the down-dip continuation of the seismogenic zone ............................... 29

Factors Controlling the Frictional Strength of Sheet-Structure Minerals ......................................................... 30

Summary of ODP Leg 190 results in the Nankai Trough.................................................................................. 31

Consolidation State and Overpressures Within the Underthrust Section, Nankai Accretionary Margin: Resultsof Uniaxial Reconsolidation Experiments .................................................................................................. 32

Splay Fault Branching Along the Nankai Subduction Zone ............................................................................. 33

Mapping the plates interface from the trench to the maximum depth of the Wadati-Benioff Zone under CostaRica ............................................................................................................................................................. 34

Seismic imaging of the megathrust in Central-Southern Costa Rica ................................................................ 35

Relative relocation of intermediate depth seismicity: A double Wadati-Benioff Zone below the Central Andes..... 36

Contrasts in veining and faulting across the aseismic to seismic transition in a sediment-rich accretionarycomplex ....................................................................................................................................................... 37

Geophysical and drilling evidence implies that a thick sandwich( 0.5->1.0 km) of sediment and detachedcontinental margins material separates the upper and lower plates; ........................................................... 38

Evaluation of the Updip Limit of the Seismogenic Zone in Central Costa Rica .............................................. 41

The Nankai Trough versus the Sagami Trough - from a viewpoint of dehydration .......................................... 42

The Cellular Shear Mesh of the Chrystalls Beach Accretionary Melange: Relevance to the Active HikurangiMargin Subduction Thrust Interface ........................................................................................................... 43

Composition of sediments on the incoming Cocos Plate, offshore Costa Rica ................................................ 44

A Coupled Hydrological and Geomechanical Study of the Nankai Trough Earthquake Recurrence ............... 45

Heat Flow and Flexure at Subduction Zones .................................................................................................... 46

Deformation at plate boundaries and related thrust faults. A comparison of ODP drilling with onland studieswhat can we infer about the aseismic-seismic transition ............................................................................ 47

Index ................................................................................................................................................................. 48

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Evolution of physical properties of the Nankai Trough plate-boundary

thrust from the trench into the seismogenic zone inferred from 3-D

seismic images

Nathan Bangs, Tom Shipley, Sean Gulick, and the Nankai 3-D seismic working group

University of Texas at Austin, Institute for Geophysics

In 1999, a group of Japanese and American scientist acquired a 3-D seismic reflection data volume toimage the plate-boundary thrust in the Nankai Trough subduction zone and investigate its physicalproperties from the trench into the seismogenic zone. These data were acquired along the Murototransect off Shikoku Island. Along this transect the decollement initiates within the same stratigraphicunit, the lower Shikoku Basin unit, that eventually develops into the seismogenic zone. This enables usto investigate the evolution of fault zone physical properties that lead to seismogensis with minimalcomplications from lithologic variation. Three-dimensional seismic reflection data, with constraintsfrom Leg 196 LWD and Legs 131 and 190 core data, provide a means to map the lateral variation ofphysical properties of the decollement from seismic reflection amplitude and waveforms to revealconsolidation patterns and fluid content of the fault zone. Seismic velocities derived from analysis of3-D prestack time migrated data also reveal the patterns of consolidation above and below thedecollement. The inferred patterns of consolidation suggest that rapid loading in the trench and be-neath the accretionary wedge preferentially consolidates the Upper Shikoku Basin section and inhibitsconsolidation of the Lower Shikoku Basin section leading to the development of overpressures alongthe decollement. The overpressured, underthrust section probably remains underconsolidated to atleast ~12 km landward of the deformation front where there is a significant increase in velocitiescoincident with the loss of decollement reflection amplitudes. Patches of high-amplitude decollementreflections that are probably overpressured fluids in the decollement extend downdip at least 30 kmlandward from the deformation front, but evidence of fluids within the fault zone does not extend muchdeeper.

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Comparison of earthquake focal mechanisms and source processes

between Nicoya and Osa Peninsulas, Costa Rica

Susan L. Bilek1, Heather DeShon2, Susan Y. Schwartz2, and Andrew V. Newman3

1.University of Michigan, Ann Arbor, MI, USA2.University of California, Santa Cruz, Santa Cruz, CA, USA3.Los Alamos National Lab, Los Alamos, NM, USA

Earthquake rupture variations for large (Mw > 6) earthquakes along the Costa Rica margin have been

related to variations in the incoming Cocos Plate. Seamounts, large plateaus, and the Cocos Ridge allsubduct at the Middle America Trench offshore Costa Rica, providing heterogeneous asperity distribu-tions. These asperity distributions have been related to large Costa Rica earthquakes in 1983, 1990, and1999. Subducting plate origin also varies along the margin. Along the Nicoya Peninsula, the origin ofthe subducting plate changes from crust formed at the East Pacific Rise subducting in the northern partof the Nicoya Peninsula to the crust formed at the Cocos-Nazca spreading center in the southern part ofthe Nicoya Peninsula. This change appears affect the temperature of the subducting crust and possiblythe updip limit of seismicity. Here we will present analyses of focal mechanisms and earthquake sourceprocess variations along this margin for comparison with the along-strike changes in the incomingplate. We will be focusing on earthquakes recorded by the Costa Rica SEIZE experiment, whichconsisted of two network deployments of seismometers on the Nicoya Peninsula and near the OsaPeninsula. Variations found with the subset of small (magnitude 2-4) earthquakes recorded with thisnetwork will be compared with variations observed for large earthquakes along the Central Americanmargin.

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Issues and Opportunities in Seismic Reflection Imaging of the Sub-

Continental Seismogenic Zone: Results of Reflection Modeling

Larry D. Brown, and Amy Kwiatkoswki

Institute for the Study of the Continents, Cornell University, Ithaca, NY USA

Although SEIZE is focused to a large extent on the submarine component of the seismogenic zone, thedeeper portions of the mega-rupture zones for most subduction complexes underlie, or must be imagedthrough, arc or continental crust. The continental element is explicit in the low angle seismogeniczones that mark continent-continent (Himalaya) and arc-continent (Taiwan) collision zones. The be-havior of intra-continental seismogenic zones are an important tectonic comparison for the oceanicsubduction zones that are the theme of SEIZE. However, the focus of this report is on seismic reflectionimaging issues common to both types of seismogenic environment. Ray trace modeling for land sur-veys was carried out to explore deeper seismogenic zone issues as inspired by a) the Nankai subductionzone, b) the Nepalese Himalaya, c) the Chi Chi earthquake of central Taiwan, and d) the subductingCocos plate beneath southeastern Mexico. Among the feasibility issues addressed were: 1) delineationof deep fault morphology to identify gross geometrical constraints on rupture dimension, 2) detectabil-ity of intrafault asperities, textures, and pore pressure changes that might influence rupture character-istics, 3) direct mapping of rupture magnitude within the fault zone, 4) the viability of low-fold seismicreflection techniques in difficult terrane for imaging the deeper seismogenic zone, and 5) the utility ofquarry blasts as a source for 3D and 4D mapping of the seismogenic zone. The modeling suggests thatwhile some of these effects are indeed subtle, they are not unrealistic targets for deep reflection sur-veys. Especially promising would seem to be the opportunities to use ongoing quarry blasts as a meansto implement low-cost, time-dependent 2D and 3D imaging of certain fortuitously located rupturezones.

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Coseismic, postseismic and interseismic deformation along the

Kamchatka subduction zone

Bürgmann, R.1, Kogan, M.G.2, Levin, V.E.3, Scholz, C.H.2, King, R.W. 4, and Steblov, G.M.5

1. Univ. of California, Berkeley, CA, USA2. Lamont-Doherty Earth Observatory, Palisades, NY, USA3. Kamchatka GPS Center KOMSP, Petropavlovsk, Russia4. MIT, Cambridge, MA, USA 5. RDAAC GSRAS, Moscow, Russia

To better determine the seismic potential of subduction zones, it is important to understand the spatialand temporal distribution of elastic strain accumulation and release of the plate boundary underthrust.Modeling of GPS measurements of interseismic deformation along subduction zones throughout theworld reveals that some plate-interface faults are currently locked over great width. Other subductionzones appear only partially coupled, and some segments show no evidence of significant elastic strainaccumulation and appear to be accommodating convergence by secular aseismic slip only. However,even a subduction zone that appears completely locked for long periods of time might accommodatesignificant amounts of slip by velocity-strengthening episodic slip events such as slow earthquakes andpostseismic afterslip. The M

w=7.8, 5 December, 1997 Kronotsky, Kamchatka subduction earthquake

was preceded by 3 days of vigorous foreshock activity. Continuously operating GPS stations inKamchatka recorded coseismic and postseismic displacements, however they did not detect significantdeformation associated with the foreshock activity. Model inversions of the GPS data show that aseismicafterslip during 2 months following the earthquake released as much moment as the earthquake itself.A logarithmic decay function fit to the cumulative afterslip curve has a relaxation time of about 3 days.The geodetic data suggest that the rapidly decaying transient slip on the subduction underthrust oc-curred near the downdip edge of the coseismic rupture and extended laterally away from it, includingthe foreshock region. The main earthquake initiated near the cluster of foreshocks and ruptured south-ward and up dip. Aftershock activity was most intense to the south of the main rupture, whereas theforeshock region was notably void of aftershocks. We will evaluate time series collected since theearthquake for evidence of any longer term transients. Recently repeated campaign GPS measurementsof a network spanning Kamchatka allows us to put first constraints on variations in interseismic lock-ing along the subduction interface. We will present models that compare the inferred locking widthacross the subduction zone in southern Kamchatka that last ruptured in a 1952, M

w = 9.0, earthquake

with that across northern Kamchatka, where smaller earthquakes and aseismic slip occur. Our modelstake into consideration changes in subduction zone geometry related to the intersection with the Aleu-tian trench in N Kamchatka.

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Measuring seafloor deformation across the Nazca/South America Plate

convergence offshore Peru: Imaging the up-dip limit

C. David Chadwell1, Edmondo Norabuena 2,4, John Hildebrand1, Fred Spiess1, Tim Dixon2,Seth Stein3

1 Marine Physical Lab, Scripps Institution of Oceanography, UCSD2 Marine Geology and Geophysics Division, Rosenstiel School of Marine and Atmospheric Science, University of Miami3 Geological Sciences Northwestern University4 Geophysical Institute of Peru

A two-year campaign of seafloor geodetic measurements has been initiated to image the up-dip limit ofthe Nazca-South America subduction thrust fault offshore Lima, Peru. Two seafloor reference pointswere established on the South America plate approximately 20 km and 50 km upslope of the Peru-Chile trench off the coast of Peru at 12°S. Using the Global Positioning System and underwater acous-tics, the horizontal position of these points were be measured with centimeter-level uncertainty in 2001and will be measured again in 2003. The measured horizontal deformation of the overriding SouthAmerica plate should delineate the slip behavior of the subduction thrust fault in the near-trench re-gion. Two fundamentally different scenarios — (i) partial locking along a fault segment from near thetrench to 50 km depth and (ii) stable sliding from the trench to 10 km with full locking from 10 to 50km — result in similar deformation at continental sites when measured with land-based GPS. Onlyseafloor points in the near-trench region can differentiate these two scenarios. Combing seafloor mea-surements with existing land GPS measurements of crustal motion, provides the first opportunity toobserve the partitioning of the total convergent plate motion across nearly the entire subduction thrustzone. This should improve understanding of the dynamics at oceanic-continental plate convergentmargins, and may lead to refinements in the estimation of earthquake hazard.

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The evolution of the forearc Sandino basin: off Nicaragua sector of the

Pacific convergent system (Central America).

K. McIntosh 1, I. Ahmed 1, C. Ranero 2, B. Taylor 3, P. Costa Pisani4 and E. Silver4

(1) Institute for Geophysics, The University of Texas at Austin, USA(2) Geomar Research Center, Kiel, Germany(3) Landmark Graphics Corporation, Denver, Colorado, USA(4) University of California at Santa Cruz, CA, USA

In June 2000, during a survey in the off Nicaragua, seismic multichannel reflection data has beencollected. The 2800 km of seismic data has been processed until the time and pre-stack depth migra-tion, with the software for seismic processing Promax (Landmark) and Geodepth (Paradigm). Theseismic lines along dip, provided imagines of the Pacific convergent system, across the Sandino basinin the east, the slope, the middle American trench and the subducting Cocos plate to the west. Theysuggested a complex vertical movements history in the forearc basin of the convergent system. Theseismic lines along strike in the Sandino basin pointed out changes occurring from the north to thesouth. Actually the forearc is relatively thin in the southern part, thinning rapidly southward against theophiolitic Nicoya complex (Upper Cretaceous) of the Santa Elena peninsula of Costa Rica, that consti-tutes the margin wedge beneath the sedimentary succession. The forearc sediments thickness ap-proaches and locally exceeds 10 km in the central and northern parts of the Sandino basin. The oldestunits (Upper Cretaceous-Middle Eocene) are very thick off northern Nicaragua, with relatively thinmiddle to late Cenozoic deposits. However,in the off central Nicaragua the latter units (Middle-UpperMiocene) attain great thicknesses and the older units appear to thin. This pattern suggests a history ofsuccessive deepening of the basin from north to south, occurred after the Paleocene, when the conver-gent system evolved from accretion to subduction erosion processes. Present efforts are devoted toquantifying this change in development and using it to understand the dynamics of forearc basin andthe Pacific convergent margin evolution.

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Hydrodynamic Response of Subduction Zones to Seismic Activity: A

Preliminary Study for the Costa Rica Margin

Paula A. Cutillo1, Shemin Ge1, and Elizabeth J. Screaton2

1. Department of Geological Sciences, University of Colorado, Boulder, CO, USA2. University of Florida, Department of Geological Sciences, Gainesville, FL, USA

Seismic faulting is generally recognized as a potential mechanism for the rapid and episodic movementof fluids along fault zones, and is frequently cited as the cause for chemical and thermal anomaliesobserved at convergent margins. Faulting produces poroelastic deformation and coseismic strain changesin the crust, disturbing pore-pressure fields and driving fluid flow. The rupturing of permeabilitybarriers due to slip, or hydrofracture may also be a principal mode of fluid transport. It has beenproposed that seismic activity at the Costa Rica convergent margin may enhance fluid circulation andheat and solute transport within the oceanic crust, causing low heat flow over the region and chemicalsignals at shallow depths. The relationship between the earthquake cycle and the generation and flowof fluids, however, remains poorly understood because pressures and temperatures within the seismogeniczone are difficult to measure due to great depths and technological limitations. Fluid flow and transportmodeling constrained by shallow observations can be an effective tool for determining the range offeasible fluid pressures and temperatures within the deeper seismogenic zone. A coupled two-dimen-sional model is utilized to investigate whether seismic events within the Costa Rica subduction zoneproduce observable thermal anomalies at the depth of shallow boreholes. An earthquake strain modelis used to generate earthquake-induced stress-strain fields and to calculate coseismic pore-pressurechanges for shallow seismic events. The postseismic diffusion of pore pressure over time, and result-ant thermal effects are then investigated using a numerical flow and transport model. Numerical simu-lations are constrained by data collected from seismic surveys, submersible dives and drilling pro-grams. The preliminary study provides a quantitative analysis of coseismic pore-pressure change andsubsequent postseismic diffusion processes and constrains the influence seismic events have over time.Preliminary results indicate that fault movement causes significant perturbation of the local pore-pres-sure field due to compressional and tensional forces. Temperature fluctuations are less prominent,suggesting that vertical-flow along high-permeability faults or fractures may be necessary to greatlyaffect geothermal gradients at the toe of the complex.

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20032003

Comparisons between seismicity and fluid flow behavior along the

western Costa Rica margin

Heather DeShon1, Kevin Brown2, Susan Schwartz1, Mike Tryon2, and LeRoy Dorman

1. University of California, Santa Cruz, Santa Cruz, CA, USA2. Scripps Institution of Oceanography, La Jolla, CA, USA

Costa Rica seismogenic zone experiment (CRSEIZE), a collaborative effort between UCSC, OVSICORI-UNA, UCSD/Scripps, Univ. of Miami, and GEOMAR, combined broadband and short-period seismicland arrays, broadband ocean bottom seismometers (OBS), oceanic fluid flux meters, and GPS receiv-ers in order to better understand the geometry and mechanical behavior of the seismogenic portion ofthe plate interface. One of the specific goals was to explore any possible relationship between mi-croseismicity and fluid expulsion activity on the forearc of the subduction system. The fluid flux meterswere co-located on the OBS offshore both the Osa and Nicoya Peninsulas from Sept. – Dec. 1999 andDec. 1999 - June 2000 respectively. Fluid flux meters located on OBS as part of the Nicoya Peninsulaexperiment recorded a number of fluid flow excursions on both the toe of the forearc wedge and on theincoming Cocos plate. These excursions lasted upwards of two weeks in some cases and had variableamplitudes at each station that correlated across three instruments along an ~30 km transect along thetoe of the wedge. Initial comparisons of local and regional seismicity rate and fluid excursions fromstations located on the toe of the forearc wedge suggests a causal link. The nature of the link remainspoorly resolved at this time and more than one process may be involved. In addition, Osa Peninsulafluid flux meters located seaward of the Middle America Trench also recorded an unexplained fluidexcursion simultaneously across 3 meters placed 15 km apart on the incoming plate. We will present amore detailed analysis of local and regional seismicity recorded by the both the Nicoya Peninsula andOsa Peninsula CRSEIZE experiments during the time period recorded by co-located fluid flux metersfocusing on comparisons of these datasets. Our objective is to determine if and how changes the localand regional seismicity patterns may be linked to the flow events recorded in the fluid flux meters.

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20032003

Seismic Attenuation in the Subduction Zone of Costa Rica

LeRoy Dorman1, Allan Sauter1, Susan Schwartz2, Heather Deshon2, Andy Newman2, MarinoProtti3, Sue Bilek4, Ernst Flueh5, Tim Dixon6

1. Scripps Institution of Oceanography, UCSD, La Jolla, CA2. University of California, Santa Cruz, CA3. OVSICORI, UNA, Heredia, Costa Rica4. University of Michigan, Ann Arbor, MI5. GEOMAR, Kiel, Germany6. RSMAS, U of Miami, Miami, FL

Newman and others (GRL, 2002) using permanent and temporary land stations as well as Ocean-Bottom Seismometers, have shown a significant change in depth to the top of the seismogenic zone atthe boundary between EPR and CNS crust. These plate partitions differ in heat flow, so their thermalstructures should be different, although there is little difference in water depth. Attenuation of seismicwaves, especially S, is increased by heating so attenuation becomes a useful proxy for temperature.Attenuation studies usually use the spectral ratio method, in which spectra observed at two stations aredivided, so that common elements in the product-of-transfer-functions equation (source spectrum, in-strument corrections) are removed. There is, however, a way of treating many observations from asingle source in a single solution, rather than pairwise. By taking the logarithm of the transfer functionequation, the product becomes a sum, and can be treated as a row in a matrix equation containing dataat multiple stations at multiple frequencies (Dorman, JGR, 1968, 1969). The bulk of the data process-ing is being done within the framework of the Antelope relational database system, which offers sev-eral programs to calculate spectra and provides comprehensive housekeeping tools. The system, how-ever, is intricate and we have not yet been successful in making all the programs operate with thewaveform and response files we generated for the OBSs. The result of this is that we have to do moreprocessing station-by-station than we had hope. The solution for the matrix equation described in thethird paragraph is stable, and eigenvector expansion techniques yield insight to the process. The solu-tion vector is tripartite, consisting of the source spectrum, the range dependent amplitude terms, and 1/Q. The value of 1/Q is associated with a large eigenvalue, so it is well-determined. Preliminary analy-sis shows values of about 500 for Q

p. This result is subject to revision as we examine more data.

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The Seismogenic Zone Along the Alaska-Aleutian Trench

Jeff Freymueller1, Chris Zweck2, Steven Cohen3, Sigrun Hreinsdottir1, and Hilary Fletcher1

1. Geophysical Institute, University of Alaska Fairbanks, AK, USA2. Alfred-Wegener-Institut, Bremerhavn, Germany3. NASA Goddard Space Flight Center, Greenbelt, MD, USA

GPS data from Alaska show evidence for dramatic along-strike variations in plate coupling. Strainrates in the upper plate forearc and arc range from nearly zero in the western Shumagin segment to~2x10-7 /year or locally higher near Prince William Sound. Dislocation models estimated from the GPSdata show that either the downdip width of the seismogenic zone or the strength of frictional couplingwithin it, or both, must vary substantially along strike. Even in the presence of complexity frompostseismic deformation and forearc transport, geodetic data can be used to identify strongly and weaklycoupled segments of a subduction zone. The along-strike distribution of strongly-coupled and weakly-coupled regions in Alaska today matches very well the regions of high and low slip, respectively,during the last series of great earthquakes along much of the Alaska-Aleutian trench. This suggests thatseismic asperities are persistent rather than ephemeral features. The available data suggest that thealong-strike transition from strong to weak coupling can occur over very short distances, 10s of km orless. Large variations in plate coupling over very short distances means that slowly-varying featureslike convergence rate, dip angle, and age or sediment thickness of the downgoing slab are not thefundamental controls on plate coupling and the seismogenic zone, although they may have a second-order influence.

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20032003

Along strike variability in the seismogenic zone and thermal models of

subduction below the Nicoya Peninsula, Costa Rica

Robert N. Harris1, Andrew V. Newman2, and Kelin Wang3

1. Unversity of Utah, Utah, USA2. Los Alamos National Laboratory, New Mexico, USA3. Pacific Geoscience Center, British Columbia, Canada

Data from the Nicoya portion of the Costa Rica SEIZE seismic network, consisting of 34 land andocean bottom seismometers, successfully image the seismogenic zone beneath the Nicoya Peninsula.Precise locations of 650 earthquakes are used to define the plate interface and delineate the updip limitof the seismogenic zone. The updip limit of seismicity occurs approximately 20 km under the northernportion of the Nicoya Peninsula and abruptly shallows to approximately 10 km under the southernportion of the peninsula. This distinct change along strike coincides with a change in the origin ofoceanic crust and an apparent change in the degree of crustal cooling through hydrothermal circulation.Oceanic crust being subducted under the northern Nicoya Peninsula was created at the East PacificRise (EPR) while oceanic crust being subducted under the southern Nicoya Peninsula is of nearly thesame age but was created at the Cocos-Nazca Spreading center (CNS). While the marine heat flowdata is extremely heterogeneous there is a correlation between low values of heat flow to the north andhigher values of heat flow to the south with the depth of seismicity. To investigate thermal influenceson the updip limit of seismicity we construct thermal models of subduction beneath the Nicoya Penin-sula. Initial geotherms of the incoming plate, representing different effective cooling depths of hydro-thermal circulation, affect the thermal structure and temperatures along the main subduction thrust.Comparisons of precise earthquake hypocenters with thermal models of subduction indicate that theupdip limit of seismicity is consistent with temperatures between 100° – 150° C, and the significantalong strike variation may be caused by changes in the thermal regime.

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20032003

Secular, Transient and Periodic Crustal Movements in Japanese

Subduction Zones, and Dynamics Underlying Them

Kosuke Heki

Earth Rotation Div., Nat. Astron. Obs., Mizusawa, Iwate, Japan

The Japanese Islands are made of two continental plates that collide with each other in central Japanand two oceanic plates subduct beneath them. In spite of such a complicated plate tectonic setting,basic crustal kinematics are fairly well understood there thanks to a dense continuous GPS networkthat covers the entire country with average distances of about 20 km. Observed phenomena includeplate movements, and inter-, co-, and postseismic crustal movements, and periodic movements. Dy-namics underlying these variety of crustal behaviors are being clarified by incorporating informationprovided by other nationwide networks such as seismometers, strainmeters and meteorological sen-sors. Basic physical processes include interplate mechanical coupling, various modes of sliding atfault surfaces, and seasonal change of surface loads. The talk includes reviews as well as new findings,and will address the question “What should we do next?”.

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Images of Seamount Subduction Beneath Nankai Margin

P. Henry1, V. Martin2, S. Lallemant3, M. Noble2, S. Kuramoto4

1. Labo de G ologie de l’ENS (CNRS UMR 8538), Paris, France2. Centre de G ophysique de l’Ecole des Mines de Paris, Fontainebleau, France3. U. de Cergy-Pontoise, France4. AIST, Tsukuba (now at Jamstec, Yokosuka), Japan

Subducting oceanic crust topography exerts important control on the tectonics and seismogenic behav-ior of Nankai margin. This has now been recognised in central Nankai were a large seamount lies in theNankai-Do 1946 rupture zone and, likely, influenced the distribution of slip. In the Tokai area, to thewest of the To-Nankai 1944 rupture zone, the subduction of a basement ridge has been suggested basedon tectonic interpretation, magnetic anomalies and seismic refraction surveys. As part of the SFJ MCS3D survey on Tokai thrust, a series of 2D regional lines have been shot and one of the subductingseamounts of the Paleo-Zenisu ridge has been imaged. The geometry of the volcano is a 2km high by10 km diameter cone. The decollement and accretionary wedge are uplifted over a wider area, reachingthe base of the Tokai splay fault system about 10 km north of the summit of the volcano.

The To-Nankai rupture zone appears limited to the present extension of the Kumano forearc basinand stops near where basement topography is subducting, suggesting a similar pattern as in CentralNankai. In the Tokai area, a similar sedimentary sequence as the one underlying the Kumano basin hasbeen uplifted and shortened by broad folding and faulting. This recent deformation is likely the conse-quence of Paleo-Zenisu ridge subduction and, hypothetically, of earlier subduction of other ridgesprotruding from the Izu-Bonin arc. Tokai thrust is the lateral extension of the splay fault in which theTo-Nankai rupture terminated. Kodaiba fault is another potentially seismogenic fault rooted in themain plate boundary. At the intersection of Kodaiba with Tenryu canyon, evidence of recent rapiduplift is found. We suggest that splay fault activity is influenced by the subduction of basement topog-raphy.

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What We Know about Subduction Thrust Faults

R.D. Hyndman

Pacific Geoscience Centre, Geological Survey of Canada & SEOS, University of Victoria, Canada

Most of the world's great earthquakes and tsunamis are generated by rupture on the "seismogenic zone"of subduction thrust faults. What we know about the seismogenic zone comes mainly from: (1) greatand smaller thrust earthquakes, (2) land geodetic data, (3) ocean drilling and exhumed subductionthrusts, (4) seismic reflection - wide angle data, (5) thermal data and models, electrical sounding etc.Some subduction zones generate maximum M7 earthquakes, some reach M9. Great thrust earthquakesand smaller thrust events usually do not extend to the trench; there is an updip aseismic zone. Thrustearthquakes also are limited to a specific maximum depth for each subduction zone, i.e., to a maximumdepth of ~10 km for some island arcs to a maximum of about 50 km for some continental subductionzones. The maximum magnitude (related to maximum fault rupture area) is related first to the maxi-mum downdip rupture width, i.e., between updip and downdip seismogenic limits. Secondly, subduc-tion zones with very smooth incoming oceanic plates may have infrequent but very large events (up toM9), whereas rough incoming plates with seamounts, fracture zones, etc., that cause stress concentra-tion, may have smaller more frequent thrust earthquakes (M<8). At some continental subductionzones, great earthquake rupture occurs within or at the base of large accretionary sedimentary prisms.For other subduction zone, there is little accreted sediment and substantial undercutting erosion of theforearc crust is inferred. For most subduction zones, the updip great earthquake rupture limit appearsto correlate with a thrust temperature of 100-150C; the physical explanation, clay dehydration, porefluid pressure, etc. is debated. The downdip seismic limit for most, but not all, subduction zones agreeswith either a maximum temperature of 350C, or the thrust intersection with the forearc serpentinizedmantle. Seismic moment release data indicate that much of the subduction plate convergence is notaccommodated in earthquakes; the earthquake fraction is large for some continental subduction zonesand small for some island arcs. However, land geodetic data seem to require almost complete thrustlocking between earthquake events in most measured locations. Thus, the aseismic motion is probablymostly in great earthquake afterslip, and in "aseismic" slip events rather than continuous aseismic slip.Some slip events are too slow for much seismic energy but fast enough to generate tsunamis ("tsunamiearthquakes"), i.e., slip duration of 5-10 min., a few other slip events are still slower and are seen onlyin geodetic data. Models for fault slip behaviour must accommodate this range of event slip rates.Such models also must accommodate evidence that subduction thrust faults are very weak, from re-gional stress estimates and earthquake data. High fluid pressure that reduces the fault normal stress aresuggested. The nature of rupture "asperities" inferred from modelling of great earthquake data andtheir association with local variability in thrust structure remains unclear. Recent GPS geodetic datahave indicated slow slip events downdip of the thrust seismogenic zone, not associated with earth-quakes. In at least one location these slip events have an associated seismic tremor. Each of these slowslip events loads the locked part of the thrust further updip, and must represent a period of increasedprobability of great earthquake rupture. Ocean drilling (ODP) has shown that the thrust in the updipaseismic region is quite thin and, at least in sedimentary prisms, has high fluid pressure. The multi-

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channel reflection character in this updip region is variable from inferred positive to negative imped-ance contrasts that may relate to lateral pore pressure variations. At greater depth, within the lockedseismogenic zone, the multichannel seismic reflection images also generally show a thin seismic re-flector and inferred sharp thrust contact. Still deeper, below the seismic zone in the slow slip region,the multichannel reflection images show a ~5 km thick shear zone. There is some support for this shearzone thickening with depth in exhumed faults. The thrust reflection thickness change may allow map-ping the downdip seismic-aseismic transition. Magnetotelluric data and receiver function analysesshow high electrical conductivity and high Poisson's ratio in the latter thick reflection band region;both types of data suggest overpressured layered fluid concentrations.

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20032003

Variations in Basement Topography and Sediment Thickness on the

Philippine Sea Plate Subducting Along the Nankai Trough

T. Ike1, J-O. Park2, G.F. Moore1, Y. Kaneda

1. Dept. Geology and Geophysics, Univ. Hawaii, Honolulu, HI USA2. IFREE, JAMSTEC, Yokohama, JAPAN

Remarkable variations in basement topography and sediment thickness characterize the northern ShikokuBasin crust that is being subducted along the Nankai Trough. These variations are reflected in dramaticalong-strike differences in structure of the accretionary prism. We have analyzed regional seismicreflection profiles collected by the Japan Marine Science and Technology Center (JAMSTEC) to de-fine basement structure and sediment sequence stratigraphy on the Shikoku Basin seaward of the NankaiTrough. Our analysis concentrates on the lower section of the stratigraphic sequence that is subductedbeneath the toe of the Nankai accretionary prism. In the southeast part of the area, south of KyushuIsland, the Shikoku Basin crust is relatively smooth, with basement relief of no more than 100 m, sosediment thickness is not highly variable. The lower Shikoku Basin sequence, identified at ODP Site1177 and DSDP Site 297 is 325-375 m thick in this region, with a basal turbidite unit that is approxi-mately 300 m thick. The Kinan Seamount chain, associated with the extinct spreading center in theShikoku Basin is being subducted southeast of Shikoku Island. Sediment thickness in this region variesgreatly and is controlled by variable topography of the seamount chain. The lower Shikoku Basinsequence varies from a minimum of 350 m on top of structural highs to greater than 1.5 km in interven-ing structural lows. The basal turbidite unit is generally absent on structural highs, but is greater than700 m thick in the lows. South and east of the Kii Peninsula, the Shikoku Basin basement is dominatedby the Zenisu Ridge, a large crustal fragment associated with the Izu-Bonin arc. Between the ridge andthe Nankai Trough, basement topography is relatively smooth and the lower Shikoku section is 300-700 m thick; the basal turbidite sequence is 250-400 m thick. Southeast of the Zenisu Ridge, the lowerShikoku Basin sequence is difficult to recognize and is generally less than 100 m thick.

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20032003

Deformation in granular aggregates: Implications for strength of porous

rocks and shear within fault zones

Stephen L. Karner

Dept. Geology and Geophysics, Texas A&M Univ., College Station, TX

Deformation in granular rocks and mature fault zones is accommodated via a complex interplay ofphysical and chemical mechanisms. A common assumption for porous rocks is that intergranular de-formation occurs by elastic strains near contacting grain boundaries, thereby altering porosity andpermeability. As such, this mechanism is often employed to explain the development of fluid overpres-sures. Yet, poro-elastic models neglect the significant strains that occur due to grain rearrangement(e.g. rolling, frictional slip at grain boundaries), grain cracking and fracture, and fluid-rock interactions(e.g. solution transfer, pressure solution, cementation, in-situ mineralization). Rock mechanics experi-ments designed to investigate these processes produce results consistent with geologic observationsand with research in other areas (e.g. soil mechanics and the material sciences).

Experiments on initially loose, granular quartz sand show that deformation occurs by a combination ofelastic and inelastic mechanisms. Microscopic observations and acoustic emission (AE) data indicatethat comminution is favored when applied mean stress is increased. Despite exhibiting permanent strainsat all stresses, samples display an interval of quasi-elastic strain (with elevated AE rates) prior to macro-scopic yield. Results from experiments on quartz sand define a failure envelope (consistent with criticalstate and CAP models of soil mechanics) that scales inversely with initial grain size and temperature.Time-dependent mechanisms also influence volumetric strain evolution in granular media. Compactionrates in quartz sand increase with the presence of pore water, effective stress, and temperature, perhapsdue to fluid-assisted mechanisms such as stress-induced dissolution and sub-critical crack growth. Whensubjected to a pressure increment, volumetric strain rates and associated AE rates are initially large anddecay logarithmically with time. After several weeks, creep data show a significant transition to greaterstrain rates while maintaining the previous AE rates. This may coincide with a switch in mechanism frombrittle deformation to acoustically silent chemical processes (e.g. fluid-assisted solution transfer).

These observations can be applied to fault studies. Seismic data indicate that coseismic stress dropsincrease with earthquake recurrence times, suggesting that fault strength evolves during the interseismicperiod. This may result from physico-chemical processes that change the real area of contact betweenopposing fault walls or, if fault gouge is present, the gouge consolidation state. Fault healing rates areknown to depend on physical parameters (e.g. stressing rate, fault stiffness) and the character of theshear zone (e.g. surface roughness, shear displacement, presence of gouge). Yet, hydrothermal labora-tory experiments indicate that fluid-rock interactions and the chemical character of the system alsoeffect fault strength and fluid transport properties (e.g. mineral diagenesis, in-situ mineral alteration,solution transfer and precipitation, solution-assisted creep compaction). A natural outgrowth of labora-tory friction experiments has been the development of several constitutive relationships that describefrictional sliding behavior. Of these, the rate- and state-dependent friction laws have received consid-erable attention and are increasingly being used for models of faulting and earthquake rupture dynam-ics. In light of their increasing popularity, it is important to validate and modify these constitutiverelations using laboratory data for a wide range of conditions.

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20032003

Comparative study on exhumed seismogenic faults and modern Nankai

seismogenic megathrust

Gaku Kimura1,2, Arito Sakaguchi2, Kohtaro Ujiie2, Airaro Kato2, J-O Park2, Yujin Kiramura1,Eisei Ikesawa1, Masayuki Matsumura1 and Yoshitaka Hashimoto3

1. University of Tokyo, Tokyo, JAPAN2. Institute of Frontier Research on Earth Evolution, JAMSTEC, Yokosuka, JAPAN3. Kochi University, Kochi, JAPAN

One of the critical questions to understanding the seismogenic subduction zone is what controls theupdip limit of the zone. The question is quite significant to address why the rupture zone stops in somecase or propagates to shallow portion and results in the tsunamigenesis in another case. The Nankaitrough is a single most unique subduction zone in the world, where historical repetition of large earth-quakes and tsumani are well recorded more than millenium and the updip limit is located enoughshallow to drill by riser vessel. That is the reason why we propose a drill into the seismogenic plateboundary in the Nankai Trough. Before drilling, we are conducting a comparative research on ex-humed seismogenic fault in ancient accretionary complex on land, which was once located in thesesimogenic depth, and modern Nankai Trough of targeted site for drilling. Several new findings fromthe exhumed rocks stimulate the understanding of the sesimogenic zone and sharpen the targets bydrilling.1) Break and involvement of oceanic basement into the accretionary complex as blocks of melange is

suggested to be a seismic process because of ubiquitous brittle (cataclastic to ultracataclastic) break-age of the oceanic fragment in contrast to ductile deformation of underthrusted sediments.

2) A boundary fault such as sandstone dominated coherent unit (might be offscraped) and mudstonedominated chatoic one (might be underthrusted) is a clear seismogenic fault and a candidate for amajor seismogenic plate boundary. A discovery of pseudotachyllyte from the fault strongly suggestsa melting lubrication for rupture propagation around the updip limit of the sesimogenic zone.

3) Detailed analysis of the fault suggests a repeated activity of pressure solution creep (interseismic?)alternated with cataclastic or melting slip (coseismic) within a narrow (>30 cm) fault zone.

4) Crack seal mineral veins are also developed in and around the fault. P-T condition from H2O-CH

4

fluid inclusions and vitrinite reflectance suggests the development of the veins within the seismogeniczone. Vein development especially within the fault and shale dominated part below the fault sug-gests the place where dehydrated fluid pool and pathway is strongly controlled by structural andlithological setting.

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20032003

Very focused expulsion of pore fluid along the western Nankai

accretionary complex detected by closely-spaced heat flow

measurements

Masa Kinoshita1, Shusaku Goto2, Sean Gulick3, Hitoshi Mikada1, KR02-10 Shipboard Scien-tific Party

1. JAMSTEC, Japan2. Eartq. Res. Inst., Japan3. Univ. Texas, USA

During the KR02-10 cruise onboard R/V KAIREI, JAMSTEC, intensive heat flow measurements werecarried out across the western and middle Nankai Trough areas, in order to reveal thermal and hydro-logical process across the frontal thrust and the Large Thrust Slice Zone (LTSZ). Previous heat flowdata suggest that the Nankai accretionary complex is basically thermal-conduction dominant, exceptfor strongly channelized flow along the faults. Heat flow was measured using two types of geothermalprobes: a 4.5m geothermal probe lowered from the ship, and two 60 cm probes manipulated by ROVKAIKO. Probe positions were controlled using SSBL acoustic navigation with the accuracy of 30-70m. We obtained 19 heat flow data across the second frontal thrust off Muroto. Heat flow is highest atthe base of the second frontal thrust. Maximum heat flow reaches up to 280 mW/m2 and its width isprobably less than 50 m. We observed no indication of seepage activity at this site. Upslope we founda cold seep site, which was distributed along a topographic contour of 4620 m. Although we measuredheat flow in the middle of seep site, no heat flow anomaly was detected. We obtained 12 heat flow dataacross the lower part of LTSZ off Muroto. Two local heat flow anomalies of up to 250 mW/m2 weredetected, both of which are related to cold seep activities. The amplitude of heat flow anomalies issimilar to that observed in the frontal thrust area, although the basal heat flow here, 60-80 mW/m2 , ismuch lower that in the frontal thrust area. Also, the width of the anomaly seems similar to frontal thrustarea. These data indicates that fluid flow is restricted within the fault or in the hanging wall, andotherwise the thermal regime in the accretionary complex is conduction dominant. On the other hand,difference in heat flow anomaly locations between two areas may provide insights into the maturity ofcold seep activity and the thrust as fluid conduits.

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20032003

Continental deformation in the Central Andes controlled by changing

subduction parameters and geologic inheritance

Jonas Kley

Universität Jena, Germany

The Central Andes are the world’s largest active non-collisional mountain range. On a large scale thearc-shaped Central Andes are highly symmetric about the plate convergence vector, suggesting thatcontinental deformation is strongly controlled by the direction of Nazca-South America convergencethat has remained nearly constant over the last 45 Ma. The present-day displacement field derived fromGPS data and a displacement field for the past 25 Ma estimated from geological reconstructions agreewell in displacement rates and directions. Their close link with the convergence direction supports thenotion that the Central Andes have been built in a rather uniform kinematic regime. The displacementpattern also indicates that the orogen’s arc shape in map view is the effect of varying displacementmagnitudes rather than directions. Displacements are highest in the center of the arc and decreasetoward the tips. These variations have been conditioned by inherited inhomogeneities of the continen-tal crust, in particular thick sedimentary cover sequences and areas of earlier continental extension.The segmentation by old discontinuities coincides in part with the present-day steeper and nearly hori-zontal segments of the subducting Nazca plate, indicating a control of upper plate deformation on slabdip. The geologic history of the Central Andes suggests a decreasing influence of subduction param-eters on continental deformation as the orogen thickened and widened. An early pulse of contractionaldeformation around 40 Ma coincides with a phase of rapid convergence. The onset of strong deforma-tion, rapid uplift and building of the Central Andean high plateau about 25 Ma ago was also triggeredby an increase in the Nazca-South America convergence rate. Andean orogenic deformation began inthe west and then migrated eastward in an unsteady way. By about 10 Ma deformation essentiallyceased in what is now the Central Andean plateau and moved to its east flank where it is still activetoday, as far as 700 km away from the trench. This switch in the locus and mode of continental defor-mation does not coincide with any obvious change in subduction parameters. The location of the pla-teau, however, is in part controlled by the areal extent of a much older folding event. Continentaldeformation rates apparently increased as the deformation shifted eastward and remain high today,contrasting with the coeval decrease in plate convergence rate and suggesting strong intrinsic controlsduring the later phases of Andean mountain building. The uplift history and mechanics of the CentralAndean plateau are not well understood. While numerical models indicate that plateaus must reachtheir final elevation before deformation becomes focussed on their margins, the available geologicevidence suggests that the Central Andean plateau stood only at about half its present elevation whendeformation shifted to the marginal thrust belt, and that uplift has since accelerated. Data on recentuplift rates would add crucial information.

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20032003

Partitioning of Seismogenic Strain in the Offshore Costa Rica Forearc

Jonathan C. Lewis1 and Susan Bilek2

1. University of Massachusetts, Amherst, MA, USA2. University of Michigan, Ann Arbor, MI, USA

We examine non-recoverable strain in the offshore forearc crust between the Osa and Nicoya Peninsu-las of Costa Rica. Treating the crust as a micropolar continuum we use new and published earthquakefocal mechanisms to invert for best-fitting partial strain-rate tensors. Our preliminary results for newfocal mechanisms west northwest of the Osa Peninsula indicate a shallow northwest-plunging maxi-mum stretching axis and a subvertical minimum stretching axis. This strain geometry suggests thin-ning of the forearc and extension nearly parallel to the Middle America Trench. The earthquakes usedto constrain this deformation geometry occurred at depths less than 5.1 kilometers in hanging wallcrust adjacent to substantial topographic relief on the downgoing Cocos plate. To date evidence fortrench-parallel stretching has only come primarily from observations of brittle faults in coastal andinland exposures in Miocene sediments and Quaternary fluvial deposits. Our inversions of existingdata from southeast of the Nicoya Peninsula yield results that are in accord with previous findings.Here, events ranging in depth between about 12 and 31 kilometers indicate nearly trench-normal short-ening and crustal thickening. Earthquakes at similar depths just west northwest of the Osa Peninsula,however, reveal subhorizontal maximum and minimum stretching axes with the latter trending south.This strain geometry is consistent with strike slip faulting and suggests partitioning of seismogenicstrain horizontally in the forearc offshore Osa Peninsula. In total, our findings suggest variations in the3-dimensional pattern of non-recoverable strain in the Costa Rica forearc. A quantitative description ofthis sort of variation is necessary in order to better understand geodetic observations, and ultimately,the relation between coupling across the seismogenic zone and forearc strain.

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20032003

Seismogenic Strain at the Costa Rica Convergent Margin

Jonathan C. Lewis1 and Allan Lopez2

University of Massachusetts, Amherst, MA, USAInstituto Costarricense de Electricidad, San Jos , COSTA RICA

We have expanded our analyses of 3-dimensional seismogenic strain at the Middle America conver-gent margin (see original abstract below) to include the Costa Rica arc inboard of subducting seafloortopography. At shallow crustal levels seismogenic strain is dominated by arc-subparallel stretchingaccompanied by either crustal thinning or horizontal shearing. Two spatially-related subsets of seismicevents characterized by horizontal shearing record non-zero components of relative vorticity (W) thatindicate block rotations about the intermediate stretching-rate axes (d

2). West of the highest part of the

arc, the relative vorticity value suggests counterclockwise rotation about a moderately steep axis whereasin the high peaks region, the relative vorticity value suggests the opposite sense of rotation. A subset ofseismic events north of the high peaks suggests clockwise north-viewed horizontal axis rotation ac-companying crustal thinning. All of our results indicate plane strain with the exception of just north-west of the highest topography where subhorizontal flattening is indicated. These preliminary findingssuggest block rotations of varying sense in a setting where significant topography is being subducted.

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20032003

Possible Forearc Sliver at the Lesser Antilles

Alberto M. López and Seth Stein

Northwestern University, Evanston, IL, USA

Boundaries of the Caribbean Plate (CA) include two subduction zones at the western and easternboundaries. Interestingly, there are two main points to note: 1)none of these have CA crust beingsubducted, and 2) both were excluded from global plate motion models due to their fitting problems.Several years have passed of research at plate boundaries, and we now know that misfits and discrep-ancies are now attributed to biased slip vectors at oblique subduction zones. Many researchers workedon the interactions between the CA, North America (NA), and the Cocos (CO) plates, yielding differentresults through different methods and data used. However, recently, the use of GPS have shed valuableinformation on the problem by comparing it to the standard procedures. DeMets (2000) have shownthese results and have suggested the presence of a forearc sliver whose translation is parallel to theMiddle America trench. Although the eastern Caribbean also suffered from misfits on global platemotions, and it is also an oblique plate scenario, plate motions are extremely slow compared to itswestern counterpart. However, slip vectors can be estimated and GPS measurements are currentlytaken. We propose the presence of a possible forearc sliver at the Lesser Antilles Arc.

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20032003

Fault Friction and the Transition From Seismic to Aseismic Faulting

Chris Marone1 and Demian Saffer2

1Penn. State University2University of Wyoming

Plate boundaries are often divided into three main zones: an aseismic updip zone, the seismogeniczone, and a deep aseismic zone. Identifying and understanding the transitions from seismic to aseismicfaulting are key goals in understanding subduction zone megathrusts. We focus on the mechanics andfrictional rheology required for stable and unstable faulting.

Two hypotheses for the updip stability transition have been proposed. The clay mineral hypothesisposits that a thermally-driven transition from dominantly smectite to dominantly illite clay produces atransition from aseismic to seismic behavior. The consolidation/lithification hypothesis posits that thestability transition is the result of a change from granular shear accommodation, in which grain crush-ing and bulk consolidation are important, to localized shear within highly consolidated (lithified) ma-terial, for which friction is dominated by adhesive contact mechanics. We review laboratory frictiondata and constitutive laws in the context of requirements for unstable faulting.

We report on laboratory experiments designed to investigate the frictional behavior of smectite-illite clays and their possible role in determining frictional stability in subduction zones. We observe acoefficient of friction (m) of 0.42-0.68 for illite shale, consistent with previous work. Over a range ofnormal stresses from 5-150 MPa and sliding velocities from 0.1–200 mm/s, this material exhibits onlyvelocity-strengthening behavior, opposite to the widely expected, potentially unstable velocity-weak-ening behavior of illite. Smectite sheared under identical conditions exhibits low friction (m = 0.15-0.32) and a transition from velocity weakening at low normal stress to velocity strengthening at highernormal stress (>35 MPa). Our data suggest that the transformation of smectite to illite results in anincrease in friction, but do not support the hypothesis that the smectite-illite transition is responsible forthe seismic-aseismic transition in subduction zones. We suggest that the other depth- and temperature-dependent processes, such as cementation, consolidation, and slip localization with increased shearing,may play an important role in changing the frictional properties of subduction zone faults, and thatthese processes, in addition to clay mineralogy, should be the focus of future investigation.

Implications of the laboratory data and the associated stability analysis are compared with fieldobservations including the spatial distribution of slip during dynamic rupture, the depth frequencydistribution of seismicity, and earthquake afterslip.

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20032003

Characteristics of the Nicaragua convergent margin and their possible

influence on seismicity and tsunami generation

Kirk McIntosh1, Imtiaz Ahmed1, Eli Silver2, Arnim Berhorst3, Ernst Flueh3, and Cesar Ranero3

1. University of Texas Institute for Geophysics2. University of California Santa Cruz3. GEOMAR, Kiel, Germany

Seismic reflection and refraction data acquired offshore Nicaragua provide key information to helpevaluate the seismogenic zone characteristics at this convergent margin. This margin is particularlyinteresting due to the tendency for tsunami generation associated with interplate earthquakes. Al-though the processes are not currently well understood, structural characteristics of both the upper andlower plates along this part of the Middle America Trench may be important for tsunami genesis. Ourdata set demonstrates the pervasive presence of large, trenchward-dipping normal faults in the subduct-ing Cocos plate, some of which expose basement at the seafloor, as well as common small to moderate-size seamounts (to ~1.5 km height). Using the seismic reflection and the seismic refraction data we canalso evaluate the tectonic and velocity structure of the upper plate margin wedge. These characteristicslikely influence or reflect the rigidity of the upper plate and hence its capability of storing elastic strain.Our work to date suggests that seismic velocities are higher here than in typical margin wedges, and wewill try to investigate what rigidity is implied by the velocity structure.

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20032003

Relationship of Fluids and Deformation at Seismogenic Depths:

Structural Study of the Rodeo Cove Thrust Zone, Marin Headlands,

California

Francesca Meneghini 1, J. Casey Moore 2

1.Universita’ di Pisa, Pisa, Italy2. University of California, Santa Cruz, Ca., USA

The thrusts associated to the subduction zones interplay with an important circulation of over pres-sured fluids generated during the dewatering of the sediment pile undergoing lithification and diagen-esis. We hypothesize that fluid flow associated with deformation cause physical and chemical changesthat influence the seismic behaviour of faults, in particular strengthening of fault surfaces throughchanges in cohesion and coefficient of friction. An investigation of rocks of exhumed subduction thrustsacting at seismogenic depth is reported, focusing on the fabric and the deformational processes devel-oped in the thrust zone. We provide geometric, lithologic, mineralogic, and structural informationabout how sediments act in the seismogenic zone, and how the fluid flow, influence the onset of thestick slip behaviour. The Rodeo Cove Thrust Zone outcropping in the Marin Headlands area, North ofthe Golden Gate Bridge of San Francisco, imbricates two underplated basalt-chert-sandstone sequencesthat are part of the Marin Headlands terrane of the Franciscan Complex. The youngest sediments in thissequence are Late Cretaceous, consistent with underplating shortly thereafter. The thrust outcrop is a200 m thick, E-W striking, S-SW dipping, complex zone, displaying a range of stratal disruption fromincipient deformation in mappable coherent units, to broken formation with high percentage of veins(Fault Core). The development of discrete fault systems synthetic to the main thrust (R and P fractures)occur at every scale. These faults, involving essentially basaltic lithologies, are marked by cataclasiteswith a shaly matrix showing scaly foliation. The cleavage is defined as a web of syndeformationalchlorite layers showing evidence of shear (S-C structures). The syndeformational recrystallization ofpumpellyte in the basalts constrains the depth of fault formation between 6 and 12 km, within theaseismic to seismic transition. A complex system of carbonate filled (rarely quartz filled) veins up toone cm thick run parallel to the discrete faults in the fault core. Veins occur both in the fragments andthe matrix parallel to the chlorite foliation. The veins are either folded and truncated along the cleavageand their microstructures indicate that they formed during multiple dilatancy episodes. The underplatingof slabs of oceanic crust and overlying sediments in the Marin Headlands occurred in a Late Creta-ceous seismogenic zone. The carbonate veins in the fault core are controlled by injection of overpres-sured fluids causing hydrofracturing, permeability increase and mineralization. Overprinting co-planarevents of veining and shearing indicate large changes in fluid pressure that may occur on the timeinterval of seismic cycles. Mineralization during interseismic periods of veining could significantlyincrease the cohesion of the fault zone and linked to strengthening of fault surfaces.

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20032003

Evolution of a ductile shear zone in the down-dip continuation of the

seismogenic zone

Laurent G. J. Montési 1 and Greg Hirth 1

(1) Woods Hole Oceanographic Institution, Woods Hole, MA

Understanding the processes that control the down-dip termination of the seismogenic zone (SZ) is oneof the goals of the SEIZE experiment. This limit may correspond to a rheological transition, fromunstable frictional sliding (velocity-weakening), to either stable frictional sliding (velocity-strength-ening) or ductile creep, possibly aided by serpentine. Structural geology provides much evidence forductile shear zones below brittle faults and geodesy indicates that a narrow region in the down-dipcontinuation of the SZ deforms at a relatively high rate after earthquakes. It is therefore reasonable topostulate the presence of a ductile shear zone below the SZ. This shear zone could be the locus ofpostseismic deformation and control the down-dip extent of the SZ. However, the formation of thisshear zone and its possible expression in geophysical and geodetic datasets must be better understoodthan at present if its presence below the SZ is to be ascertained. For ductile materials to form a localizedshear zone, they must be weaker than the surrounding materials. However, the strength of ductile rocksis higher at the higher strain rate that prevails in a localized shear zones than at slower distributed strainrate. The weakness of the ductile shear zone is thought to result from a difference of microstructure,principally a diminished grain size. However, grain size reduction can only weaken a rock if its micro-structure is sufficiently far from equilibrium. Near the SZ, an obvious source of stress perturbation isprovided by earthquakes. During, and immediately after earthquakes, the stress in the down-dip exten-sion of the SZ increases, so that a ductile shear zone accelerates and its grain size decreases by dynamicrecrystallization. This produces postseismic deformation, which may explain geodetic studies. We de-scribe how this origin of postseismic creep can be differentiated from alternative mechanisms (e.g.stable frictional sliding or Newtonian creep) using the shape of the decay curve of postseismic creeprate and how characteristic time scales of postseismic creep change with depth. An implication ofductile creep triggered by earthquake is a permanent reduction in grain size below the SZ if graingrowth is inhibited by the presence of a second phase. We use a simple shear zone model includingdislocation creep, diffusion creep, and grain size evolution to show how repeated earthquakes influ-ence the microstructure of a ductile shear zone.

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20032003

Factors Controlling the Frictional Strength of Sheet-Structure Minerals

Diane E. Moore 1 and David A. Lockner 1

(1) U. S. Geological Survey, Menlo Park, CA, USA

We compare the frictional strengths of 17 sheet-structure minerals, measured under oven-dried andwater-saturated conditions, to identify the factors that cause them to be relatively weak compared tomost other rock-forming minerals. We ran room-temperature frictional sliding experiments on sawcutrock samples containing a 1-mm layer of gouge of a given mineral separate. The samples were vacuum-dried at 120 degrees C overnight and then immediately tested in a triaxial apparatus at 100 MPa normalstress and 0.0005 mm/sec sliding velocity. The samples were sheared dry to 4 mm axial displacement,then water was introduced to a pressure of 10 MPa and sliding was resumed at 100 MPa effectivenormal stress to 9 mm axial displacement. Additional samples were sheared dry to 9 mm displacement.A preliminary study (Morrow et al., 2000, Geophys. Res. Lett., v. 27, pp. 815-818) had indicated apossible correlation between the coefficient of friction of a dry gouge and the strength of its interlayerbonds, and the new results verify this correlation. The values of dry coefficient of friction rangeupwards from 0.18 for graphite, levelling off at 0.80-0.85 for margarite, clintonite, gibbsite, kaolinite,and lizardite. SEM examinations support the hypothesis that for those gouges whose dry coefficient offriction is less than 0.8, shear occurs by breaking through the interlayer bonds to form new cleavagesurfaces. For a mineral whose dry coefficient of friction equals 0.8, consistent with Byerlee’s law, theinterlayer bonds are sufficiently strong that other frictional processes dominate. This correlation sug-gests that the energy input associated with a given value of the coefficient of friction may be calibratedagainst the surface energy of that mineral. The addition of water causes the coefficient of friction todecrease for every mineral except graphite. Water-saturated coefficients of friction range from about0.2 for talc and graphite to about 0.7 for the brittle micas margarite and clintonite. If these minerals areseparated into groups with similar crystal structures, the water-saturated coefficient of friction increaseswith increasing interlayer bond strength in each group. We propose that the water in a saturated gougeexists as thin, structured films that are bonded to the plate surfaces proportional in part to the mineral’ssurface energy; the water-saturated coefficient of friction would then reflect the stresses required toshear through the water films. Increasing temperature and pressure would tend to reduce the width ofthe water films and cause the strength to increase towards the limiting dry value.

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20032003

Summary of ODP Leg 190 results in the Nankai Trough

G.F. Moore1, Asahiko Taira2, Adam Klaus3

1. Dept. Geology and Geophysics, Univ. Hawaii, Honolulu, HI USA2. Center for Deep Earth Exploration, JAMSTEC, Yokosuka, JAPAN3. Ocean Drilling Program, Texas A&M Univ., College Station, TX, USA

The Nankai Trough accretionary prism is considered an “end-member” prism accreting a coarse terrig-enous sediment section in a setting with structural simplicity, unparalleled resolution by seismic andother geophysical techniques, and large historic earthquakes. It therefore has been the focus of OceanDrilling Program (ODP) drilling to address several unresolved questions concerning accretionary pro-cesses and prism evolution. At six sites cored along two transects across the Nankai Trough accretion-ary prism during ODP Leg 190, lithostratigraphy and sediment diagenesis vary markedly. For the firsttime, reference sites at the seaward ends of the two transects defined the stratigraphic framework of theaccreting/subducting Shikoku Basin sedimentary section. A thick section of Miocene turbidites andsmectite-rich mudstone is present within the subducting section at the Ashizuri site. The turbidites andmudstones are absent in the correlative section at the Muroto site; variations in lithology, mineralogy,and hydrologic properties of the incoming sediments probably contribute to the difference in prismwedge taper between the two transects, while possibly controlling the seismic character of the activeplate boundary. The décollement in both transects is localized within a common stratigraphic unit(~5.9 Ma) within the lower Shikoku Basin facies. The décollement is also a major boundary for bothphysical and mechanical properties. A broad low-chloride porewater anomaly in the lower ShikokuBasin unit, first identified at Site 808, progressively decreases in magnitude from prism to basin alongthe Muroto Transect. Physical properties relationships, evidence for mineralogic changes in the sedi-ments, and pore fluid chemistry suggest that the chloride anomaly results primarily from in situ diage-netic reactions in the sediments, possibly augmented by flow of freshened fluid from depth. Newconstraints on stratigraphy and age of units along more landward parts of the Muroto Transect havedramatically changed our ideas about the tectonic evolution of the prism in this area. Growth of theseaward-most part of the prism took place very rapidly, with 40 km of accretion within the past 2 Myr.This rate is at least three times greater than growth rates in a comparable prism.

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Consolidation State and Overpressures Within the Underthrust Section,

Nankai Accretionary Margin: Results of Uniaxial Reconsolidation

Experiments

Julia K. Morgan 1, and Maria V.S. Ask 2

1. Department of Earth Science, Rice University, Houston, TX 77005, USA2. Division of Rock Mechanics, Luleå University of Technology, SE-971 87 Luleå, Sweden

Uniaxial reconsolidation experiments conducted on Ocean Drilling Program drill cores along the MurotoTransect of the Nankai Accretionary Margin provide improved estimates for in-situ effective verticalstresses and excess pore within the underthrust sedimentary section. Tests were conducted on samplescollected from similar stratigraphic levels below the basal décollement and its age-equivalent seawardprojection. Sediments within the underthrust package are moderately lithified, a consequence of con-current diagenesis and consolidation. Nonetheless, initial yield strengths appear to be sensitive indica-tors of ambient in-situ effective vertical stress conditions. Consolidation state increases landward fromthe reference site, but records progressive increase in excess pore pressures, from ~0.35 MPa at theseaward reference Site 1173, to more than 3 MPa beneath the active prism. The seaward reference Site1173, and Site 1174, which passes through the protothrust zone, record normal consolidation historiesdespite their overpressured state. In contrast, underthrust sediments at Site 808 exhibit relatively lowyield stresses, despite considerable strength due to consolidation and matrix structure or “cementa-tion”. This behavior appears to reflect a reduction in in-situ effective stress conditions due to increasedpore pressures localized along the décollement zone. Concurrent diagenesis leads to secondary miner-alization and matrix cementation during stable, lower effective stress conditions, essentially “lockingin” the lower stress conditions. Rapid pore pressure drops, such as might accompany co-seismic slipalong the décollement and frontal thrust, would lead to renewed consolidation and break-down ofintergranular cement in the underthrust sediments, until pore pressures rebuild and interparticle bondscan be regenerated. The estimated yield strengths suggest that within ~50 m below the base of thedécolleme! nt at Site 808, in-situ pore pressures may be in excess of 6.9 MPa, or nearly 70% of effec-tive vertical stress in the presence of hydrostatic pore pressures. Similar reconsolidation experimentsmay help to constrain in-situ pore pressures at deeper levels within the prism, proximal to the seismogeniczone.

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Splay Fault Branching Along the Nankai Subduction Zone

Jin-Oh Park, Tetsuro Tsuru, Shuichi Kodaira, and Yoshiyuki Kaneda

(All at: Institute for Frontier Research on Earth Evolution, JAMSTEC)

Seismic reflection profiles reveal steeply landward-dipping splay faults in the rupture area of the mag-nitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These splay faults branch upwardfrom the plate-boundary interface (that is, the subduction zone) at a depth of ~10 kilometers, ~50 to 55kilometers landward of the trough axis, breaking through the upper crustal plate. Slip on the activesplay fault may be an important mechanism that accommodate the elastic strain caused by relativeplate motion.

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Mapping the plates interface from the trench to the maximum depth of

the Wadati-Benioff Zone under Costa Rica

Marino Protti1, Victor González1, Susan Schwartz 2, Andy Newman3, Heather DeShon2 andLeRoy Dorman4

(1) OVSICORI-UNA, Heredia, Costa Rica.(2) University of California, Santa Cruz, CA, US(3) Los Alamos National Laboratory, Los Alamos, NM, US.(4) Scripps Institution of Oceanography, UCSD, CA, US.

We will present a three-dimensional map of the Cocos-Caribbean and Cocos-Panama Block plate inter-faces under Costa Rica. This 3-D view will cover from the Middle America Trench to the maximumdepth of the Wadati-Benioff zone. We are integrating results from active experiments off-shore as wellas seismic events located both, by the permanent telemetric seismic network of OVSICORI-UNA aswell as the temporal networks installed in southern and northern Costa Rica, as part of a SeismogenicZone Experiment from September 1999 to June 2001. Over this map of the plates interface we willoverlap the upper and lower limits of the seismogenic zone as preliminary results of the above men-tioned experiments. These results will be useful for seismic risk assessments in Costa Rica.

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Seismic imaging of the megathrust in Central-Southern Costa Rica

Cesar R. Ranero 1, Roland von Huene 1, Soenke Neben 2, Udo Barckhausen 2

1. GEOMAR, Kiel, Germany2. BGR, Hannover, Germany

We present prestack depth migrated sections from multichannel seismic reflection data collected alongthe Pacific convergent margin of Costa Rica. The data are located across the margin in the area wherethe flank of the Cocos Ridge is subducting. The Cocos ridge creates a unique tectonic setting due to thelow subduction angle and the high heat flow associated to the thick crust. Those two characteristicspotentially bring processes that ussually occur at depth of more than 6-8 km depth to less than ~ 4 kmdepth. Thus it provides an ideal scenario for drilling. We will discuss the depth seismic images in termsof the geometry of tectonic elements, seismic atributes like amplitude and relation to lateral variabilityin the character of the incoming plate. Also, we will present data on how the macro-velocity models forthe prestack depth migration are built and thus on the accuracy of the geometry of the structure and thedepth to the plate boundary. In addition the velocities produced during pre-stack depth migration aregeologically meaningful and thus can be used for geological interpretation. The seismic depth sectionswill be discussed integrated with high resolution swath bathymteric, sidescan sonar data and seafloorobservations from the area. The data will be presented to foster discussion on the drilling targets for theIODP proposal on drilling the seismogenic zone in Middle America.

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Relative relocation of intermediate depth seismicity: A double Wadati-

Benioff Zone below the Central Andes

Andreas Rietbrock 1, and Felix Waldhauser 2

1. University of Liverpool, UK2. Lamont-Doherty Earth Observatory, Columbia University, New York, USA

In the last several years numerous temporary local networks have been installed in northern Chile andwestern Bolivia to image the intermediate depth seismicity. While a double Wadati-Benioff-Zone (WBZ)could be clearly observed at approximately 19°S, its existence further to the south is discussedcontroversely. Even by using 3D velocity models and local earthquake tomography techniques theabsolute location error in these seismicity studies is in the range of a few kilo meters. Using relativerelocation techniques the relative accuracy can be pushed in the 100 m scale and revealing structures inthe seismicity pattern, which could not been identified before. In 1996 a temporal seismic array wasinstalled in northern Chile and western Bolivia to image the seismicity of the WBZ between 20°S and24°S (ANCORP experiment). We use manual determined onset times to peform a relative location ofthe intermediate depth seismicity in a depth range between 90 km and 130 km. Relative relocations areperformed in a previously determined minimum 1D model and compared to the ones performed in a3D velocity model based on local earthquake tomography. We observe a clear double WBZ between21.5°S and 22.5°S with a depth seperation of about 9 km to 10 km. We associate the upper band withthe plate interface and earthquakes occuring in the subducted oceanic crust. The thicknes of the incom-ing oceanic crust is about 7 km, as determined by former refraction and refelction experiments. There-fore the lower band of seismicity is situated at the top of the subducted oceanic mantle. We can notobserve a clear magnitude correlation between upper and lower band seismicity.

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Contrasts in veining and faulting across the aseismic to seismic

transition in a sediment-rich accretionary complex

Christie Rowe 1, Eric Thompson 1, and J. Casey Moore 1

1. Earth Sciences Dpt, University of California, Santa Cruz, California, USA

Subduction thrust systems produce the world‚s largest earthquakes. The transition from aseismic toseismogenic faulting occurs at approximately 5-8 km depth. The chemical and physical controls onthis transition are not well understood, but previous research indicates that phase transformations, fluidpressure changes, and formation of authigenic minerals and cements may produce changes in cohesionand coefficient of friction that control fault behavior. We have described and sampled areas of paleofaulting and fluid flow in n early Tertiary subduction thrust system, Kodiak Islands, Alaska. We com-pare two formations: the Eocene Sitkalidak Formation, from the shallow decollement, and the upperPaleocene Ghost Rocks Formation, which experienced burial to seismogenic depths. Our goal is todefine the explicit differences in structural style and vein/cement development in these two formationsthat were emplaced above and below the aseismic to seismic transition. The Sitkalidak Formation ischaracterized by wide zones (meters to 10s meters) of webs of shear surfaces and very fine (<1mm)veins of carbonates and zeolites. Conjugate shear fracturing and associated lustrous, scaly black resi-dues on striated surfaces, indicate the onset of stratal disruption. Bedding-normal dilational carbonateveining follows, indicating rising fluid pressures. In contrast, the Ghost Rocks Formation containsdiscrete, focused, heavily veined fault zones only a few meters wide that crosscut well-developedmélange fabric. Individual quartz and clean calcite veins commonly reach thicknesses of several cmand display evidence of multiple episodes of dilational crystal growth. Brecciated wall rock and evenquartz vein material completely supported by calcite cements indicate high fluid pressures followed byrapid precipitation, suggesting episodic veining events.

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Geophysical and drilling evidence implies that a thick sandwich( 0.5->1.0

km) of sediment and detached continental margins material

separates the upper and lower plates;

So what does this mean for forearc seismicity and

expected deep drilling encounters?

David W. Scholl

Department of Geophysics, Stanford University; [email protected]

INTRODUCTIONEvidence accumulating over the past 30 years indicates that the by-passing process of sediment sub-duction and the crustal thinning process of subduction erosion strongly influence the tectonic evolutionof most (at least 80 %) convergent margins. In recent decades the combination of offshore drilling andgeophysical data have made it possible to estimate the volume and thickness of ocean floor and slopesediment and larger chunks of upper plate crust traveling beneath the margin as material within thesubduction channel. The underthrusting material of the subduction channel (MSC) separates the morecoherent rock framework of the margin from the igneous ocean crust of the downgoing plate. Presum-ably, the active interplate boundary (decollement) occurs within the MSC, the mechanical and physicalproperties of which will affect seismicity in the upper reaches of the seismogenic zone.

TERMS OF REFERENCEThe upper plates of convergent margins are constructed of three principal rock and sediment bodies:

(1) an interior, seaward tapering framework of consolidated material or basement rock˜the marginwedge,

(2) an overlying sequence of continental slope deposits, and(3), at the base of the margin, a frontal prism of tectonically disrupted rock and sedimentary material.

The frontal prism is constructed of accreted ocean-floor sediment (trench section) of the lower plate(accretionary prism, e.g. Barbados, Cascadia, Nankai), upper plate continental margin sediment androck debris transported to the base of the slope (slope-material prism, e.g., Costa Rica), or, conceptu-ally, a combination of accreted and slope material (composite prism). Tectonic ownership of upper andlower plate material is defined by the subsurface position and seaward extent of the active interplatedecollement that, near the base of the margin, separates the domains of the two plates. At subductionzones, three processes interact to steer the tectonic evolution of the submerged forearc:

(1) subduction accretion,(2) sediment subduction, and(3) subduction erosion.

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These terms are used in this abstract to mean the following:

(1) Subduction accretion is the tectonic addition of lower plate material to the rock and sediment bodyof the upper plate;

(2) Sediment subduction is the tectonic by-passing of the frontal prism by lower plate sediment under-thrusting the margin‚s backstop of older basement rock, i.e., the seaward end of the margin wedge(e.g., for northern Chile, underthrusting beneath the coastal basement of Mesozoic and older agethat extends seaward to near the inner trench wall, and, for the Lesser Antilles, sediment under-thrusting beneath the seaward terminus of its arc massif, which lies deeply buried inboard of thetrench axis and a wide frontal prism of accreted ocean floor sediment.

(3) Subduction erosion is the detachment of upper plate material—rock or sediment˜and the transportof this material toward the mantle with the underthrusting lower plate.

THICKNESS OF THE MSCDrawing on drilling, geophysical, and coastal mapping information, the thickness of the MSC can beestimated for both regional and local sectors of convergent margins. Good estimates for the efficiencyof subduction of lower plate ocean sediment are available for all but those margins fronted by largeaccretionary prisms, (e.g., Nankai, Cascadia, Lesser Antilles), which border about 20 % of all subduc-tion zones. For margins edged by small to moderate size frontal prisms (5-40 km wide)(80 % of allmargins), the efficiency of ocean floor sediment subduction is ~80 to 100 %. Globally, the averagevolume of subducted ocean floor sediment is probably about 30 km3/Myr/km of margin, which in-cludes a guestimate that 40% ocean floor sediment subduction occurs at margins fronted by frontalprisms that are massive accretionary bodies.

Rates of crustal thinning attributed to subduction erosion have been estimated for the Japan, Tonga,South Sandwich, Chile, Peru, Costa Rica, Guatemala, and Mexico margins. Erosion rates are bestderived from evidence documenting substantial (4-5 km) subsidence of a former shoreline or subaerialunconformity. For example, the coring or dredging of nearshore and subaerial deposits near the base ofthe lower landward trench slope requires the removal of at least 10-12 km of formerly underlying crustsince the deposits formed. Crustal thinning can also be calculated based on the progressive seawardtilting of slope deposits, and the landward migration of the axis of arc magmatism. The global long-term (> 15-20 Myr) average rate of subduction erosion is presently estimated at ~40 km3/Myr/km ofmargin (range = 28-62). This rate includes the volume of continental slope sediment and rock debrisstreamed into the MSC from a frontal prism (frontal erosion) and that removed from the base of themargin‚s rock framework (basal erosion) landward of the frontal prism.

In combination, beneath the submerged forearc, the long-term, global average volume of oceansediment and upper plate material shunted along the subduction channel toward the mantle is thus ~70km3/Myr/km of margin. The thickness of the MSC is ~1 km, 60 percent of which is removed upperplate crust and sediment and 40 percent subducted ocean floor sediment. These number make it clearthat subduction erosion contributes the greater bulk of the MSC in transport between the plates andthus along the length of the seismogenic zone.

LOCAL IMPLICATIONSLittle information is available for the Nankai margin, which is bordered by a large accretionary prism,concerning rates at which ocean sediment by-passes the frontal prism or the amount of subduction

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erosion occurring here. However, drilling and acoustic imaging of the Nicoya sector of the Costa Ricamargin document that sediment subduction is virtually 100 % efficient and that the long-term (16.5Myr) rate of subduction erosion has averaged 40-45 km3/Myr/km of margin. More significantly, be-cause of the underthrusting of the thickened ocean crust flanking the Cocos Ridge, forearc erosionduring the past 6 Myr has increased for periods of time to ~100 km3/Myr/km of margin. Thus, since thelatest Miocene, at the present orthogonal convergence rate, the thickness of the MSC contributed byeroded debris alone has been at least 1 km. Including underthrusting ocean sediment dewatered to zeroporosity, the MSC is roughly 1.3 km thick, at least 75% of which is eroded material from the upperplate.

Deep crustal drilling of the Costa Rica margin in the proposed Osa Peninsula region will reach,beneath a blanket of slope sediment, into the forearc‚s unsampled framework of basement rock, pre-sumably largely mafic igneous rock of Late Cretaceous age widely exposed in the coastal area, andthen deeper still to the unsampled material of the underlying UMC. New research along the coastalregion of adjacent Panama implies that early Tertiary arc magmatic rocks may be encountered in theforearc basement and thus included in the MSC.

The proposed Complex Drilling Proposal for the Costa Rica margin will thus provide ground-truthinginformation about the evolution of the forearc, the role (importance) of subduction erosion, and thephysical and compositional nature of the MSC through which the active interplate boundary runs.

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Evaluation of the Updip Limit of the

Seismogenic Zone in Central Costa Rica

S.Y. Schwartz 1, H.R. DeShon 1, and S.L. Bilek 2

1. Univ. of California, Santa Cruz, CA, USA 2. Univ. of Michigan, Ann Arbor, MI, USA

The shallow seismogenic portion of subduction zones generate damaging large and great earthquakes.We provide geometric constraints on the seismogenic zone of the Middle America Trench offshoresouthern central Costa Rica based on aftershocks of the M

w 6.9 August 20, 1999, Quepos, Costa Rica

underthrusting earthquake. We locate 300 aftershocks using a 3D model based on available P-wavevelocity information and a temporary local network of land and ocean bottom seismometers. We useaftershock locations to define the geometry and characteristics of the seismogenic zone in this region.The majority of aftershocks occur below 13 km depth, 40-45 km from the trench and above 30 kmdepth, 95 km from the trench. These events define a plane dipping at 19° that marks the interfacebetween the Cocos and Caribbean Plates. Although we are confident that the 1999 event and its after-shocks did not rupture the plate interface at depths shallower than about 10 km, we would like toestablish whether or not this shallower part of the plate interface can behave seismogenically (exhibitstick-slip behavior). Preliminary locations of a moderate underthrusting earthquake (M

w 6.4) on June

16, 2002 just southeast of the 1999 Quepos event place it close to the trench, at the updip edge of theseismogenic zone. We propose to model teleseismic waveforms from this event to obtain its depth andestablish its position on the plate interface. We will also relocate its aftershock seismicity by combiningdata from several networks in Costa Rica and Panama to evaluate whether this event ruptured a shal-lower portion of the plate interface than the 1999 Quepos event. Establishing the updip edge of seis-micity in this region is especially important since it is the location of a proposed deep drill hole into theseismogenic zone.

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The Nankai Trough versus the Sagami Trough - from a viewpoint of

dehydration

Tetsuzo Seno

Earthquake Res Inst, Univ of Tokyo

Between the Nankai Trough and the Sagami Trough, there is a difference in mode of occurrence ofgreat earthquakes. In the Nankai Trough, the recurrence of great earthquakes is regular, with an intervalof 90-150 years for the past 600 years. The amount of the fault slip for historical earthquakes (Ando,1975) indicates that the relative plate motion is consumed almost in the seismic slip, i.e., the seismiccoupling ratio is ~1. There are two types of great earthquakes, i.e., the 1923 Taisho and 1703 GenrokuKanto earthquakes in the Sagami Trough region. They shared the fault plane of the Taisho earthquake.The Holocene marine terrace data give the recurrence of the faulting of this common plane as 400 years(Kayane and Yoshikawa, 1986). Because the amount of fault slips of these events is on the order of 6 m(Ando, 1971) and the convergence rate of 3 cm/yr (Seno et al., 1993), the seismic coupling ratio is 0.5.This contrast between the Nankai and Sagami Troughs might be a corollary of the contrast in dehydra-tion from the subducting Philippine Sea slab as stated below. Recently, Obara (2002) found that lowfrequency tremor is aligned within the forearc upper plate around the Moho depth along the strike ofthe subducting Philippine Sea slab. Interestingly, there are places where no such tremor is found; theyare Kanto-Izu Peninsula, E. Shikoku and S. Kyushu. I note that these locations are where non-normaloceanic crust is subducting; beneath Kanto, the Izu-Bonin Ridge, beneath S. Kyushu, the Kyushu-Palau Ridge, and beneath E. Shikoku, the Kinan seamount chain are subducting, respectively. Theformer two have island arc crust. The last one is part of the Shikoku Basin. Near the axis of the spread-ing center, volcanism occurred after cessation of the spreading, producing the seamounts with 3 kmheight relative to the ocean floor. Significant amount of melting of the oceanic crust would have oc-curred, producing crust different from the normal oceanic crust. If the main part of island arc crust iscomposed of granitic rocks, it is expected that dehydration from the subducted island arc crust wouldbe minor, compared with from the typical metamorphosed oceanic crust. Then forearc low frequencytremor would not be expected where island arc crust is subducting. Minor dehydration would alsomake the occurrence of great earthquakes in the thrust zones rare due to low pore fluid pressure in thethrust zone. If this is true, based on the dehydration embrittlement hypothesis, it is expected that earth-quakes do not occur within the crustal part of the subducting Philippine Sea slab, where low frequencytremor is not found. I will show that in fact the slabs in these regions show no activity in the crust. Thesubduction of normal oceanic crust vs. island arc crust would then be the largest contrast between theNankai Trough and the Sagami Trough.

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The Cellular Shear Mesh of the Chrystalls Beach Accretionary Melange:

Relevance to the Active Hikurangi Margin Subduction Thrust Interface

Richard H. Sibson

Department of Geology University of Otago P.O. Box 56, Dunedin New Zealand

The seismogenic thrust interface of the active Hikurangi Subduction Margin in southeastern NorthIsland, New Zealand, dips ~10° NW over a depth range of 10-25 km and exhibits variable couplingalong strike (Reyners, 1998: N.Z. J. Geol. Geophys. 41, 343-354.). Evidence that seismic rupturingalong the thrust interface and in its hanging wall occurs within strongly overpressured crust includesthe low taper of the offshore accretionary prism, the presence of active mud volcanoes in the immediatehanging wall of the seismogenic interface, and the distribution of highly overpressured (<95% lithostatic)oil exploration wells up to 4 km deep along the forearc hanging-wall. Additionally, portions of theseismogenic thrust interface are defined by belts of microseismic activity ~1-2 km thick associatedwith anomalous V

P/V

S ratios and other parameters diagnostic of extreme localized overpressuring

(Reyners et al. 1999: Geophys. J. Int., 137, 873-890). Deformation within an accretionary mélangedeveloped in a Triassic subduction complex exposed at Chrystalls Beach, SE Otago, took place in asubgreenschist environment (P = 2-4 kbar; T = 200-300°C) broadly comparable to the active Hikurangisubduction thrust interface. The mélange, composed predominantly of sandstone phacoids set in asheared and cleaved pelitic matrix, is disrupted by a cellular shear zone mesh comprising multiple slipsurfaces subparallel to cleavage coated with incrementally developed quartz/calcite slickenfibres. Slidingsurfaces splay and amalgamate and are also commonly interlinked by sets of extension veins localisedwithin dilational jogs. These, together with numerous extension veins cutting the more competentsandstones suggest that the tensile overpressure condition (P

f > s3) was widely achieved with deforma-

tion occurring under near-lithostatic fluid overpressures. Individual slip surfaces can be traced formetres to many tens of metres and petrographic investigations of microstructures suggest that theslickenfibres developed incrementally in sub-millimetre to millimetre displacements, consistent withmicroearthquake slip increments. A cellular shear mesh of this kind forms a potential analog to stronglyoverpressured portions of fault zones such as the subduction thrust interface along the Hikurangi Mar-gin, accounting for the association of clustered microseismicity with repeating microearthquakes atspecific structural sites along fault segments that are also undergoing aseismic creep.

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Composition of sediments on the incoming Cocos Plate, offshore Costa

Rica

Glenn Spinelli and Michael Underwood

University of Missouri, Department of Geological Sciences

We have determined the biogenic silica (opal) content, clay content, and clay composition of sedimentsoffshore Costa Rica on the incoming Cocos Plate. Within subduction zones, the diagenesis and dewa-tering of opal and smectite may affect fluid pressure, fluid flow patterns, and frictional properties.Fluids recovered from the decollement along the toe of the Costa Rica margin were generated at tem-peratures from 80-150°C, suggesting both opal and smectite dehydration at depth may be sources forthe fluids. Although the temperature ranges for opal and smectite diagenesis overlap, most of thereaction progress for opal dewatering occurs at lower temperatures (<100°C) than for smectite dewa-tering (60-160°C). Therefore opal dewatering will likely occur shallower in the subduction zone (closerto the trench) than smectite dewatering. The location of dewatering reactions in the subduction zonewill affect the distribution of fluid pressures and flow patterns along the plate interface. Preliminaryanalysis of shallow sediments overlying the Cocos Plate offshore the Nicoya Peninsula indicate a highopal content and a relatively low clay mineral content, however most of the clay is smectite. Given therelatively low clay content, the mechanical effect of altering the coefficient of internal friction duringthe smectite to illite transistion (a ~2x increase) may be limited, but a combination of dehydrationreactions will act as fluid sources within the subducted sediments. The relatively high opal content andlow clay content of the sediments suggest that the effect of diagenetic fluid sources on fluid pressurewithin the subduction zone may be greatest relatively close to the trench (<100°C) and diminish sharplywith distance into the subduction zone (100-160°C). The measured amounts of opal and smectite withinthe subducted sediments will be used for both evaluations of fluid flow and frictional properties withinthe Costa Rica margin subduction zone. Future research will include modeling heat and fluid flowwithin the subducted sediments in response to sediment compaction, heating from the underlying oce-anic crust, and diagenetic fluid sources. With the volume and distribution of fluid release duringdehydration reactions well constrained, we will calculate the proportion of the pore fluid chemistryanomalies observed at the decollement near the toe of the sedimentary wedge that can be explained byopal and smectite dewatering.

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A Coupled Hydrological and Geomechanical Study of the Nankai Trough

Earthquake Recurrence

Eyal Stanislavsky 1, and Grant Garven 1

1. Johns Hopkins University, Baltimore, MD, USA

The interplay between groundwater and solid deformation is important in the mechanics of faultingand in fluid migration. Fluid pressures in the crust are close to lithostatic in many active geologicalsetting implying on the connection between fluid pressure and tectonic stress. Over-pressured fluid isproposed as a mechanism responsible for overthrust faulting, hydrocarbon migration, ore precipitation,and earthquake recurrence. High fluid pressure is known to trigger seismic faulting and in return,seismic events change pore pressure distribution, cause aftershocks and hydrological phenomena.

In this work I propose a fully coupled fluid-deformation numerical model for earthquake recur-rence. I want to apply the model to the Nankai Trough, southwestern Japan (figure 1), which is one ofthe best places for modeling poroelastic processes because fluid pressure are known to be high, thehistory of earthquake recurrence in Nankai Trough is well documented, the mechanical boundary con-ditions are known, and the geometry of the Nankai Trough and the mechanical properties of the differ-ent layers are known from extensively seismic and drilling exploration. This model will have thecapacity to predict the fluid flow, pore pressure and tectonic stress changes associated with futureearthquakes.

Episodic changes in Earth’s crust permeability are common in tectonically active areas. Accordingto the “Fault Valve” model, the faults are nearly impermeable barriers for fluid flow except immedi-ately after rupture, after which they briefly become highly permeable channelways for fluid discharge.At the seismogenic zone, failure will trigger an earthquake that will rupture the fault. Even though theearthquake nucleates at the seismogenic zone, it might rupture the upper shallow part of the faultpromoting upward discharge of fluids.

The poroelasticity equations are solved using the finite element method for a two-dimensional,plane strain, cross-section. The finite element code has already been written as a part of my Ph.D.thesis and was successfully applied to two generic scenarios where different poroelastic effects wereinvestigated (Stanislavsky and Garven, 2002, 2003). In the next phase of my thesis I want to apply thiscode to the Nankai thrust where the model could be constrained by geophysical measurements. Cur-rently, I am programming a new numerical technique for simulating more accurately the fault behavior.This technique involves adding a contact element that can transfer pore pressure and normal stress butcannot transfer shear stress.

Kodaira et al. (2000) present a detailed cross-section of the Nankai Trough seismogenic zone withlithological and mechanical properties. I plan to discretize this cross-section and use it to simulatecoupled hydromechanical processes of thrust faulting in the Nankai Trough.

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Heat Flow and Flexure at Subduction Zones

Carol A. Stein

University of Illinois at Chicago, Chicago, IL USA

At subduction zones it is observed that compared to the incoming plate, heat flow is generally lowerarcward of the trench and increases over the volcanic arc. On the incoming plate, seaward of the trench,faulting starts about 50-75 km away [Masson, 1991] and flexing of the plate may extend up to 300 kmaway. It has been assumed that flexing and faulting of oceanic lithosphere seaward of the trench in-creases hydrothermal circulation in the incoming plate and thus lower measured heat flow would beexpected over much of this region. To test this, the global heat flow data, 300 km oceanward of thetrench to up to 200 km arcward of the trench, are examined. However, for the global average heat flowthere is no clear decrease in heat flow in the oceanic crust approaching the trench when compared withthe average measured heat flow value for the same age crust. Many of the higher heat flow valuesarcward and adjacent to the trench may be due to the expulsion of fluids through the accretionaryprisms/forearc complexes, but on average, heat flow arcward of the trench is about 60% of that in theincoming plate.

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Deformation at plate boundaries and related thrust faults. A comparison

of ODP drilling with onland studies what can we infer about the

aseismic-seismic transition

Paola Vannucchi1, Cesar R. Ranero2 and, Don Fisher3

1. Universita' di Firenze, Italy2. GEOMAR, Germany3. PennState University, USA

Plate boundary faults produce the world most intense earthquakes starting at a depth of approximately4 to 6 km in response to chemical, physical and structural transformations of the rock which allow thetransition from creeping to stick-slip behavior. The onset of processes driving these changes are poorlyunderstood since the direct observation of active faulting mechanisms at plate boundaries is limited tofew hundreds of meters below the sea floor, and the information on deformation processes at seismicdepths are inferred from ancient, exhumed subduction thrusts. Until the seismogenic zone will bereached in active subduction zones, what can we expect to drill through? Here we focus on the erosiveCosta Rica convergent margin and compare to the accretionary Nankai Trough to examine what is thedifference in terms of incoming section, sediment properties at the décollement zone, fluid flow asso-ciated with the deformation and what is the result in deformation features and strain localization. Allthese parameters, on the other side, define these two plate boundaries at shallow depth, while theoccurrence of the major processes of erosion and accretion might turn to be the leading mechanisms indefining the onset of seismogenesis. On land accretion is widely recognized, while erosion is moredifficult to infer. The seismic-aseismic transition is defined on the PT conditions as resulting from thebulk of the rock or from the veins paragenesis, and assume a quasi-equilibrium state of the mechanicalbehavior with the background setting. Here we present examples of deformation structures across theseismic-aseismic transition coming from well-exposed subduction related faults of the NorthernApennines as well as examples of subduction deformation from Costa Rica and compare them toseismic images of the Middle America convergent margin.

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Index

A

Ahmed 8, 27

Ask 32

B

Bangs 3

Barckhausen 35

Berhorst 27

Bilek 4, 11, 23, 42

Brown, K 10

Brown, L 5

Bürgmann 6

C

Chadwell 7

Cohen 12

Costa Pisani 8

Cutillo 9

D

DeShon 4, 10, 34, 42

Deshon 11

Dixon 7, 11

Dorman 10, 11, 34

F

Fisher 47

Fletcher 12

Flueh 11, 27

Freymueller 12

G

Garven 45

Ge 9

González 34

Goto 21

Gulick 3, 21

H

Harris 13

Hashimoto 20

Heki 14

Henry 15

Hildebrand 7

Hirth 29

Hreinsdottir 12

Hyndman 16

I

Ike 18

Ikesawa 20

K

Kaneda 18, 33

Karner 19

Kato 20

Kimura 20

King 6

Kinoshita 21

Kiramura 20

Klaus 31

Kley 22

Kodaira 33

Kogan 6

KR02-10 Shipboard Scientific Party 21

Kuramoto 15

Kwiatkoswki 5

L

Lallemant 15

Levin 6

Lewis 23

Lockner 30

López 25

M

Marone 26

Martin 15

Matsumura 20

McIntosh 8, 27

Meneghini 28

Mikada 21

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Montési 29

Moore, D 30

Moore, G 18, 31

Moore, J.C. 37

Moore, JC 28, 37

Morgan 32

N

Nankai 3-D seismic working group 3

Neben 35

Newman 4, 11, 13, 34

Noble 15

Norabuena 7

P

Park 18, 20, 33

Protti 11, 34

R

Ranero 8, 27, 35, 47

Rietbrock 36

Rowe 37

S

Saffer 26

Sakaguchi 20

Sauter 11

Scholl 38

Scholz 6

Schwartz 4, 10, 11, 34, 41

Screaton 9

Seno 42

Shipley 3

Sibson 43

Silver 8, 27

Spiess 7

Spinelli 44

Stanislavsky 45

Steblov 6

Stein, C 46

Stein, S 7, 25

T

Taira 31

Taylor 8

Thompson 37

Tryon 10

Tsuru 33

U

Ujiie 20

Underwood 44

V

Vannucchi 47

von Huene 35

W

Waldhauser 36

Wang 13

Z

Zweck 12

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Documents

MARGINS Theoretical Institute on the2 MARGINS Theoretical Institute on the Seismogenic Zone Experiment (SEIZE) Snowbird, Utah, 16-21 March, 2003 MARGINS web site: