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Strain Release Along the Northern Costa Rica Seismogenic Zone. Susan Y. Schwartz Department of Earth and Planetary Sciences UC Santa Cruz. Modes of Strain Release. 1950 M~7.7. Costa Rica Subduction Zone-Instrumenting the Plate Boundary with a Seismic, GPS and Fluid Flow Network - PowerPoint PPT Presentation
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Strain Release Along the Northern Costa Rica Seismogenic Zone
Susan Y. Schwartz
Department of Earth and Planetary Sciences
UC Santa Cruz
Modes of Strain Release
1950 M~7.7
Costa Rica Subduction Zone-Instrumenting the Plate Boundary with a Seismic, GPS and Fluid Flow Network
Collaborators:
Tim Dixon, LeRoy Dorman, Kevin Brown, Marino Protti, Victor Gonzales, Heather DeShon, Edmundo Norabuena, Andy Newman, Sue Bilek, Ernst Flueh
EPR interface seismicity: 17-28 km
CNS interface seismicity:12-26 km shallower dip
% Locked
0 20 40 60 80 100
1. Up-dip limit of seismogenic zone is defined by shallowest geodetic locking and is consistent with models of thermal control (100-150o C)
2. Plate boundary earthquakes begin deeper than start of seismogenic zone
3. Plate boundary earthquakes terminate shallower than continental Moho and 350 C isothermThermal Modeling by Spinelli and Saffer ( 2004)
300o C isotherm from Harris and Wang (2002)
Implications
1. Locked portion of plate boundary is accumulating strain to be released in next large earthquake
2. Something is weakening the plate boundary to allow the transition from locked with no seismicity to unlocked with seismicity at ~250o C
What causes the onset of microseismicity at 15-17 km depth where modeled temperatures on the plate interface are ~200-250oC
Stable sliding
Stick-slip
Fault zone weakening by increased pore-fluid pressure from low grade metamorphic reations in basalt
Fluid flow excursions caused by 3 episodes of slow slip on the plate interface
Brown et al. (2005)
Interseismic Strain Accumulation
Slow Earthquake Signal
2003 Geodetically observed slow slip event
% Locked
0 20 40 60 80 100
Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior
Global Distribution of Episodic Aseismic Slip
Modified from Dragert and Rogers [2004]Modified from Obara and Hirose [2005]
DIFFERENT FRICTIONAL PROPERTIES CONTROL FAST VS. SLOW SLIP
Velocity Weakening Velocity Weakening/ Strengthening Transition
Strain Accumulation Slip- Abundant Microseismicity
Coseismic Slip- Asperity Afterslip
Slow Slip Events
2005 Nias Earthquake AfterslipNortheast Japan strain accumulation,
asperity and afterslip patterns
Different frictional properties control fast vs. slow slip
No observations of slow slip in strongly coupled regions exist
Hot or warm subduc-tion zones have deep slow slip
Cascadia
SW Japan
Mexico
Cooler subduction zones or with thin overriding crust have shallow slow slip
NE Japan (afterslip)
Boso Japan
Costa Rica
Deep slow slip may require frictional transition at shallow depth (low pressure) or fluids generated from dehydration reactions (baslate-eclogite).
New GPS/Seismic/Tilt Nicoya Network for Detection of Slow Slip Events
Collaborators:
Tim Dixon, Kim Psencik (UM), Marino Protti, Victor Gonzales (OVSICORI-UNA)
Technical Support:
Dan Sampson (UCSC), Jacob Sklar & Freddy Blume (UNAVCO)
SFB574 Borehole Seismic:
Ernst Flueh, Wolfgang Rabbel, Martin Thorwart & Nilay Dinc
CONCLUSIONS:
Strain along the northern Costa Rica plate boundary is released in large earthquakes and slow slip.
Spatial separation exists between these two modes of strain release with strain presently accumulating just offshore the Nicoya Peninsula and slow slip occurring in regions up and down-dip of this.
A dense network of cGPS, seismic and tilt stations has been installed to improve our understanding of slow slip at this plate boundary.