3-Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone..pdf

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ders the sinking of this higher-density material,

this process further increases the gravity anomaly

at the basin center. The net effect is that isostatic

uplift of the surrounding depressed surface to-

pography and crustal collar, combined with cool-

ing and contraction of the melt pool, create the

central positive free-air anomaly. The flexural

strength that enables the inner basin to rise into

a superisostatic state prevents the outer basin

from fully rising to isostatic equilibrium, leaving

the observed ring of negative free-air anomalythat surrounds the inner basin.

Isostatic uplift raised the surface topography

of the Freundlich-Sharonov basin by ~2 km at

the center of the basin (Fig. 3A). These effects

place the final basin depth at just over 4 km, a

value which is consistent with LOLA elevation

measurements (11, 21). For the Humorum basin,

the inner basin was calculated to rise ~3 km (Fig.

3B). This uplift distribution would have left the

Humorum basin ~4 km deep before mare fill.

Infilling of a 3-km-thick mare unit and associated

subsidence brings the floor depth of the Humorum

basin to just over 1.5 km deep, modestly deeper

then the 1 km depth measured by LOLA (21).The free-air gravity anomalies of both basins

increased markedly after crater collapse as a re-

sult of cooling and isostatic uplift. The free-air

anomaly of the Freundlich-Sharonov basin is

predicted to have risen to a positive 80 mGal in

the inner basin and – 200 mGal in the outer basin

above the thickened crust, which are figures in

excellent agreement with GRAIL observations

(Fig. 1C, red line) (1). Furthermore, the model

predicts an outer annulus of positive anomalies,

which is also in agreement with observations. A

similar post-impact increase in the free-air anom-

aly is observed in our model of Humorum basin

(Fig. 1D, red line), although this gravity anomalycannot be verified because the Humorum basin

was subsequently partially filled with mare ba-

salt. Our results support the inference that lunar

basins possess a positive gravity anomaly in

excess of the mare load (5). As a final step in our

analysis, we emplaced a mare unit 3 km thick and

150 km in radius (tapered to zero thickness

over the outermost 50 km in radial distance)

within the Humorum basin. The addition of the

mare increases the mascon at the center of the

Humorum basin to 320 mGal (Fig, 1D, blue line),

matching GRAIL measurements (1).

This basin evolution scenario depends pri-

marily on the energy of the impactor, the thermal

gradient of the Moon at the time of the impact,

and the thickness of the crust. A high thermal

gradient enables weaker mantle to flow more

readily during the collapse of the transient crater,

resulting in less inward motion and thinning of

the crust. In contrast to hydrocode parameters

that control crater excavation and collapse, such

as the energy of the impactor and the initial ther-

mal gradient, the close match of our predicted

free-air gravity anomalies to those observed by

GRAIL is not a product of finding a special com-

bination of finite-element m odel parameters as-

sociated with isostatic uplift and cooling. These

processes are controlled by the evolution of the

density and viscosity structure in the model, which

follow from the mineralogy of the lunar crust and

mantle and the evolution of temperature as the

region conductively cools.

References and Notes1. M. T. Zuber et al., Science 339 , 668 (2013).

2. P. M. Muller, W. L. Sjogren, Science 1 61, 680

(1968).

3. D. E. Smith et al., J. Geophys. Res. 98, 20,871 (1993).

4. D. E. Smith et al., Science 336 , 214 (2012).

5. G. A. Neumann, M. T. Zuber, D. E. Smith, F. G. Lemoine,

J. Geophys. Res. 101 , 16,841 (1996).

6. M. A. Wieczorek, R. J. Phillips, Icarus 13 9, 246

(1999).

7. E. Pierazzo, H. J. Melosh, Icarus 145 , 252 (2000).

8. J. C. Andrews-Hanna, Lunar Planet. Sci. 43 , 2804

(2012).

9. W. L. Sjogren, R. N. Wimberly, W. R. Wollenhaupt, Moon

9, 115 (1974).

10. M. T. Zuber, D. E. Smith, F. G. Lemoine, G. A. Neumann,

Science 266, 1839 (1994).

11. D. E. Smith et al., Geophys. Res. Lett. 37 , L18204

(2010).

12. “Isostatic adjustment” as used here is the process by

which the stresses imparted in a non-isostatic

crust–mantle volume are relieved as they drive densityboundaries toward mass balance (isostasy). The level of

isostasy achieved depends on viscosity-controlled flow

and also on the finite strength of the system as

characterized by lithospheric flexure. This “isostatic

adjustment” includes the uplift of the basin center to

a superisostatic position as a result of its flexural

coupling to the subisostatic annulus.

13. H. J. Melosh, D. M. Blair, A. M. Freed, Lunar Planet. Sci.

43, 2596 (2012).

14. A. A. Amsden, H. M. Ruppel, C. W. Hirt, LANL Rep.

LA-8095, 101 pp., Los Alamos Natl. Lab., Los Alamos,

N. M. (1980).

15. G. S. Collins, H. J. Melosh, B. A. Ivanov, Meteorit. Planet.

Sci. 39 , 217 (2004).

16. K. Wünnemann, G. S. Collins, H. J. Melosh, Icarus 18

514 (2006).

17. The precise value of the impact velocity is not critica

this computation because a lower impact velocity can

be compensated by a larger impactor, and vice versa

The impact velocity distribution on the Moon is stron

skewed toward high velocities, with a mode at 10 km

and a median of ~15 km/s ( 25).

18. M. A. Wieczorek et al., Science 339, 671 (2013).

19. B. A. Ivanov, H. J. Melosh, E. Pierazzo, in Large Meteo

Impacts and Planetary Evolution IV , W. U. Reimold,

R. L. Gibson, Eds. (Special Paper 465, Geological Soc

of America, Boulder, Colo., 2010), pp. 29–49.

20. G. Schubert, D. L. Turcotte, P. Olson, Mantle Convect

in the Earth and Planets (Cambridge Univ. Press,

Cambridge, 2001).

21. More detailed descriptions of these models and meth

are available as supplementary materials on Science On

22. A. M. Freed, S. C. Solomon, T. R. Watters, R. J. Philli

M. T. Zuber, Earth Planet. Sci. Lett. 285, 320 (2009

23. A. M. Freed et al., J. Geophys. Res. 117, E00L06 (20

24. S. R. Taylor, Planetary Science: A Lunar Perspective

(Lunar and Planetary Institute, Houston, TX, 1982).

25. M. Le Feuvre, M. A. Wieczorek, Icarus 214, 1 (2011)

Acknowledgments: The GRAIL mission is supported by NAS

Discovery Program and is performed under contract to the

Massachusetts Institute of Technology and the Jet Propulsi

Laboratory. The Lunar Reconnaissance Orbiter LOLA

investigation is supported by the NASA Science Mission

Directorate under contract to the NASA Goddard SpaceFlight Center and Massachusetts Institute of Technology.

Data from the GRAIL and LOLA missions have been depos

in the Geosciences Node of NASA’s Planetary Data Syste

Supplementary Materialswww.sciencemag.org/cgi/content/full/science.1235768/DC1

Supplementary Text

Figs. S1 to S6

Tables S1 to S4

References ( 26–43)

28 January 2013; accepted 16 May 2013

Published online 30 May 2013;

10.1126/science.1235768

Continuous Permeability MeasurementRecord Healing Inside the WenchuanEarthquake Fault ZoneLian Xue,1,2* Hai-Bing Li,2 Emily E. Brodsky,1 Zhi-Qing Xu,2 Yasuyuki Kano,3 Huan Wang,2

James J. Mori,3 Jia-Liang Si,2 Jun-Ling Pei,4 Wei Zhang,2,5 Guang Yang,2,6

Zhi-Ming Sun,4 Yao Huang7

Permeability controls fluid flow in fault zones and is a proxy for rock damage after an earthquakWe used the tidal response of water level in a deep borehole to track permeability for 18 months the damage zone of the causative fault of the 2008 moment magnitude 7.9 Wenchuan earthquake.The unusually high measured hydraulic diffusivity of 2.4 × 10−2 square meters per second implies amajor role for water circulation in the fault zone. For most of the observation period, the permeabilitdecreased rapidly as the fault healed. The trend was interrupted by abrupt permeability increasesattributable to shaking from remote earthquakes. These direct measurements of the fault zone reveal aprocess of punctuated recovery as healing and damage interact in the aftermath of a major earthqua

The initiation and propagation of earth-

quakes depend critically on the hydrogeo-

logic properties of the fault zone, including

the fracture-dominated damage zone (1 – 6 ). Fault

zone permeability serves as a proxy for fractur-

ing and healing, as the fault regains strength

during one of the most unconstrained phases of

earthquake cycle (7 ). In addition, permeabi

and storage help to govern the pore pressure a

effective stress on a fault. Because earthquak

generate fractures in a damage zone aroun

fault, it is reasonable to expect that after a lar

www.sciencemag.org SCIENCE VOL 340 28 JUNE 2013

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earthquake, the fault zone permeability transi-

ently increases. Over time, the permeability may

decrease as a result of a combination of chemical

and mechanical processes (7 ). However, measur-

ing in situ fault zone hydrogeologic properties

requires post-earthquake rapid-response drill-

ing, and appropriate data have not previously

been recorded continuously immediately after a

large earthquake.

The devastating moment magnitude 7.9

Wenchuan earthquake occurred on 12 May 2008

and was the largest seismic event in China in the

past 50 years. Shortly afterward, the Wenchuan

earthquake Fault Scientific Drilling Project (WFSD)

constructed a series of boreholes penetrating the

main rupture zone. The first borehole (WSFD-1;

31.1°N, 103.7°E) is 1201 m deep and nearly ver-

tical, in a locale with 6 m of vertical displacement

at the surface (8). The borehole is open to fluid

flow in the formation below 800 m (Fig. 1) and provides a unique opportunity to directly measure

fault zone permeability over time. The borehole in-

tersects the likely principal slip zone at a depth of

590 m, which is a major lithological boundary be-

tween the upthrust Pre-Cambrian Pengguan gra-

nitic and volcanic complex and the underlying

Triassic sediments (8, 9). The fault breccia extends

to 760 m, and the fracture density remains high to

the bottom of the borehole (8). Mature faults have

damage zones extending at least ~100 m from the

edge of the fault core (10). Therefore, the dama

zone of this site is expected to extend into t

open interval beginning at 800 m.

We measured the water level response to ti

forcing in WFSD-1 to constrain the average h

drogeologic properties of the damage zone b

tween 800 and 1200 m below the ground surfa

[~200 to 600 m below the principal slip zo

(8, 9)]. We used these measurements to infer

hydraulic diffusivity and permeability variatio

inside the Wenchuan earthquake fault zone fr1 January 2010 to 6 August 2011. The WFSD

pressure transducer recorded data with a sam

rate of 2 min and at a resolution of 6 mm (Fig.

Data gaps occurred every month or two, when

instruments were removed from the well to

trieve the data and measure temperature profil

The raw records show clear tidal oscillations sup

imposed on the long-term recharge trend (Fig.

The tidal oscillations serve as probes of

fault ’s hydrogeologic properties. The tidal forc

Fig. 1. Location and sketch of the WFSD-1 site.Red lines in the inset indicate the main rupturezone; the red star is the epicenter of the Wenchuanearthquake. In the sketch, the black line is the faultcore, which is surrounded by the damage zone. Theborehole is 1201 m deep, and 800 to 1201 m isthe open interval where water can flow into thehole from the formation (white arrows). The faultthat was most likely active during the Wenchuanearthquake is the major lithological boundarybetween the pre-Cambrian Pengguan complex andthe Triassic sediments at 590 m.

Water level

Pre-CambrianPengguanComplex

5 9 0 m

8 0 0 m

Triassicsediments

d a m a g

e z o n

e

d a m a g e z o n e

Fault core

Chendu

Wenchuan

EAST TIBET

L o n g m e n

S h

a n

SICHUAN

BASIN

103°E 104°E 105°E

33°N

32°N

31°NDujiangyan

WFSD-1

Beichuan

1

Department of Earth and Planetary Sciences, University ofCalifornia, Santa Cruz, CA 95064, USA. 2State Key Laboratory ofContinental Tectonic and Dynamics, Institute of Geology, ChineseAcademy of Geological Sciences, Beijing 100037, China. 3Di-saster Prevention ResearchInstitute, Kyoto University, Gokasho,Uji,Kyoto 6110011, Japan. 4Instituteof Geomechanics, ChineseAcademy of Geological Sciences, Beijing 100081, China.5Shandong Provincial Lunan Geo-engineering Exploration In-stitute, Yanzhou, Shandong 272100, China. 6Guangdong ZhuhaiEngineeringInvestigationInstitute, Zhuhai 519000, China. 7No.6Brigade of Jiangsu Geology and Mineral Resources Bureau,Lianyungang, Jiangsu 222023, China.

*Corresponding author. E-mail: [email protected]

28 JUNE 2013 VOL 340 SCIENCE www.sciencemag.org56

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Fig. 2. Water levels from WFSD-1recorded from 1 January2010 to 6 August 2011. The oscillations in the inset are gen-erated by Earth tides. The precision of the water level measure-ment is 6 mm. Water level is assumed to be continuous acrossthe data gaps. The measured water level is the height of waterabove the pressure transducer.

2010−01 201 0−04 2010−07 2010−10 2011−01 201 1−04 2011−520

522

524

526

528

530

532

534

536

538

Date

W a t e r l e v e l ( m )

0 4/ 14 /10 04 /16 /1 0 04 /1 8/1 0 0 4/ 20/ 10525.05

525.1

525.15

525.2

525.25

525.3

W a t e r l e v e l ( m )

Date

2010−01 2010−04 2010−07 2010−10 2011−01 2011−04 2011−07

−30

−25

−20

−15

Date

P h a s e

L a g

( d e g r e e s )

2010−01 2010−04 2010−07 2010−10 2011−01 2011−04 2011−076

6.2

6.4

6.6

6.8

7

7.2

Date

A m p l i t u d e R e s p o n s e A ( 1

0 - 7 m

- 1 )

A

B

Fig. 3. Water level response relative to semidiurnal tidal dilatationstrain. (A) Phase lag; (B) amplitude response. Values were calculatedusing a Bayesian Monte Carlo Markov chain inversion method in the time

domain (13). The inversion was applied by 29.6-day segments overlaping by 80%, respectively. The error bars represent the 95% confideninterval.

www.sciencemag.org SCIENCE VOL 340 28 JUNE 2013

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are another objective of post-earthquake studies

(15). Previous modeling work has shown that

hydraulic diffusivities comparable to the ob-

served effective D could suppress the temper-

ature anomaly by at most a factor of 2 relative to

a conductively cooled model (13, 16 ), and there-

fore thermal anomalies in the fault zone could be

observable if the fault friction is comparable to

laboratory values.

The effective permeability k is related to trans-

missivity T by

k ¼ m

r gd T ð1Þ

where m is the fluid dynamic viscosity, d is the

thickness of the open interval of the well, and r is

the density of fluid. Using m = 10−3 Pa·s at 20°C,

r = 103 kg m−3, g = 9 . 8 m s−2, andd = 400m, the

average value of the effective permeability is 1.4 ×

10−15 m2 (Fig. 4). Permeability errors are esti-

mated by propagating the range of phase errors.

This approach is appropriate for measuring the

precision of the inversion, and these errors are

useful for assessing time variability. The absolute

value of the permeability is more strongly affected by the limitations of the flow model (13). The ob-

servation constrains the effective permeability aver-

aged over the entire open interval and is therefore

a lower bound for the effective permeability of

the highly fractured regions.

Our observed fault zone permeability is much

larger than core-scale laboratory measurements

of permeability from active-fault core samples

(17 ), which range from 10−19 to 10−18 m2; it is

also larger than the previously measured average

permeability of 1.9 × 10−16 m2 for the intact

upper Triassic rock near the Wenchuan drilling

site (18). The difference is likely due to mesoscale

fractures and highlights the importance of damagein determining the field-scale behavior (6 ).

There arealso substantial temporal changes in

transmissivity, which we interpret as permeability

changes because the formation thickness and fluid

properties are unlikely to vary during the obser-

vation period. During most of the study period, the

permeability trends downward and is most easily

interpreted as a reduction in fracture aperture and

connectivity during the continuous evolution

since the original earthquake. Seismic studies

in Wenchuan suggest that damage healed over

a protracted time after the earthquake (19), and

permeability does not generally evolve in time

in the absence of a disturbance (20). The only

candidate perturbation besides the earthquake

is the drillingitself, which could potentially produce

transient damage. However, drilling-induced fac-

tures are expected to extend at most a few bore-

hole radii away from the hole (21) (i.e., <0.3 m) and

cannot account for the phase change of the long-

period tidal response that senses average proper-

ties up to ~40 m from the borehole.

This decreasing permeability may reflect

the healing process of the fault zone after the

Wenchuan earthquake due to a combination of

fracture closure, sealing, precipitation, biogenic

growth, and pressure solution (7 ). The heal-

ing rates range from 4.1 × 10−16 m2 year −1 to 2.1 ×

10−17 m2 year −1, using linear fits to each interval

between perturbation events (Fig. 4) (13). Pre-

vious work (22, 23) modeled fault zone healing as

an exponential recovery process with decay times

on the order of decades or longer. However, our

data are best fit with much shorter exponential

decay times of 0.6 to 2.5 years, indicating a much

morerapid process than anticipated (table S2). The

short exponential decay times might indicate a fast healing process, such as removal of props trapped

in fractures, or crack sealing with a strongly dis-

equilibrated fluid to allow mass transfer with the

observed characteristic times.

Fault zone healing has been documented in

seismic velocity changes (24, 25) and has been

suspected on the basis of discrete repeated ac-

tive formation tests (26 ). After the 1995 Kobe

(Hyogoken-Nanbu) earthquake, water injection

experiments in 1997 and 2000 tracked fluid flow

in the hanging wall 50 m from the Nojima fault

core and found that the permeability in 2000 had

decreased to 50% of the value in 1997 (26 ). Seis-

mic studies document seismic velocity decreasesaround the fault after an earthquake continuing

for years, which can also be interpreted as a

consequence of fracture closure (19, 24, 25). In

Wenchuan, the repeated seismic velocity measure-

ments made in the first year are consistent with

such healing (19).

The sudden increases in permeability result in

an overall rate of decrease that is more gradual

than the short-term trends by a factor of 1.5 to

7.5. Previous work suggests that permeability

might be enhanced by remote or regional earth-

quakes (20, 27 ). Plausible mechanisms include

fracture unclogging due to the rapid, oscillatory

flow driven by the seismic waves as they passthrough the fault zone (27 , 28). The times of the

four permeability increases in Fig. 4 are cor-

related with the four teleseismic earthquakes

that produced the largest integrated seismic

shaking at the drilling site during the observa-

tion period (table S2). However, like many

hydrogeologic observations, the magnitudes of

the perturbations are not simply proportional to

that of the peak amplitude of the seismic wave

(27 ). Most important, our observations imply

that any physical modeling of precipitation,

fracture closure, or any other healing process of

a fault zone needs to match the much more rapid

healing rate that is only visible in the continu-

ously recorded data.

An interplay between permeability evolution

and fault strength has previously been suggested

on geological and theoretical grounds (29). The

Wenchuan earthquake Fault Scientific Drilling

Project captured the permeability evolution in the

critical post-earthquake period, when damage

heals and the stage is set for the next earthquake.

The unexpectedly high average hydraulic diffusiv-

ity (2.4 × 10−2 m2 s−1) measured here also implies

substantial fluid circulation in the evolving fault

zone. If this value represents the hydrogeologic

properties during the earthquake, fluid flow sho

take place during the earthquake rupture.

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Acknowledgments: Supported by the National Science an

Technology Planning Project in China (H.-B.L.) and NSF grEAR1220642 (E.E.B.). Seismic data from the Chinese nationa

network are archived and distributed by Incorporated Resea

Institutions for Seismology (IRIS) Data Management System

Supplementary Materialswww.sciencemag.org/cgi/content/full/340/6140/1555/DC1

Materials and Methods

Supplementary Text

Figs. S1 and S2

Tables S1 and S2

References ( 31– 34)

1 March 2013; accepted 17 May 2013

10.1126/science.1237237

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3-Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone..pdf