ders the sinking of this higher-density material, thi s pro cess fur the r inc re ase s the gra vit y ano mal y at the basin center. The net effect is that isostatic uplif t of the surro undin g depre ssed surface to- pogra phy and crus tal colla r, combined wit h cool- ing and contraction of the melt pool, create the centra l positive free-air anomaly . The flexur al 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 anomaly that surrounds the inner basin. Isostatic uplift raised the surface topography of the Freundlich-Sharonov basin by ~2 km atthe 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 ( 1 1, 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. Inf ill ing of a 3-k m-t hic k mar e uni t and ass oci ate d subsidence brings the floor depth of the Humorum basin to just over 1.5 km deep, modes tly deeperthen 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-airanomaly of the Freundlich-Sharonov basin is predicted to have risen to a pos itive 80 mGal in the inner basin and–200 mGal in the outer basin above the thickened crust, which are figures in exce llent agre ement with GRAIL obse rvations (Fig. 1C, red line) (1). Furthermore, the model pred icts an outer annulus of positi ve anoma lies, 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 ), altho ugh this gr avity an omaly cannot be verified because the Humorum basin was subsequently partially filled with mare ba- salt. Our results support the inference that lunarbasins possess a positive gravity anomaly in excess of the ma re load ( 5). As a fina l step in ouran al ys is, we empl ac ed a ma re un it 3 km thic k and 15 0 km in radius (taper ed to zer o thi ckness 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 ofthe 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 model parameters as- sociated with isostatic uplift and cooling. These processes are controlled by the evolution of t he den sity and visc osi ty stru ctu re in the mod el, whi ch foll ow fr om th e mi nera lo gy of the luna r cr ust an d mantle and the evolution of temperature as the region conductively cools. References and Notes 1. M. T. Zuber et al., Science339, 668 (2013). 2. P. M. Muller, W. L. Sjogren, Science1 61, 680 (1968). 3. D. E. Smithet al., J. Geophys. Res. 98, 20,871 (1993). 4. D. E. Smithet al., Science336, 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. Pieraz zo, H. J. Melosh, Icarus 145, 252 (2000). 8. J. C. Andrews-Hanna, Lunar Planet. Sci. 43 , 2804 (2012). 9. W. L. Sjogren, R. N. Wimber ly, W. R. Wollen haupt, Moon 9, 115 (1974). 10. M. T. Zuber, D. E. Smith, F. G. Lemoine, G. A. Neumann, Science266, 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 density boundaries 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 couplin g to the subisostati c 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 180, 514 (2006). 17. The precise value of the impact velocit y is not critic al for this computation because a lower impact velocity can be compensated by a larger impactor, and vice versa. The impact velocity distribu tion on the Moon is strongly skewed toward high velocities, with a mode at 10 km/s and a median of ~15 km/s ( 25). 18. M. A. Wiecz oreket al., Science 339, 671 (2013). 19. B. A. Ivanov, H. J. Melosh, E. Pierazzo, inLarge Meteorite Impacts and Planetary Evolution IV, W. U. Reimold, R. L. Gibson, Eds. (Special Paper 465, Geological Society of America, Boulder, Colo., 2010), pp. 29–49. 20. G. Schuber t, D. L. Turcot te, P. Olson,Mantle Convection in the Earth and Planets (Cambridge Univ. Press, Cambridge, 2001). 21. More detaile d descriptio ns of these models and methods are available as supplementary materials on ScienceOnline. 22. A. M. Freed, S. C. Solomon, T. R. Watters, R. J. Phillips, M. T. Zuber, Earth Planet. Sci. Lett. 285, 320 (2009). 23. A. M. Fre edet al., J. Geophys. Res.117, E00L06 (2012). 24. S. R. Taylor , Planetary Science: A Lunar Perspective (Lunar and Plane tary Institute , Houston, TX, 1982). 25. M. Le Feuvre, M. A. Wieczorek , Icarus 214, 1 (2011). Acknowledgments:The GRAIL mission is supported by NASA’s Discovery Program and is performed under contract to the Massac husetts Institute of Techno logy and the Jet Propul sion Laboratory. The Lunar Reconnaissance Orbiter LOLA investigation is supported by the NASA Science Mission Directorate under contract to the NASA Goddard Space Flight Center and Massachusetts Institute of Technology. Data from the GRAIL and LOLA missions have been deposited in the Geosciences Node of NASA’s Planetary Data System. Supplementary Materials www.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 Measurements Record Healing Inside the Wenchuan Earthquake Fault Zone Lian Xue, 1,2 * Hai-Bing Li, 2 Emily E. Brods ky, 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 Huang 7 Permeability controls fluid flow in fault zones and is a proxy for rock damage after an earthquake. We used the tidal response of water level in a deep borehole to track permeability for 18 months in 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 a major role for water circulation in the fault zone. For most of the observation period, the permeability decreased rapidly as the fault healed. The trend was interrupted by abrupt permeability increases attributable to shaking from remote earthquakes. These direct measurements of the fault zone reveal a process of punctuated recovery as healing and damage interact in the aftermath of a major earthquake. T he initiation and propagation of earth- quakes depend critically on the hydrogeo- lo gi c pr ope rti es of the fau lt zon e, inc lud ing the fracture-dominated damage zone ( 1 –6). Faultzone permeability serves as a proxy for fractur- ing and healing, as the fault regains strength during one of the mos t uncon str ain ed pha se s of the ea rthq uake cycl e (7). In addit ion, perme abili ty and storage help to govern the pore pressure and effective stress on a fault. Because earthquakes generate fractures in a damage zone around a fault, it is reasonable to expect that after a large www.sciencemag.org SCIENCE VOL 340 28 JUNE 2013 1555 REPORTS
5
Embed
3-Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone..pdf
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
8/10/2019 3-Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone..pdf
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
REP
8/10/2019 3-Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone..pdf
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.
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.
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
REP
8/10/2019 3-Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone..pdf