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| Titel |
Continuous Record of Permeability inside the Wenchuan Earthquake Fault Zone |
| VerfasserIn |
Lian Xue, Haibing Li, Emily Brodsky |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250073530
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| Zusammenfassung |
| Faults are complex hydrogeological structures which include a highly permeable
damage zone with fracture-dominated permeability. Since fractures are generated by
earthquakes, we would expect that in the aftermath of a large earthquake, the permeability
would be transiently high in a fault zone. Over time, the permeability may recover
due to a combination of chemical and mechanical processes. However, the in situ
fault zone hydrological properties are difficult to measure and have never been
directly constrained on a fault zone immediately after a large earthquake. In this
work, we use water level response to solid Earth tides to constrain the hydraulic
properties inside the Wenchuan Earthquake Fault Zone. The transmissivity and storage
determine the phase and amplitude response of the water level to the tidal loading. By
measuring phase and amplitude response, we can constrain the average hydraulic
properties of the damage zone at 800-1200 m below the surface (~200-600 m from
the principal slip zone). We use Markov chain Monte Carlo methods to evaluate
the phase and amplitude responses and the corresponding errors for the largest
semidiurnal Earth tide M2 in the time domain. The average phase lag is ~ 30o,
and the average amplitude response is 6Ã10-7 strain/m. Assuming an isotropic,
homogenous and laterally extensive aquifer, the average storage coefficient S is
2Ã10-4 and the average transmissivity T is 6Ã10-7 m2 using the measured phase and
the amplitude response. Calculation for the hydraulic diffusivity D with D=T/S,
yields the reported value of D is 3Ã10-3 m2/s, which is two orders of magnitude
larger than pump test values on the Chelungpu Fault which is the site of the Mw 7.6
Chi-Chi earthquake. If the value is representative of the fault zone, then this means the
hydrology processes should have an effect on the earthquake rupture process. This
measurement is done through continuous monitoring and we could track the evolution for
hydraulic properties after Wenchuan earthquake. We observed the permeability
decreases 35% per year. For the purpose of comparison, we convert the permeability
measurements to into equivalent seismic velocity. The possible range of seismic
wave velocity increase is 0.03%~ 0.8% per year. Our results are comparable to the
results of the previous hydraulic and seismic studies after earthquakes. This temporal
decrease of permeability may reflect the healing process after Wenchuan Earthquake. |
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