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Z. Xu and P. Chen
Poster Abstracts
We cross-correlate the vertical components of ambient seismic noise data recorded on USArray broadband stations in the
northern California area to estimate inter-station Green's functions.These ambient-noise Green’s functions are then compared
with synthetic Green’s functions computed using the finite-difference method in a hybrid 3D reference model obtained by
combining the California state-wide 3D seismic velocity model provided by Lin et al. (2010) with the USGS 3D seismic velocity
model for the San Francisco Bay Region. The adjoint method is adopted to construct the gradient of the misfit functional,
which is defined in terms of the frequency-dependent phase-delay measurements made on time-localized surface waves on
the ambient-noise Green’s functions and the synthetics computed using our 3D starting model. The first iteration of our
inversion involves nearly 3200 inter-station paths that provide good coverage of northern California. After the first iteration,
the updated 3D seismic velocity model provides nearly 50% reduction in the misfit functional. By carrying out more iterations
and including more waveforms from ambient-noise Green’s functions as well as waveforms from natural earthquakes, our
studies will gradually improve the regional seismic velocity model in northern California.
THE PERMEABILITY VARIATIONS ON THE WENCHUAN FAULT MEASURED ON THE WATER LEVEL
RESPONSE TO SOLID EARTH TIDES (A-088)
L. Xue, E.E. Brodsky, H. Li, H. Wang, and J. Pei
The mechanics of slip during an earthquake depends critically on the hydrologic properties. The in situ fault zone
hydrological properties are difficult to measure and have never directly been constrained on the fault zone immediately after a
large earthquake. In this work, we analyze 1.5 years of continuous data from the Wenchuan Fault Zone which was the site of
the Mw 7.9 Wenchuan earthquake. We find that the hydraulic diffusivity D of Wenchuan Fault Zone is 0.03 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. This measurement at Wenchuan was made by continuously monitoring the response of the well to the solid Earth
tides. The solid earth tides impose a dilatational strain on the formation that pumps water cyclically in and out of the well. By
measuring the phase and amplitude response, we can constrain the transmissivity and storage of the damage zone at 200-600
m form the principal slip zone assuming an isotropic, homogenous and laterally extensive aquifer. We evaluated the phase
and amplitude responses for solid Earth tide in both frequency domain and time domain, and these two separate methods
yield almost identical results. The average phase lag is 25 degree, and the average amplitude response is 6x10-7 strain/m.
According to the Heish model, we solve for storage coefficient S 2.2x10-4 and transmissivity T 6.6x10-6 m2.
Calculation for the hydraulic diffusivity D with D=T/S, yields the reported value of D is 0.03 m2/s,which is a high number
compared with the thermal diffusivity 10-6 m2/s.If the value is representative of the fault zone,then this means the hydrology
processes should have an effect on the earthquake rupture process.One advantage of this tidal response method is that it is
passive and so the D is accurately recording the in situ permeability undisturbed by the potential effects of a pump test. In the
previous studies, Kitagawa et al.,[2002] reported the permeability of Nojima Fault decreased 50% over three years and
Brenguier et al.,[2008] observed seismic velocity decreased by 0.08% immediately after the Parkfield earthquake and
posetseismic velocity remained low for 3 years.We did not observe a significant healing signal recorded in the well after the
2008 Wenchuan earthquake. It seems that permeability of Wenchuan Fault does not follow expected healing behavior. This
poses constraints on any geochemically-based model of fault zone healing.
COMPUTING A LARGE REFINED CATALOG OF FOCAL MECHANISMS FOR SOUTHERN CALIFORNIA
(1981 – 2010) (B-077)
W. Yang, E. Hauksson, and P.M. Shearer
We calculate focal mechanisms for earthquakes that occurred in the southern California region from 1981 to 2010 using the
HASH method and locations derived from waveform cross-correlation. Each focal mechanism is obtained from grid searching
for the best-fitting double-couple focal mechanism solution to both the P-wave first motion records that were picked by
network analysts, and the S/P amplitude ratios computed from three-component seismograms. We process more than 410,000
earthquakes, and analyze the statistical features of the whole data set. As more S/P amplitude ratios become available after
2000, the average focal plane uncertainty decreases significantly compared with solutions that include only P-wave first
motions. We filter the preliminary data set with criteria associated with mean fault plane uncertainty and azimuthal gap, and
obtain a high quality focal mechanisms catalog with approximately 179,000 earthquakes. In general the parameters of the focal
mechanisms have been stable during the three decades. The dominant style of faulting is high angle right-lateral strike-slip
faulting with the most likely strike angle centered at 150°/330°. For earthquakes of M < 2.5, there are more normal faulting
2011 SCEC Annual Meeting | 255