High-Resolution Imaging of San Andreas Fault at Parkfield, California, Using Seismic Velocity and Anisotropy Tomography and Seismic Interferometry
Abstract
We characterized the detailed structure of the San Andreas fault zone at multiple scales using an extensive dataset collected around the SAFOD site from our long-term deployments of PASSCAL and USArray seismic instruments, and the USGS Northern California and UC Berkeley HRSN networks, SAFOD borehole logs, borehole seismometers, and several active-source projects. A suite of techniques are employed to better constrain the internal structure of the fault zone, including seismic travel-time tomography, shear-wave splitting tomography and seismic interferometry. Adaptive-mesh double-difference tomography is used to derive high-resolution Vp and Vs models around the fault zone with the waveform cross-correlation derived differential times. Knowing three-dimensional (3-D) Vp/Vs variations is helpful to have a more complete characterization of the mechanical properties and geological identity of fault zone materials. Vp/Vs variations are reliably determined by the inversion of S-P time differences constructed only from similar P and S ray paths. Our velocity models show the high-velocity granitic rocks on the southwest side of the fault, a complex low-velocity zone beneath and southwest of the surface fault trace, and an extensive low-velocity zone overlying deeper bedrock on the northeast side. We systematically analyzed shear wave splitting for seismic data observed at PASO and UC Berkeley HRSN networks. Although polarization direction of the fast shear wave and the delay time show substantial scatter for different events observed at a common station, there are spatially consistent patterns when projecting them to various depths along corresponding ray paths, derived from a 3-D shear velocity model. We developed a 3-D shear-wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths. The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The fault zone is highly anisotropic down to a depth of ~4 km and then becomes less anisotropic at greater depths. Seismic interferometry means the process of generating new seismic responses by cross-correlating seismic observations at different receiver locations. Based on the principle of seismic interferometry and reciprocity, we created pseudo shot profiles as if a shot was excited at one earthquake location and observed by a set of receivers at other earthquake locations. Preliminary results show that several reflectors may exist inside the fault zone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.T53C..01Z
- Keywords:
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- 7205 Continental crust (1219);
- 7250 Transform faults;
- 7290 Computational seismology;
- 8180 Tomography (6982;
- 7270)