Shallow Low-Velocity Zone of the San Jacinto Fault from Local Earthquake Waveform Modeling

Shallow Low-Velocity Zone of the San Jacinto Fault from Local Earthquake Waveform Modeling, Figure 1 Figure 1:
S and Sdiff wave data (black) and synthetic waveforms (red). From left to right, two events, (a) 4527, (b) 5054, recorded by the BRF array. Event locations are shown in Fig. 2.Black arrows point to the Sdiff phase.
The damaged zone associated with a fault zone (FZ) can amplify the ground motion and may control the earthquake rupture process and slip localization during earthquakes. It may hold the key for us to understand the earthquake physics. Therefore, it is important to image the structure of the FZ. We developed a method to determine the depth extent of low-velocity zone (LVZ) associated with a FZ using S-wave precursors from local earthquakes. The precursors are diffracted S waves around the edges of LVZ and their relative amplitudes to the direct S waves are sensitive to the LVZ depth. We applied the method to data recorded by three temporary arrays across three branches of the San Jacinto fault zone. The FZ dip was constrained by differential travel times of P waves between stations at two side of the FZ. Other FZ parameters (width and velocity contrast) were determined by modeling waveforms of direct and FZ-reflected P and S waves [Li et al, 2007]. We found that the LVZ of the Buck Ridge fault branch has a width of ~150 m with a 30-40% reduction in Vp and a 50-60% reduction in Vs. The fault dips 70 to southwest and its LVZ extends to 2 km in depth. The LVZ of the Buck Ridge fault is not centered at the surface fault trace but is located to its northeast, indicating asymmetric damage during earthquakes.
</p><p>Li, H., L. Zhu, and H. Yang, High-resolution structures of the Landers fault zone inferred from aftershock waveform data, Geophys. J. Int., 17, 2007.
</p><p>Yang, H., and L. Zhu, Shallow low-velocity zone of the San Jacinto fault from local earthquake waveform modeling, Geophys. J. Int., in revision, 2010
</p><p>Acknowledgements: This work is supported by NSF grant EAR-0609969. The field work was assisted by IRIS and PASSCAL.</p>


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