Variations of Crustal Anisotropy along the San Jacinto Fault Zone, Southern California

Tuesday, 16 December 2014
Zefeng Li, Georgia Institute of Technology Main Campus, Atlanta, GA, United States, Zhigang Peng, Georgia Tech, Earth and Atmospheric Sciences, Atlanta, GA, United States, Zachary Ross, University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States, Frank Vernon, University of California San Diego, La Jolla, CA, United States and Yehuda Ben-Zion, University of Southern California, Los Angeles, CA, United States
Shear waves splitting (SWS) occurs when polarized shear waves enter an anisotropic medium, and hence provide useful information about the polarization directions and anisotropic intensity. In this study we examine crustal anisotropy in the San Jacinto Fault Zone (SJFZ), using seismic data recorded by five small-aperture linear arrays (BB, RA, SGB, DW and JF) as well as other permanent and temporary seismic networks. Each array contains 7-12 stations with a length at 180-450 m, straddling different segments of the SJFZ, including the Anza seismic gap. We compute delay times and fast directions using the MFAST code (Savage et al., 2010), which is based on the method of Silver and Chan (1991) and cluster analysis (Teanby et al, 2004). Preliminary results from manual S picks of the data recorded by the RA, JF and DW arrays in 2012 show that the fast directions are generally around the north. This is consistent with the regional stress field from focal mechanism inversion. However, we observe clear variations of fast directions across the SJFZ. From the RA01 on the SW to the RA11 on the NE, the fast directions change from NNE to NNW. Similarly, the fast directions change from NNE to NNW between JFS4 on the SW and JFN4 on the NE. Given the small array apertures, such variations likely result from shallow fault structures beneath the arrays, e.g., spatial changes of crack density and preferential alignment. To fully explore the anisotropy patterns in the SJFZ, we plan to expand the SWS analysis to all the five arrays and include all available seismic data. Updated results will be presented in the meeting.