T21E-08:
Imaging the internal structure of the San Jacinto Fault Zone with high Frequency noise
Tuesday, 16 December 2014: 9:45 AM
Dimitri Zigone1, Yehuda Ben-Zion1, Michel Campillo2, Gregor Hillers2, Philippe Roux2 and Frank Vernon3, (1)University of Southern California, Los Angeles, CA, United States, (2)Université Joseph Fourier Grenoble, Grenoble, France, (3)University of California San Diego, La Jolla, CA, United States
Abstract:
Regional tomography studies in the SJFZ area gives detailed images up to the top 500 meters or so of the crust (Zigone et al., 2014; Allam et al., 2014). To obtain additional high resolution information on local structures at the shallower crust, we use cross correlation of ambient seismic noise between 10 Hz and 70 Hz recorded by several linear arrays that cross the SJFZ with typical inter-station distances around 40 m. Pre-processing techniques involving earthquakes removal and whitening on 15 minutes time windows are used to obtained the 9-component correlation tensors associated with all station pairs. The obtained cross correlations exhibit coherent waves up to 30-40 Hz that travel between the station pairs. Polarization and dispersion analysis show that both body and surface waves are reconstructed with Rayleigh group velocity around 400-500 m/s. The results likely include also body waves and trapped fault zone signals. After rejecting paths without sufficient signal to noise ratios, we invert the Rayleigh group velocity measurements using the Barmin et al. (2001) approach on a 20m grid size. The obtained group velocity maps reveal complex structures with very low velocity damage zones around the surface traces of the SJFZ. The group velocities are inverted to 3D images of shear wave speeds using the linear inversion method of Hermann & Ammon (2002). The results show local flower-type damage structures in the top 200m, with Vs values at 30m depths around 250-300 m/s in agreement with the available Vs30 values in the literature. This high-resolution study will be complemented with a more detailed analysis using a dense rectangular array with 1108 vertical-component seismometers separated by 10-30 m centered on the fault. This geometry allows the use of very high frequency seismic noise up to 200 Hz that can be used to image the small-scale features (~10-20 m) in the top few hundred m of the crust. Updated results will be presented in the meeting.
