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Three-Dimensional Seismic Attenuation Structure around the SAFOD Site, Parkfield, California

Department of Geology and Geophysics, University of Wisconsin-Madison

Steve Roecker

Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute

Online Material: Comparison of methods to estimate site response, synthetic tests of Q_p and Q_s models, and insights from Q_p/Q_s models.

We present models of the three-dimensional (3D) seismic attenuation structure, both Q_p and Q_s, for a 16 km² area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P- and S-wave t^*-values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t^*, and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t^*-values were inverted to obtain 3D frequency-independent Q_p and Q_s models using 3D V_p and V_s models and associated event locations. A shallow low-Q area (Q_p and Q_s about 50–75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high-Q feature (Q_p and Q_s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low-Q feature (Q_p and Q_s about 50–80) attributed to a wedge of sedimentary rocks; uniformly low Q_p- and Q_s-values suggest that the wedge is fluid rich. A low-Q basin feature (Q_p and Q_s about 50–75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high-Q feature (Q_p and Q_s about 220–300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q-values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF.