Temporal Changes in S-Wave Velocity at Different Depths Near Parkfield, CA

Abstract:

The 2003 Mw6.5 San Simeon earthquake and the 2004 Mw6.0 Parkfield earthquake have been found to cause significant seismic velocity decreases along the San Andreas Fault (SAF). However, the depth range of the velocity decreases is hard to constrain based on traditional approaches and is still inclusive. In this study, we used noise interferometry (MSNoise) and surface wave inversion to measure the S-wave velocity changes at different depths near Parkfield after the two large earthquakes. We processed continuous seismic recordings from 15 stations near Parkfield from 2001 to 2011 to obtain the noise cross-correlation functions, and measured the temporal variations in Rayleigh wave phase velocities at 6 different frequency bands. We then invert the Rayleigh wave phase velocity changes at different frequencies using a series of Rayleigh wave sensitivity kernels, for the S-wave velocity changes at different depths. Our results indicate that the S-wave velocity decreases caused by the San Simeon earthquake are relatively small (up to ~0.1%), and they access depths of at least 6 km in the region of Parkfield. On the other hand, the S-wave velocity decrease caused by the Parkfield earthquake is larger (up to ~0.3%), but is dominated by elastic changes in the top 1-2 km of the crust. Our ongoing work is focused on constraining and understanding the physical mechanisms for the different depth ranges of velocity changes cause by the two large earthquakes, and characterization of the recovery processes at different depths after the Parkfield earthquake.