Variations of Velocity Contrasts and Fault Zone Damage Along the Parkfield Section of the San Andreas Fault Using Fault Zone Trapped Waves

We investigate variations of velocity contrasts and damage zones in the Parkfield section of the San Andreas fault (SAF), using fault zone trapped waves generated by constructive interference of waves within a low velocity fault zone layer. The trapped waves follow the S or P wave arrival, are dispersive, characterized by higher amplitudes and lower frequencies than the body waves, and are particularly apparent in the vertical or fault parallel components of motion. The specific features of these waves are highly dependent on the fault zone structure within which they are generated, and hence they can be used to obtain high resolution images of fault zone layers. This study is part of a larger project on imaging bimaterial interfaces and damage zones in the Parkfield region using data from multiple seismic networks. Much of our previous work has concentrated on using fault zone head waves that refract along bimaterial interfaces. These observations have established the strength and variations of the velocity contrast across the SAF. Direct wave travel time inversion and head wave moveout studies indicate that the northeast side of the fault is generally slower and the southwest side is generally faster with a about 5-10% difference in velocity. However, towards the southeast, near the seismic station at Gold Hill and the hypocenter of the 2004 Parkfield earthquake, the velocity contrast is reduced to about 0-2%. Here we expand and develop upon the results from the head waves studies with observations of fault zone trapped waves. We make observations of trapped waves in near fault stations of the high resolution seismic network and other networks in the Parkfield region. Synthetic waveforms generated using a model of two quarter spaces with differing velocities separated by a low velocity layer, show a separation between the direct body (S or P) wave and trapped wave onset in a fault zone station. In contrast, when the velocities in the quarter spaces are the same, the trapped and body waves are continuous. We find strong variation in the strength and character of trapped waves at the different stations and events along the fault and attempt to correlate the variations with changes of the velocity contrast indicated by the head wave analysis.