Depth of Rock Damage from Strong Seismic Ground Motions near the 2004 Parkfield Mainshock

Changes in the S - P delay of repeating earthquakes near Parkfield, California, after strong shaking in the 2004 mainshock, occur at surface stations but not at borehole stations. This result indicates that rock damage occurs mainly in the upper few tens of meters in fractured rock with a low seismic velocity. In addition, changes in coda-primary delay are comparable to changes in S - P delay. This observation along with the lack of S - P delay changes at borehole stations yields a simple model for the coda. Direct waves pass through the shallow damaged layer once. Coda waves scatter or refract in the deep undamaged subsurface and hence also pass through the subsurface just once. The coda, however, contains some brief reverberations within the shallow subsurface where the additional path length scales to the thickness of the shallow layer. The coda-primary delay measured by correlation includes both the zero delay change of direct coda and the increased delay of reverberating coda. Published changes in Rayleigh-wave group velocity before and after the Parkfield mainshock are compatible with their main cause being from the shallow velocity changes inferred from repeating earthquakes but have little spatial resolution. Stacking of earthquake seismograms resolved surface reflections at one shallow (63 m deep) borehole station GHIB. A change in P -wave travel-time body of ∼8 msec was marginally resolved and is compatible with the repeating earthquake S - P delay changes. Autocorrelation passive seismology resolved ∼32 msec P -wave reverberation delay change above the 251 m deep borehole station CCRB.