We investigate temporal variations in teleseismic receiver functions using 11 yr of data at station PKD near Parkfield, California, by stacking power spectral density (PSD) functions within 12-month windows. We find that PSD levels for both radial and transverse components drop by ∼5 dB following the 2003 San Simeon (M 6.5) earthquake, with a persistent reduction in background levels of ∼2 dB, relative to the pre-2003 levels, after the 2004 Parkfield (M 6) earthquake, corresponding to an estimated decrease in shear-wave velocity of ∼0.12 and forumla, respectively, or equivalent negative changes in Poisson’s ratio of ∼0.02 and ∼0.01. Our results suggest that the perturbation originates at middle to lower crustal levels, possibly caused by the redistribution of crustal pore fluids, consistent with increased and sustained tremor activity near Parkfield following both earthquakes. This study shows that we can resolve temporal variations in crustal scattering structure near a major seismogenic fault using the receiver function method.

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