Temporal Variations in Crustal Scattering Structure Near Parkfield, California, Using Receiver Functions

Temporal Variations in Crustal Scattering Structure Near Parkfield, California, Using Receiver Functions Moho piercing points of receiver functions grouped in two back-azimuth ranges of incoming wave fields (black, 0–200°; gray, 225–360°) at station PKD near Parkfield, California, with respect to study region (inset, major faults in gray). This grouping also corresponds to sampled volumes exhibiting different contrasts in VP/VS of the upper to middle crust with lower values in the west (Ozacar and Zandt, 2009) . San Andreas Fault is indicated by the gray lines near Parkfield. Stars in inset indicate the location of the 2003 San Simeon (SS) and 2004 Parkfield (PK) earthquakes.
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We investigate temporal variations in teleseismic receiver functions using 11 yrs 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 ~0:06 km/sec, 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.
</p><p>References
</p><p>Audet, P., 2010. Temporal variations in crustal velocity structure near Parkfield, California, using receiver functions, Bull. Seismol. Soc. Amer., 100.
</p><p>Ozacar, A. A., and G. Zandt (2009). Crustal structure and seismic anisotropy near the San Andreas fault at Parkfield, California, Geophys. J.
Int., 178, 1098–1104
</p><p>
Acknowledgements: This work was supported by the Miller Institute for Basic Research in Science (UC Berkeley). Data was made available by
the Northern California Earthquake Data Center (NCEDC).</p>

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