Crustal attenuation and site effects at Parkfield, California

At Parkfield, California, the San Andreas fault separates strongly contrasting rock types: high velocity Salinian granite to the SW and lower velocity Franciscan basement to the NE. I use the recordings of nine earthquakes in 10 boreholes (less than or equal to 1000 m deep) and calculate P and S wave spectral ratios between borehole recordings at different depths to eliminate the source. I then invert for Q on each side of the San Andreas fault. I find that the attenuation on the NE side df the fault (Q(NE)similar to 100) is approximately twice that on the SW side (Q(SW)similar to 200) in the depth range similar to 200 m to 5 km. Q(P) and Q(S) are similar, but their ratio is not well resolved. I estimate Q(P)similar to 50 and Q(S)similar to 80 in the fault zone itself by combining recordings at a site within the fault zone with the inversion results and confirm that it is a low -Q zone. Q is also observed to increase with depth, and the rate of this increase decreases as the depth increases. In the upper 1 km at the Varian well (on the NE side of the San Andreas fault), Q(P)similar to 30 and Q(S)similar to 20, with Q(P) increasing from similar to 20 between 0 and 300 m to similar to 55 between 600 and 900 m. This study demonstrates that the borehole seismometers at Parkfield are not deep enough for the effects of attenuation to be ignored in analysis of small earthquakes. Also, the attenuation below 1 km on the NE side of the fault is actually larger than that below similar to 200 m on the SW side. The results of this study suggest that the attenuation at depths of a few hundred to a few thousand meters should be considered when estimating site effects in seismic hazard studies.