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 (≤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 of the fault ( Q NE ∼100) is approximately twice that on the SW side ( Q SW ∼200) in the depth range ∼200 m to 5 km. Q P and Q S are similar, but their ratio is not well resolved. I estimate Q P ∼50 and Q S ∼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 ∼30 and Q S ∼20, with Q P increasing from ∼20 between 0 and 300 m 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 ∼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.