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High-frequency site effects at two Parkfield downhole and surface stations

EXXON PRODUCTION RESEARCH, P.O. BOX 2189, HOUSTON, TEXAS 77252-2189
DEPARTMENT OF GEOLOGY DUKE UNIVERSITY, DURHAM, NORTH CAROLINA 27706

Abstract

Using stacked ratios of microearthquake source spectra recorded at the surface and 200 m downhole, we have established the P-and S-wave site effect at two locations in Parkfield, California. The frequency responses of both sites are bimodal, being completely dominated by multiples and amplification below 10 Hz and by attenuation above 30 Hz. The low-frequency amplification factors show roughly three to five times more uphole motion compared to that downhole. The multiples produce peaks with a maximum of nearly 10 times more uphole motion. Above 50 Hz the downhole motion is always larger, approaching a factor of 5 at the high-frequency limits of our study. When fit with a linear attenuation model, the high-frequency ratios yield compressional and shear Q values between 6 and 19. The data were divided into subsets based on event location and time of year. Statistical comparison of these subsets shows no systematic variation with azimuth or season. The results of our study accounts for the lack of high-frequency surface reflections in the Parkfield downhole data, but not for the lack of source scaling of Parkfield microearthquakes.

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