Comparing Weak- and Strong-Motion Spectral Ratios at the Turkey Flat Site Effects Test Area, Parkfield, California: Possible Nonlinear Soil Behavior

The prediction of soil response at a site is an important aspect of estimating ground motion from future earthquakes. The soil response at the Turkey Flat Site Effects Test Area from the 2004 Parkfield M6.0 earthquake (0.3g) is compared via spectral ratios to the response from the 1995 weak-motion based predictions of Cramer (1995) and from 0.1g and less strong-motion recordings from seven M3-5 Parkfield and the M6.5 San Simeon earthquakes. At Turkey Flat four recording sites are spaced across the two-kilometer wide, shallow, stiff-soil valley: Rock South, Valley Center, Valley North, and Rock North. Rock South is located 5 km from the San Andreas Fault and the 2004 Parkfield rupture. Near-field and azimuthal effects in the 0-5 Hz frequency band are present in the 2004 Rock South records, but not in the records at the other sites 6–7 km from the San Andreas Fault. Thus the 2004 Rock North mainshock record provides better predictions of the 2004 strong ground motions at both valley sites. Comparisons of the mainshock spectral ratios to both sets of weaker-motion mean spectral ratios suggests nonlinear soil behavior in the shallow soils at the Valley Center site and linear soil behavior elsewhere.