Weak Ground Motion and Amplifications Predicted From Shear-Wave Velocities at Precarious Rocks, near the 1857 Rupture of the San Andreas Fault

Robert Abbott, John N. Louie, James N. Brune, & Abdolrasool Anooshehpoor

In Preparation 2001, SCEC Contribution #601

The effects of local geology at fields of precarious rocks are inadequate to explain the persistence of untoppled rocks near the San Andreas fault, given current attenuation relationships. As evidence, we present weak ground motion spectral ratios at six locations from 61 earthquakes, in situ velocity measurements using both refraction-microtremor and tomographic techniques, and synthetic spectral ratios based on our velocity measurements. We compare the „known‰ toppling acceleration of the rocks and the accelerations predicted by current USGS/CDMG probabilistic seismic hazard maps at the rock locations. We modify the predicted USGS/CDMG accelerations by accounting for the measured shallow shear-wave velocity structure at the rock sites and the assumed velocity structure used for the maps. Comparing the synthetics generated using our velocity modeling to synthetics generated using the soft rock/soil interface velocity structure (NEHRP BC boundary), we find ground motions at the precarious rock sites 0.75 to 1 times the ground motion at an assumed BC boundary location at 3 to 5 Hz. Above approximately 5 Hz, the ground motion at the precarious rock sites is actually amplified above the ground motions at the BC boundary site. Velocity modeling shows that the precarious rock sites are characterized by velocities significantly slower the than the BC boundary from 0 to 10 meters, and significantly greater below 10 m. Strong impedance contrasts at the precarious rock sites in the upper 10 m caused by seismically slow dry sand over fast fractured granite explains the amplification pattern.

Citation
Abbott, R., Louie, J. N., Brune, J. N., & Anooshehpoor, A. (2001). Weak Ground Motion and Amplifications Predicted From Shear-Wave Velocities at Precarious Rocks, near the 1857 Rupture of the San Andreas Fault. Bulletin of the Seismological Society of America, (in preparation).