Effects of local geology on ground motion near San Francisco Bay

TLDR

Ground motion from Nevada nuclear explosions was measured at 37 sites near San Francisco Bay and compared with 1906 earthquake intensities and 1957 strong‑motion records. Amplitude variations correlate with local geology, with bay mud or fill sites exhibiting up to tenfold higher horizontal velocities and a ~1‑second dominant period, while older consolidated sediments show no dominant period and only about twice the bedrock velocities; these correlations imply that small‑motion amplification zones predict high‑intensity areas in future earthquakes.

Abstract

abstract Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, California. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of March 22, 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. For sites underlain by a layer of younger bay mud or artificial fill, maximum horizontal ground velocities generally increased with thickness of the layer and were as much as ten times greater than those recorded on nearby bedrock. The maximum vertical velocities for these sites were between 1 and 3.5 times greater. Spectral amplification curves clearly define a “dominant ground period” of about 1 second for sites underlain by younger bay mud. For sites underlain by older, more consolidated sediments, no clearly defined “dominant ground period” was found. Maximum ground velocities for the older bay sediment sites were about twice those recorded on bedrock. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.