The effects of unconsolidated sediments upon the ground motion during local earthquakes

abstract A vertical seismic array at the Richmond Field Station on the margin of San Francisco Bay, California, has provided some excellent experimental data showing the effects of unconsolidated sediments on ground motion during local earthquakes. The experiments are well controlled in that good estimates of the material properties of the sediments are available, with the largest uncertainty being in the attenuation. Three components of acceleration have been recorded in bedrock below the sediments, within the sediments, and at the surface of the sediments from two local earthquakes. The maximum accelerations at the surface are between 1.5 and 4.3 times those in the bedrock. Analytical calculations which assume vertically propagating waves and which are suitable for highly attenuating materials explain the major differences between the bedrock records and the surface records for both vertical and horizontal components. Maximum accelerations, duration, frequency content, general character of the records, and response spectra are all in approximate agreement. Some of the energy arriving late in the records can best be explained by surface waves propagating horizontally in the sediments.