Recent Data Recorded from Downhole Geotechnical Arrays

by Vladimir Graizer, Anthony Shakal and Pat Hipley

Graizer, Vladimir, Anthony Shakal and Pat Hipley (2000). Recent Data Recorded from Downhole Geotechnical Arrays. SMIP2000 Seminar on Utilization of Strong-Motion Data, p. 23 - 38.

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Data recorded by downhole arrays with sensors installed at different depths and geologic layers provide critical information for studies of local site amplification effects.

The soft-soil/rock array at Treasure Island near San Francisco was installed by the California Strong Motion Instrumentation Program in cooperation with other agencies. Analysis of the recorded low amplitude data shows that the average amplification factor from the bedrock to the surface of the soft soil reaches factor of 10 at periods of 1.2-1.3 seconds.

Geotechnical arrays at La Cienega in Los Angeles, Meloland in El Centro, in Eureka and the newly instrumented arrays near the Vincent Thomas Bridge in Long Beach represent deep soft alluvium sites. A comparison was made of the average site amplifications calculated for a number of M
The Tarzana downhole is located on the top of a small hill, and represents a soft-rock site. The downhole data from small events recorded so far demonstrate a significantly higher amplification effect for the component perpendicular to the hill than for the component parallel to the hill.

Large (up to 10 cm) long-period (up to 8 seconds) displacements were recorded at the La Cienega, El Centro, Tarzana and Long Beach arrays during the Hector Mine earthquake at the distances of more than 200 km from the epicenter. In contrast to the small events, the data recorded during the Hector Mine earthquake show that for the displacements and velocity curves there is practically no near-surface site amplification.