The attenuation of seismic shear waves in quaternary alluvium in Santa Clara Valley, California

Bulletin of the Seismological Society of America
By: , and 

Links

Abstract

We used shear waves, generated by an air-powered source at the ground surface and recorded in a borehole, to estimate the shear-wave quality factor at strong-motion station Gilroy no. 2. We find similar values of Q using both the decay of the spectra with depth and the slope of the spectral ratio at two depths; we find no evidence of a frequency dependence of Q. The mean value of Q over the depth range 10 to 115 m is close to 10. The use of this value over the depth of the borehole and the observed travel time of 0.358 sec gives a cumulative attenuation factor t* of 0.036 sec for the upper 180 m of the Quaternary alluvium. This is comparable to the differential decay between Gilroy no. 2 and a rock site 1.9 km away (Gilroy no. 1), as measured from the decay of the high-frequency spectra of accelerograms from large earthquakes, plotted on a log-linear scale: t* = 0.05, 0.04, and 0.03 sec for the 1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta earthquakes, respectively. The similarity between the attenuations measured from the low-strain surface source and those from the larger amplitude earthquake sources suggests that increases of damping due to nonlinear wave propagation effects are limited.

Study Area

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title The attenuation of seismic shear waves in quaternary alluvium in Santa Clara Valley, California
Series title Bulletin of the Seismological Society of America
Volume 84
Issue 1
Year Published 1994
Language English
Publisher Seismological Society of America
Description 15 p.
First page 76
Last page 90
Country United States
State California
Other Geospatial Santa Clara Valley
Google Analytic Metrics Metrics page
Additional metadata about this publication, not found in other parts of the page is in this table