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Short baseline variations in site response and wave-propagation effects and their structural causes: Four examples in and around the santa clara valley, California

Bulletin of the Seismological Society of America

By:
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DOI: 10.1785/0120090278

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Abstract

Ground motion records of local and regional events from a portable array are used to investigate the structural causes of variations in ground motion over distances of a few hundred meters to a few kilometers in the sedimentary basin environment of the Santa Clara Valley, California, and its margins. Arrays of portable seismic stations are used to target four study areas with different ground motion patterns: (1) an edge of the alluvial basin extending up onto a marginal ridge (Blossom Hill), (2) a Cenozoic basin with a nearly flat bottom (Cupertino Basin), (3) a long, narrow Cenozoic basin with a steep V profile (Evergreen Basin), and (4) a line perpendicular to the trace of the Hayward fault. Average peak velocities on Blossom Hill from local earthquakes are a factor of 2.5 times higher than nearby valley sites. Three-dimensional (3D) modeling is used to conclude that the majority of the amplification is due to lower shear-wave velocities along a local fault zone (Shannon-Berrocal). Site amplification over the Cupertino Basin in the frequency band 0.5-4 Hz is generally low (less than 2.0 relative to aMesozoic rock site) and spatially uniform. This response is attributed to the shallow, flat-bottomed shape of the basin and the uniform, flat-laying sedimentary fill. In contrast, site amplification in the Evergreen Basin generally exceeds 3.0 and is attributed to the deep, V-shaped geometry of the basin and younger sedimentary fill. 3D waveform modeling shows the elongated shape of the Evergreen Basin causes more efficient trapping of long-period waves for sources along the long axis of the basin. A low-velocity zone is postulated along the Hayward fault with a width between 100 and 200 m, based on elevated site response along the fault trace and 4.5-Hz fault zone guided waves on the horizontal components of stations near the fault.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Short baseline variations in site response and wave-propagation effects and their structural causes: Four examples in and around the santa clara valley, California
Series title:
Bulletin of the Seismological Society of America
DOI:
10.1785/0120090278
Volume
100
Issue:
5 A
Year Published:
2010
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Bulletin of the Seismological Society of America
First page:
2264
Last page:
2286
Number of Pages:
23