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On the Coherence of Ground Motion in the San Fernando Valley

Hough, S. E. and Field, E. H. (1996) On the Coherence of Ground Motion in the San Fernando Valley. Bulletin of the Seismological Society of America, 86 (6). pp. 1724-1732. ISSN 0037-1106. https://resolver.caltech.edu/CaltechAUTHORS:20140806-154508516

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Abstract

We present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the San Fernando Valley, California. Using aftershocks of the 17 January 1994 M_w6.7 earthquake recorded at a quasi-dense array of portable stations, we analyze the coherence of three well-recorded magnitude 3.7 to 4.0 events over the frequency range 0.5 to 15 Hz and a distance range of 0.5 to 5.3 km. All stations are located at sites with broadly similar near-site geology, characterized by medium to fine-grain Quaternary alluvial sediments. On average, relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 Hz; coherence drops sharply at frequencies near and above 3 Hz. Although average coherence functions are described reasonably well by a log-linear relationship with frequency, the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites. The distance decay of coherence is also markedly less strong, with high coherence values observed over station separations corresponding to multiple wavelengths. This may reflect fundamental differences in shallow-wave propagation in the two environments, with high-frequency scattering relatively more dominant in regions of hard-rock near-surface geology. Within a sedimentary basin or valley, the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions. However, previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater Los Angeles region; our results undoubtedly do not quantify the full range of ground-motion variability at all sites, but rather represent the level of that variability that can be expected, and quantified, for typical source/receiver paths.


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Additional Information: © 1996, by the Seismological Society of America. Manuscript received 12 October 1995. We thank Jim Mori for spear-heading the array deployment, Adam Edelman for providing the data and instrument response characteristics, Jennifer Scott for her help with the RefTek data, Walter Smith for GMT tips, Paul Spudich for a thoughtful review, and two anonymous reviewers for their comments. We thank the many families who graciously allowed use of their backyards for the deployment and Bill Curtis and Darryl Baisley for their work in site permitting and installation. The Southern California Earthquake Center is supported by grants from the National Science Foundation and the United States Geological Survey.
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Issue or Number:6
Record Number:CaltechAUTHORS:20140806-154508516
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20140806-154508516
Official Citation:S. E. Hough and E. H. Field On the coherence of ground motion in the San Fernando Valley Bulletin of the Seismological Society of America, December 1996, v. 86, p. 1724-1732
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48144
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:06 Aug 2014 22:56
Last Modified:03 Oct 2019 07:00

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