The Whittier Narrows, California Earthquake of October 1, 1987—Preliminary Analysis of Peak Horizontal Acceleration

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Abstract
The Ml=5.9 Whittier Narrows, California, earthquake triggered several hundred accelerographs in the greater Los Angeles area. One-hundred and sixty-eight of these were used to develop attenuation relationships for peak horizontal acceleration. The analysis indicates that the attenuation of peak acceleration during the earthquake was generally consistent with that predicted from the attenuation relationships of Campbell (in press). However, the acceleration amplitudes were about 65-percent higher than predicted. An analysis of residuals clearly showed that the ground motions recorded during this earthquake were influenced by a complex interaction of source mechanism, building embedment, site geology, and geography. Source effects may have been responsible for the higher-than-expected accelerations as well as some of the observed azimuthal variation. The correlation of peak acceleration with geography may have been caused in part by the gross geologic structure of the region. Buildings with basements were observed to have lower accelerations than ground-level sites, consistent with previous results. Accelerations from rock sites—especially those from hard rock sites—were found to have lower amplitudes and greater variability than those from soil sites. The larger variability may be due in part to topographic effects. All sites located within about 20 km of the fault recorded about the same level of acceleration whether they were sited on deep soil, soft rock, or hard rock. Shallow-soil sites, however, had higher-than-average accelerations at relatively short distances, but lower-than-average accelerations at longer distances. Their behavior at long distances was more consistent with that of the underlying rock rather than that of the overlying soil, no doubt reflecting the longer wavelengths of the more distant ground motions.

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