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OSMS 91-03

by M. Huang, A. Shakal, R. Darragh, C. Ventura, T. Cao, R. Sherburne, P. Fung, J. Wampole, M. DeLisle and C. Petersen

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Introduction

Strong-motion records were recovered from 94 stations of the California Strong Motion Instrumentation Program (CSMIP) after the Sierra Madre earthquake of June 28, 1991 which occurred under the San Gabriel Mountains, approximately 20 km northeast of downtown Pasadena. These 94 stations include 48 ground response stations and 46 extensively-instrumented structures. The 46 structures include 38 buildings, six dams, an airport control tower and a power plant. The epicentral distance of the stations triggered by the earthquake ranges from 4 km for the closest (Cogswell Dam) to about 80 km for the farthest (Neenach, north of the San Andreas fault in the Antelope Valley). About half of the stations triggered were within 40 km of the epicenter. Although the area within which stations were triggered is smaller than that for the 1987 Whittier earthquake (Shakal et al., 1987), the number of stations triggered by both earthquakes is about the same. This is due to the fact that numerous new stations were installed by CSMIP in the Los Angeles region since the Whittier earthquake. This report includes all CSMIP data from this earthquake and supersedes the quick report distributed on July 1 (CSMIP, 1991).

The estimated earthquake location and magnitude are:
           Epicenter: 34.262N, 118.002W. Depth: 11 km (Caltech)
           Origin Time: 14:43:54.5, 28 June 1991 GMT (7:43 am, 28 June 1991 PDT)
           Magnitude: 5.8 ML (Caltech)

Analysis by the USGS and Caltech indicates that the earthquake had a thrust mechanism with the San Gabriel mountains on the upthrown block. Damage was moderate in the Arcadia, Monrovia, Pasadena, and Sierra Madre areas.