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Organization Title

OSMS 94-07

by A. Shakal, M. Huang, R. Darragh; T. Cao, R. Sherburne, P. Malhotra, C. Cramer, R. Sydnor; V. Graizer, G. Maldonado, C. Petersen and J. Wampole

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Introduction

The magnitude 6.7 Mw earthquake that occurred near Northridge, California on January 17, 1994 produced an extensive set of strong-motion recordings. The epicenter is located about 32 km northwest of Los Angeles in the densely populated San Fernando Valley. No surface rupture that can be associated unequivocally with the mainshock has yet been found by field geologists from CDMG, USGS and other agencies. Analysis by the USGS and Caltech indicates that the earthquake had a thrust mechanism on a fault plane striking N60°W and dipping 35-45°S. The rupture initiated about 19 km below the San Fernando Valley and propagated upward toward the northeast on this plane. It is inferred that the rupture stopped about 5 km below the surface. The aftershock zone lies along this plane located mainly under the San Fernando Valley. The location of mainshock epicenter is indicated on the map in Figure 1.

The estimated location and magnitude of the Northridge earthquake are:
           Epicenter: 34.209°N, 118.541°W. Focal Depth: 19 km (Caltech/USGS)
           Origin Time: 12:30:55.4, 17 January 1994 UTC (4:30 AM, PST)
           Magnitude: 6.4 ML (Caltech), 6.7 Mw (Caltech); 6.8 Ms (NEIC)

Extensive damage occurred in the San Fernando Valley, Los Angeles basin and adjoining areas, although injuries were confined mainly to the epicentral area. A collection of damage studies by various individuals and institutions is being prepared by the Earthquake Engineering Research Institute. The U.C. Berkeley Earthquake Engineering Research Center published a preliminary report on the seismological and engineering aspects of the earthquake (Moehle, 1994).

This report includes all CSMIP data from the Northridge earthquake. It supersedes the five Quick Reports, CSMIP (1994a through 1994e), released during January 17 through January 25. These data are being processed and the first report containing five ground-response records was released on February 7 (Darragh and others, 1994a).