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Organization Title

OSMS 92-05

by A. Shakal, R. Darragh, M. Huang, T. Cao, R. Sherburne, R. Sydnor, P. Malhotra, C. Cramer, J. Wampole, P. Fung and C. Petersen

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Introduction

Some of the highest accelerations ever recorded were obtained at stations of the California Strong Motion Instrumentation Program (CSMIP) during the Ms 6.9 April 25 earthquake near Petrolia. The Cape Mendocino station recorded a peak acceleration of near 2 g, the largest acceleration ever recorded in California. One of the highest accelerations ever recorded on a structure, 1.23 g, occurred on the deck of a freeway overpass near Rio Dell.

The accelerograms in this report were recorded during three earthquakes with magnitude over 6 that occurred in an 18-hour time span between 11:06 PDT on April 25 and 04:18 PDT on April 26. The largest event, the first, had a magnitude of 6.9 Ms. The epicenters of these three earthquakes were within about 25 km of each other.

The preliminary earthquake locations and magnitudes estimated by the USGS and UC Berkeley are:

Earthquake Date Time N. Lat. W. Long. Depth Ms ML
Mainshock 4/25/92 11:06:04 PDT 40.37 124.31 15 km 6.9 6.4
Aftershock No. 1 4/26/92 00:41:40 PDT 40.44 124.58 18 km 6.2 6.2
Aftershock No. 2 4/26/92 04:18:26 PDT 40.40 124.56 21 km 6.5 6.4


The April 25th event is the largest earthquake to occur in California since the 7.1 Ms Loma Prieta earthquake of 1989. It caused significant damage in Ferndale. The second large event, at 00:41 PDT on April 26, caused greater damage in Rio Dell, Fortuna, and Scotia, according to press reports. The USGS source mechanism indicates that the mainshock had pure thrust motion on a fault striking N45°W (approximately parallel to the San Andreas fault) and dipping 30° to the east. The epicenters of the first and second large aftershocks are northwest of the mainshock. The source mechanism of these events indicates strike-slip faulting on a fault striking northeast.