OSMS 87-04

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"The Cerro Prieto, Baja California Earthquake of February 6, 1987"

by Luis Munguia, John G. Anderson and James A. Brune

The Cerro Prieto earthquake of February 6, 1987 (ML=5.4) occurred approximately 6 km south of the Cerro Prieto volcano in northern Baja California, Mexico (see Figure 1). The mainshock was preceded by two foreshocks having magnitudes of about 3.0, and followed by many aftershocks with magnitudes as large as 3.8. The mainshock was felt strongly in the Mexicali Valley area; it was also felt at El Centro, Tijuana, Ensenada, San Diego, and Yuma, Arizona.

Three strong-motion records were recovered following the earthquake. One of the records was obtained by an analog accelerograph (SMA-1) at the Cerro Prieto station, the other two were obtained by digital accelerographs (DCA-310). An unusually large peak horizontal acceleration of approximately 1.4 g was recorded by the SMA accelerograph.

Minor damage was reported at the city of Mexicali (approximately 34 km north of the earthquake epicenter) where a few windows were cracked, local power was briefly disrupted and items fell from shelves. In the epicentral region the only engineered structure is the Cerro Prieto geothermal power plant. The plant is located about 6 km from the epicenter. Despite the high levels of acceleration recorded at the Cerro Prieto station, no damage was reported at the plant.

This section of the report briefly describes the earthquake characteristics, regional seismicity, and the strong-motion instrumentation in the area.

"Processed Strong-Motion Data From Cerro Prieto Station"

by A.F. Shakal, M.J. Huang, D.L. Parke and R.W. Sherburne

Following the ML 5.4 Cerro Prieto earthquake of February 6, 1987 three strong-motion records were recovered from a network maintained and operated by the Center of Scientific investigation and Higher Education at Ensenada (CICESE) and the Institute of Geophysics and Planetary Physics (IGPP) at the University of California, San Diego. A large horizontal acceleration of approximately 1.4 g was recorded at the Cerro Prieto station. Because of the unusually large horizontal acceleration and the importance of these data not only to Mexico but also to California, the California Strong Motion Instrumentation Program (CSMIP) digitized and processed the Cerro Prieto accelerogram for distribution to engineers, seismologists, and others concerned with the seismic safety problem.

This section of the report is focused on the digitization and processing of the analog accelerogram recovered from the Cerro Prieto strong-motion station. In the appendix the results of digitization and processing are presented through a series of plots of the acceleration, velocity, and displacement, and of the absolute acceleration and relative velocity response spectra.