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OSMS 94-31

"Earthquake Engineering Aspects of Strong Motion Data From Recent California Earthquakes"

by R.B. Darragh, M.J. Huang and A.F. Shakal

Darragh, R.B., M.J. Huang and A.F. Shakal (1995). Earthquake Engineering Aspects of Strong Motion Data From Recent California Earthquakes. Proceedings Volume 3 of Earthquake Awareness and Mitigation Across the Nation. Fifth U.S. National Conference on Earthquake Engineering, July 10-14, 1994, Chicago, Illinois, p. 99-108.

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Six magnitude 6 and greater earthquakes, with important earthquake engineering results, occurred in California in 1992. The Cape Mendocino earthquake sequence in northern California includes a magnitude 7.0 mainshock and aftershocks with magnitudes of 6.2 and 6.3. The Landers sequence in southern California includes the Joshua Tree, Landers and Big Bear earthquakes of magnitude 6.1, 7.4 and 6.4, respectively. We present three significant results obtained from the California Strong Motion Instrumentation Program (CSMIP) data for these earthquakes. First, the strong motion record from the Cape Mendocino station has one of the highest accelerations ever recorded, near 2 g. Second, recordings from the Cape Mendocino and Landers mainshocks have more long period energy in the ground motion than seen in previous strong motion recordings. Third, the Landers earthquake records are of long duration compared to most records that have been obtained in California. The duration of strong shaking for Landers was 2-3 times longer than for the magnitude 7 Loma Prieta earthquake.

Important data for geotechnical engineering was recorded at the new NSF/CSMIP Treasure Island Geotechnical Array near San Francisco from a magnitude 5.3 earthquake 120 km away. In this site-response array accelerometers are installed in 5 boreholes and at the surface. The borehole accelerometers are located below the bedrock surface and at 4 intermediate locations in the soil profile. Peak accelerations ranged from 0.0032 g in the bedrock to 0.0143 g at the surface with an amplification of peak acceleration of greater than 4. Spectral amplification of the horizontal bedrock motion at the surface is greater than 10 near 0.8 Hz (1.2 second).