Strong Motion Data From the Large California Earthquakes of 1992

by R.B. Darragh, T.Q. Cao, C.H. Cramer and A.F. Shakal

Darragh, R.B., T.Q. Cao, C.H. Cramer and A.F. Shakal (1993). Strong Motion Data From the Large California Earthquakes of 1992. SMIP93 Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, p. 13 - 26.

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From April to July, 1992 six earthquakes occurred in California with magnitude greater than 6. The Cape Mendocino earthquake sequence in northern California includes a magnitude 7.0 mainshock and two aftershocks with magnitudes of 6.2 and 6.3. The Landers sequence in southern California includes the Joshua Tree, Landers and Big Bear earthquakes with magnitudes of 6.1, 7.3 and 6.2, respectively. Strong-motion records were recovered from more than 500 stations of the California Strong Motion Instrumentation Program (CSMIP) following these earthquakes. For example, the Landers earthquake produced an extensive set of strong motion accelerograms at 144 CSMIP stations that recorded the largest earthquake to occur in California since 1952.

We present four results obtained from the CSMIP strong motion data. First, the strong motion records from the Cape Mendocino mainshock have some of the highest accelerations ever recorded. The Cape Mendocino station recorded a peak acceleration near 2 g, the largest acceleration ever recorded in California. Also, one of the highest accelerations ever recorded on a structure, 1.4 g, occurred on the ground near the abutment of a freeway overpass near Rio Dell. Second, the most significant aspect of the records from the Landers earthquake is their long duration, compared to most records that have been obtained in California. For example, the duration of strong shaking was 2-3 times longer than for the magnitude 7 Loma Prieta earthquake of 1989. Third, recordings from both mainshocks have significantly more long period energy in the ground motion than seen in previous strong motion recordings. Fourth, the strong motion records from these earthquakes have larger peak accelerations than most existing attenuation models would predict. Also, the Landers peak accelerations show less attenuation with distance.