Selected Accelerograms from the Redlands, California Earthquake of October 2, 1985

OSMS 85-02


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On October 2, 1985 an earthquake occurred approximately 8 kilometers (5 miles) southeast of San Bernardino near the community of Redlands, which is located about 110 km east of Los Angeles. According to the Associated Press, telephone service was disrupted for a few minutes and some plate glass windows were cracked in Redlands. High-rise buildings in Los Angeles and chandeliers in Palm Springs swayed during the earthquake. The earthquake may be associated with the San Jacinto fault zone. No known surface rupture was associated with the earthquake.

Strong-motion records were obtained at six stations instrumented by the California Strong Motion Instrumentation Program (CSMIP). This report presents copies of the analog records recovered as of October 5, 1985 and the instrumentation layout for each of the structures. Perhaps the most noteworthy of the records obtained from this earthquake are those from the San Bernardino County Law and Justice Center in Rancho Cucamonga. This building incorporates a system of seismic isolators at the foundation level. Seismic sensors are attached both to the foundation slab and to the building above the isolators. Building instrumentation was completed by CSMIP only last month. Construction of the building itself is still underway. Although the records are of low amplitude (4% g), their uniqueness makes them important.

Preliminary hypocentral parameters determined at the California Institute of Technology (Hutton, personal communication) are:
           Origin time: 2 October 1985, 23:44:15 GMT (16:44:15 PDT)
           Epicenter: 34.03 N, 117.25 W
           Depth: 15 km, approximate.
           Magnitude: 4.9 ML

The approximate epicenter and station locations are shown on the map in Figure 1. The stations are also listed in Table 1.