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OSMS 87-06

by M. J. Huang; T.Q. Cao; C .E. Ventura; D.L. Parke; A.F. Shakal;

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Strong-motion records were recovered from fifteen stations of the California Strong Motion Instrumentation Program (CSMIP) following the earthquakes of 23 and 24 November 1987 which occurred approximately 15 km west of Westmorland in Imperial County, California. The stations include 10 ground-response stations , 4 buildings, and one bridge. In total , 7 records were recovered from the November 23 earthquake and 16 records were from the November 24 earthquake. This report includes all CSMIP data from the Superstition Hills earthquakes and supersedes the brief compilation distributed immediately after the earthquakes (CDMG, 1987). In addition to the CSMIP records described here, records were also recovered by other agencies such as the U.S. Geological Survey.

Current estimates of the earthquake location and magnitude are (Caltech):

·November 23 Earthquake

           Hypocenter: 33.O83 N, 115.775 W , 5 km depth.
           Origin Time: 01:54:14.5 GMT, 24 November 1987 (17:54:14.5 PST, 23 NOV.)
           Magnitude: 5.8 ML ( Caltech ) , 6.2 Ms (NEIC)

·November 24 Earthquake

           Hypocenter: 33.013 N, 115.838 W, 2 km depth.
           Origin Time: 13:15:56.5 GMT, 24 November 1987 (05:16:56.5 PST, 24 NOV.)
           Magnitude: 6.0 ML (Caltech), 6.6 Ms (NEIC)

According to Jim Kahle (CDMG, personal communication), rupture was observed along the Superstition Hills fault west of Westmorland. Kahle reported that an offset of 15 - 20 cm was observed where the Superstition Hills fault crosses Imler Road after the second event but t h a t no rupture was observed there after the first event. CDMG field work has documented more than 20 km of rupture along the northwest trending Superstition Hills fault ; up to 60 cm of right slip was measured along with a few cm of vertical displacement (Gray, CDMG, personal communication).

Most of the damage from both earthquakes was confined to the cities of Calexico in California and Mexicali in Baja California, Mexico. Little damage was reported in the rest of the Imperial Valley (ENR, 1987).