Historic Earthquakes
West of Westmorland, California
1987 November 24 UTC 01:54 (local November 23)
Magnitude 6.5
1987 November 24 UTC 13:15
Magnitude 6.7
The Superstition Hills earthquakes caused an estimaged $3 million property damage in Imperial County. Epicenters of the shocks were in the western Imperial Valley on a fault system comprising the northwest-striking Superstition Hills fault and a previously unknown northeast-striking structure. The earthquake sequence consisted of foreshocks, the first main shock, and aftershocks on the northeast trend, followed by the second main shock about 11 hours later and aftershocks on the northwest trend. Significant surface ruptures occurred along the Superstition Hills Fault.
Damage reported in the epicentral region at El Centro, Imperial, and Westmoreland included fallen chimneys, broken underground pipes, broken windows, and large displacements in highways or streets. The Worthington Road Bridge across the New River needed to be replaced, owing to liquefaction damage to the approach fill on both sides of the bridge. Damage at the Desert Test Range Control Center near Westmorland, which included equipment falling through a window and small water tanks tipping against the building, was sufficient to stop operations for several days.
Damage to canal facilities in the Southern California Irrigation District was estimated to be between $600,000 and $750,000. The first main shock caused minor buckling of the concrete lining in canals of the west side of the valley; the second main shock collapsed thousands of feet of concrete canal lining, mainly in the northwest corner of the valley nearest the earthquake epicenters.
The shock at 01 54 UTC was associated with left-lateral surface rupture on many faults in and near the Superstition Hills. A maximum surface slip of 12.5 cm was observed on the Elmore Desert Ranch fault. The shock at 13 15 UTC ruptured the surface of the right-lateral Superstition Hills fault for a distance of 27 km southeastward from its epicenter. This right-lateral movement continued to increase over the following 339 days. The maximum vertical slip observed on the southernmost ruptured part of the Superstition Hills fault zone (named the Wienert fault) was 25 cm, but that rupture also continued to increase. Both shocks were felt over most of southern California and in parts of western Arizona and southern Nevada.
Abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.
