Abstract

On 28 and 30 November 1987, we found new surface ruptures along the central break of the Coyote Creek fault's 1968 rupture. Mapped slip that may have been triggered by the Superstition Hills earthquakes of 24 November 1987 was dominantly right-lateral and ranged up to 15 mm. Total length of the new ruptures was about 3 km, distributed along two quasi-continuous segments of the main fault, plus one segment of a secondary fault, all of which ruptured in 1968. The longest segment of new rupture followed the northeast side of the Ocotillo Badlands for 2.3 km. Newly mapped slip in 1987 occurred along parts of the Coyote Creek fault that were characterized in 1968 by several features. These were: large total dextral slip, a prominent vertical slip component, rupture along single fractures instead of en-echelon cracks, and large afterslip in the few years following the 1968 earthquake. Past episodes of triggered slip on the San Andreas, Imperial, and Superstition Hills faults were hypothesized by others to be precursory phenomena. The 1987 slip on the Coyote Creek fault, however, may be an episode of afterslip. This suggestion is supported by the decrease in creep rates since 1968 shown by data from two alinement arrays on the Coyote Creek fault. The correct interpretation of triggered slip events, and their value as earthquake precursors, therefore depends upon a good knowledge of the slip history of a fault.

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