Abstract

Surficial slip along the entire mapped length of the Superstitition Hills fault in southern California occurred in association with the Superstition Hills earthquake (Ms 6.6) of 24 November 1987. We made repeated measurements of surface slip at 36 sites along the fault and occupied 64 sites at least once. At our sites, dextral slip was as high as 48.5 cm 1 day after the earthquake and 71 cm 2 months after. The measurements show that slip during the period from hr to several hundred hr following the event is described by a simple power law of time. Extrapolation to t = 1 min indicates that co-seismic slippage ranged from 5 to 23 cm at 10 of our best recorded sites, suggesting that finite co-seismic slippage occurred along the length of the fault. These extrapolations are supported by a measurement made at Imler Road 30 min after the shock.

Measurements are complete through October 1988. At many sites, the form of slip-rate was decay changed from power law to a function of log time during the interval between 300 and 500 hr after the earthquake. Logarithmic slip-rate decay in time was observed for a period of several yr after the Parkfield, Borrego Mountain, and Imperial Valley earthquakes. Those measurements may have begun too late to resolve power-law behavior at early times. If current logarithmic behavior of the Superstition Hills fault persists, right-lateral slippage will approach 90 cm 10 yr after the rupture.

Changes in the along-fault displacement profile correlate well with geometric features including a fault bend and a major fault step. Moreover, slip behavior appears to be correlated to the thickness of sedimentary cover along the fault. Also, the northern half of the fault is bounded by a large block of continental crystalline basement. The presence of this block may have contributed to the relatively uniform early slip behavior observed there.

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