Rupture process of the 1987 Superstition Hills earthquake from the inversion of strong-motion data
A pair of significant earthquakes occurred on conjugate faults in the western Imperial Valley involving the through-going Superstition Hills fault and the Elmore Ranch cross fault. The first event was located on the Elmore Ranch fault, M_s = 6.2, and the larger event on the Superstition Hills fault, M_s = 6.6. The latter event is seen as a doublet teleseismically with the amplitudes in the ratio of 1:2 and delayed by about 8 sec. This 8-sec delay is also seen in about a dozen strong-motion records. These strong-motion records are used in a constrained least-squares inversion scheme to determine the distribution of slip on a 2-D fault. Upon closer examination, the first of the doublets was found to be itself complex requiring two episodes of slip. Thus, the rupture model was allowed to have three separate subevents, treated as separate ruptures, with independent locations and start times. The best fits were obtained when all three events initiated at the northwestern end of the fault near the intersection of the cross-fault. Their respective delays are 2.1 and 8.6 sec relative to the first subevent, and their moments are 0.4, 0.9, and 3.5 × 10^(25) dyne-cm, which is about half of that seen teleseismically. This slip distribution suggests multi-rupturing of a single asperity with stress drops of 60, 200, and 15 bars, respectively. The first two subevents were confined to a small area around the epicenter while the third propagated 18 km southwestward, compatible with the teleseismic and afterslip observations.
Additional Information© 1990, by the Seismological Society of America. Manuscript received 26 January, 1990. We thank Lorraine Hwang and Harold Magistrale for informative and useful discussions related to this paper. Reviews by Tom Heaton, Hiroo Kanamori, and Lisa Wald improved the original manuscript and critical reviews by Arthur Frankel and Paul Spudich helped clarify the presentation significantly. This work was supported by the USGS under contract 14-08-0001-21912. Contribution No. 4830, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
Published - 1079.full.pdf