Surface ruptures on cross-faults in the 24 November 1987 Superstition Hills, California, earthquake sequence
Left-lateral slip occurred on individual surface breaks along northeast-trending faults associated with the 24 November 1987 earthquake sequence in the Superstition Hills, Imperial Valley, California. This sequence included the M_s = 6.2 event on a left-lateral, northeast-trending "cross-fault" between the Superstition Hills fault (SHF) and Brawley seismic zone, which was spatially associated with the left-lateral surface breaks. Six distinct subparallel cross-faults broke at the surface, with rupture lengths ranging from about Formula to 10 km and maximum displacements ranging from 30 to 130 mm. About half a day after the M_s = 6.2 event, an M_s = 6.6 earthquake nucleated near the intersection of the cross-faults with the SHF, and rupture propagated southeast along the SHF. Whereas right-lateral slip on the SHF occurred dominantly on a single trace in a narrow zone, the cross-fault surface slip was distributed over several stands across a 10-km-wide zone. Also, whereas afterslip accounted for a large proportion of total slip on the SHF, there is no evidence for afterslip on the cross-faults. We present documentation of these surface ruptures. A simple mechanical model of faulting illustrates how the foreshock sequence may have triggered the main rupture. Displacement on other cross-faults could trigger an event on the southern San Andreas fault by a similar mechanism in the future.
Additional Information© 1989, by the Seismological Society of America. Manuscript received 28 July 1988. We thank H. Kanamori and the staff at the CIT/USGS office in Pasadena, especially L. Jones and D. Given, for epicenter locations soon after the earthquakes. We also thank T. Hanks at the USGS for providing us with the new airphotos. We thank R. Sharp, M. Clark, M. Rymer, and J. Lienkaemper at the USGS, J. Kahle at CDMG, P. Williams at L-DGO/CIT, and others for their advice and comments in the field. L. Sykes pointed out the April 1988 seismicity on the Extra fault to us. Reviews by R. Wallace, C. Scholz, and D. Simpson greatly improved this paper. Research was supported by U. S. Geological Survey grant No. 14-08-00001-G1330. Lamont-Doherty Geological Observatory contribution No. 4429.
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