Triggered slip along the San Andreas fault after the 8 July 1986 North Palm Springs earthquake
In addition to minor surface cracks in the region of the 8 July 1986 North Palm Springs earthquake, minor aseismic surficial rupture occurred along three segments of the San Andreas fault, 44 to 86 km southeast of the epicenter. Data from a creepmeter and a tiltmeter at one locality suggest that triggered slip occurred coseismically beneath the instruments but took 33 hr to propagate to the surface. That slippage occurred coseismically at depth favors mechanisms for triggered slip that involve dynamic or static strain changes rather than creep migrating from the source region. The distribution of slip along the San Andreas fault associated with the North Palm Springs earthquake differed significantly from that recorded after the moderate 1968 Borrego Mountain, California and 1979 Imperial Valley, California, earthquakes. During these earthquakes, triggered slip occurred along the San Andreas fault in the Durmid Hill area and in the Mecca Hills. Triggered slip associated with the North Palm Springs earthquake occurred in these two areas again, but also extended farther northwest into the Indio Hills, where as much as 9 mm of dextral slip occurred. In the Mecca Hills, surface cracks in 1986 appeared over a shorter fault length than in previous events, and the dextral displacement was smaller, with maximum values of only 2 to 3 mm. On Durmid Hill, surface cracks in 1986 were localized along a 200-m-long stretch of the fault spanning the Mecca Beach creepmeter and extending about 150 m to the southeast. Right-lateral displacements on surface cracks in this area were 1.4 to 2.0 mm, smaller than those observed in previous events. Although the mechanism of triggered aseismic slip is poorly understood, examination of displacement rates for the past several decades to centuries may indicate whether the aseismic slip rate is constant or represents accelerating premonitory failiure of the southernmost San Andreas fault.
Additional Information© 1989 Seismological Society of America. Manuscript received 24 August 1987. We are grateful to D. Johnson and the staff of the Caltech Seismological Laboratory who have assisted him in the installation and maintenance of creepmeters and alignment arrays. Discussions with K. Evans and C. Scholz illuminated some of the roles of creep and episodic aseismic fault slippage. We appreciate critical reviews of this manuscript by K. Evans, K. Hudnut, and an anonymous reviewer; these significantly improved the manuscript. D. Donovan, S. Phelps, and A. Thomas assisted us in the field. Our thanks to J. Mayne and K. Nagao for drafting and annotating figures. This study was supported by funds from the Caltech Earthquake Research Affiliates and by U.S. Geological Survey Grant 14-08-0001-G1177.
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