Abstract

We present evidence for several temporally related premonitory movements preceding the Landers earthquake of 28 June 1992. Premonitory movements were measured at alignment arrays spanning the northeast, Painted Canyon, and San Andreas faults in Mecca Hills, California. A transitory fault normal extension of 30 ± 3 mm to eight targets was measured twice, on 8 and 9 March 1992 and 7 and 8 June 1992, across the northeast and Painted Canyon faults at the Hidden Spring Wash site (HSWS). Four kilometers northwest of the HSWS simultaneous fault normal extensions of 24, 21, and 6 ± 6 mm were measured across a previously locked 12-km segment of the San Andreas fault on 6 June 1992, 22 days prior to the Landers earthquake. A linear regression fit of horizontal displacement at HSWS showed a distinct increase in horizontal angles for those targets spanning active faults on the same dates as the fault normal extension. For the first time, other than a few small events on the North Shore creepmeter, dextral slip was measured on a nontranspressive, parallel slip segment of the San Andreas. On 6 June 1992, we measured offsets of 14, 8, and 3 arc sec ±3 sec at the Indian Stone Ring site (ISRS) on the San Andreas fault, which translates into movement of 20, 18, and 9 mm. Triggered slip was not observed on the main trace of the San Andreas fault after the Joshua Tree earthquake, but 17 mm of dextral slip, possibly coseismic with the Landers earthquake, was measured along cracks on 5 July 1992. This amount of slip supports the magnitude of premonitory movement. Taken together, substantial simultaneous fault normal extension and lateral slip at three alignment arrays across three faults followed by a large triggered slip event provide strong evidence for the occurrence of precursors to the Landers earthquake.

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