MULTIPLE PRECURSORS TO THE LANDERS EARTHQUAKE

We present evidence for several temporally related premonitory movemen ts preceding the Landers earthquake of 28 June 1992. Premonitory movem ents were measured at alignment arrays spanning the northeast, Painted Canyon, and San Andreas faults in Mecca Hills, California. A transito ry 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 northea st and Painted Canyon faults at the Hidden Spring Wash site (HSWS). Fo ur kilometers northwest of the HSWS simultaneous fault normal extensio ns of 24, 21, and 6 +/-6 mm were measured across a previously locked 1 2-km segment of the San Andreas fault on 6 June 1992, 22 days prior to the Landers earthquake. A linear regression fit of horizontal displac ement at HSWS showed a distinct increase in horizontal angles for thos e 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 N orth Share creepmeter, dextral slip was measured on a nontranspressive , parallel slip segment of the San Andreas. On 6 June 1992, we measure d offsets of 14, 8, and 3 are sec +/-3 sec at the Indian Stone Ring si te (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 meas ured along cracks on 5 July 1992. This amount of slip supports the mag nitude of premonitory movement. Taken together, substantial simultaneo us fault normal extension and lateral slip at three alignment arrays a cross three faults followed by a large triggered slip event provide st rong evidence for the occurrence of precursors to the Landers earthqua ke.