Triggered surface slips in the Coachella Valley area associated with the 1992 Joshua Tree and Landers, California, earthquakes

The Coachella Valley area was strongly shaken by the 1992 Joshua Tree (23 A pril) and Landers (28 June) earthquakes, and both events caused triggered s lip on active faults within the area. Triggered slip associated with the Jo shua Tree earthquake was on a newly recognized fault, the East Wide Canyon fault, near the southwestern edge of the Little San Bernardino Mountains. S lip associated with the Landers earthquake formed along the San Andreas fau lt in the southeastern Coachella Valley, Surface fractures formed along the East Wide Canyon fault in association wi th the Joshua Tree earthquake. The fractures extended discontinuously over a 1.5-km stretch of the fault, near its southern end. Sense of slip was con sistently right-oblique, west side down, similar to the long-term style of faulting. Measured offset values were small, with right-lateral and vertica l components of slip ranging from 1 to 6 mm and 1 to 4 mm, respectively. Th is is the first documented historic slip on the East Wide Canyon fault, whi ch was first mapped only months before the Joshua Tree earthquake. Surface slip associated with the Joshua Tree earthquake most likely developed as tr iggered slip given its 5 km distance from the Joshua Tree epicenter and aft ershocks. As revealed in a trench investigation, slip formed in an area wit h only a thin (<3 m thick) veneer of alluvium in contrast to earlier docume nted triggered slip events in this region, all in the deep basins of the Sa lton Trough. A paleoseismic trench study in an area of 1992 surface slip revealed eviden ce of two and possibly three surface faulting events on the East Wide Canyo n fault during the late Quaternary, probably latest Pleistocene (first even t) and mid- to late Holocene (second two events). About two months after the Joshua Tree earthquake, the Landers earthquake t hen triggered slip on many faults, including the San Andreas fault in the s outheastern Coachella Valley, Surface fractures associated with this event formed discontinuous breaks over a 54-km-long stretch of the fault, from th e Indio Hills southeastward to Durmid Hill. Sense of slip was right-lateral ; only locally was there a minor (similar to 1 mm) vertical component of sl ip. Measured dextral displacement values ranged from 1 to 20 mm, with the l argest amounts found in the Mecca Hills where large slip values have been m easured following past triggered-slip events.