Paleoseismology of the Johnson Valley, Kickapoo, and Homestead Valley faults: Clustering of earthquakes in the eastern California shear zone

Paleoseismic data from 11 trenches at seven sites excavated across the southern Johnson Valley, Kickapoo, and Homestead Valley faults that ruptured in the 1992 Landers earthquake, as well as the northern Johnson Valley fault which did not fail in 1992, indicate that the return period for large surface rupturing events in this part of the eastern California shear zone is in the range of 5-15 ka. The inferred slip rates, based on their respective recurrence intervals, are in the range of 0.2-0.6 mm/yr for each of the faults studied. A previous large earthquake ruptured the southern Johnson Valley and Kickapoo faults about 5 ka B.P. The northern Johnson Valley fault: also failed at about this time at 5.8 ka B.P. and may have been part of the same rupture. In contrast, the penultimate large earthquake that we identify on the Homestead Valley fault occurred about 15 ka B.P., much earlier than other faults involved in the 1992 rupture. From these observations, combined with paleoseismic work by others after the 1992 earthquake, it appears that previous events along the southern Johnson Valley and Kickapoo faults were different than those of 1992 and may have involved other fault segments. It has been over 5 ka since the most recent rupture on the northern Johnson Valley fault. Therefore, it is surprising that it did not fail in the 1992 rupture. From our observations, dextral shear appears to be distributed across the entire eastern California shear zone. with individual faults taking only a small proportion of the overall slip. Release of this regional strain appears to occur in temporal clusters of large (?) earthquakes, with the 1992 event apparently the most recent of a sequence of late Holocene (0-1 ka) earthquakes that have ruptured the nine faults we have trenched in the southwestern Mojave desert. Previous dusters of earthquake activity occurred in the early (8-9 ka) and middle (5-6 ka) Holocene, and possibly the latest Pleistocene (similar to 15 ka).