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 sout hern 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 surfa ce 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 r ecurrence intervals, are in the range of 0.2-0.6 mm/yr for each of the faul ts studied. A previous large earthquake ruptured the southern Johnson Valley and Kickap oo 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 Home stead Valley fault occurred about 15 ka B.P., much earlier than other fault s involved in the 1992 rupture. From these observations, combined with pale oseismic work by others after the 1992 earthquake, it appears that previous events along the southern Johnson Valley and Kickapoo faults were differen t than those of 1992 and may have involved other fault segments. It has bee n over 5 ka since the most recent rupture on the northern Johnson Valley fa ult. Therefore, it is surprising that it did not fail in the 1992 rupture. From our observations, dextral shear appears to be distributed across the e ntire eastern California shear zone. with individual faults taking only a s mall proportion of the overall slip. Release of this regional strain appear s to occur in temporal clusters of large (?) earthquakes, with the 1992 eve nt apparently the most recent of a sequence of late Holocene (0-1 ka) earth quakes that have ruptured the nine faults we have trenched in the southwest ern Mojave desert. Previous dusters of earthquake activity occurred in the early (8-9 ka) and middle (5-6 ka) Holocene, and possibly the latest Pleist ocene (similar to 15 ka).