Variable rates of Late Quaternary strike slip on the San Jacinto Fault Zone, southern California

Abstract

Three strike slip displacements of strata with known approximate ages have been measured at two locations on the San Jacinto fault zone. Minimum horizontal offset between 5.7 and 8.6 km in no more than 0.73 m.y. northeast of Anza indicates 8–12‐mm/yr average slip rate since late Pleistocene time. Two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the Coyote Creek fault in western Imperial Valley. Horizontal slip of 1.7 m has been calculated for the youngest sediment of Lake Cahuilla since its deposition 275–510 yr B.P. The corresponding slip rate is 2.8–5.0 mm/yr. Right lateral offset of 10.9 m measured on a buried stream channel older than 5060 yr B.P. but younger than 6820 yr B.P. yields average slip rates for the intermediate time periods, 400± to 6000± yr B.P., of 1–2 mm/yr. The average rates of slip for these three time intervals suggest a relatively quiescent period for the San Jacinto fault zone from about 4000 B.C. to about 1600 A.D. To the extent that long‐term variations in seismic activity of major strike slip faults elsewhere are known, fluctuations in slip rate for the San Jacinto fault would not appear to be abnormal. If the San Jacinto and adjacent segments of the San Andreas fault alternately assume dominant roles in absorbing motion between the Pacific and American plates, perhaps even more recently than 400 yr ago, the San Andreas fault south of the Transverse Ranges expressed most of the motion but has since become relatively inactive.