Abstract

A seismically active, blind fold and thrust belt underlies the Coast Range-Central Valley geomorphic boundary (CRCV), California. Earthquakes associated with this fault system include the 1983 Mw 6.5 Coalinga and 1985 Mw 6.1 Kettleman Hills events. Published estimates of the CRCV slip rate derived from published studies range from 1 to 10 mm/yr, and estimated recurrence intervals of Coalinga-type events range from 200 to 2000 yr. Relative consistency of geomorphic expression and regional geologic relationships suggests that deformation rates are similar along the 500-km length of the CRCV.

We evaluated historical seismicity to further constrain recurrence interval and slip-rate estimates. Based on structural geology, geomorphology, and historical seismicity, the CRCV may comprise 18 to 25 segments from 12 to 57 km in length, with most segments between 20 and 30 km. The characteristic earthquake for the average-length segment may be Mw 6.3 to 6.4, or slightly smaller than the Coalinga earthquake, and the longest segments may yield earthquakes as large as Mw 6.8 to 7.1. During the approximately 150-yr period of historical record, 11 earthquakes of Mw 5.8 to 6.8, of which eight may be characteristic events, have taken place on the CRCV. The characteristic earthquakes appear to have ruptured single segments. Using the estimates of co-seismic rupture length for the 1983 Coalinga and 1985 Kettleman Hills earthquakes to calibrate earlier events, the historical earthquake record corresponds to rupture of 32 to 39% of the length of the CRCV in 142 yr, a recurrence interval of 360 to 440 yr for the average segment. Combining the recurrence estimates with estimates for co-seismic displacement yields slip rates of 1.8 to 8.0 mm/yr. Because recent VLBI studies suggest that the total convergence rate normal to the Pacific-North American plate boundary at the latitude of the CRCV is ≦ 3 mm/yr, the lower end of the slip rate range (1.8 to 3 mm/yr) is favored for the CRCV. If these estimates are correct, the CRCV accommodates most of the plate convergence between the North American and Pacific Plates, indicating that the historical earthquakes efficiently released strain accumulated on the CRCV and suggesting that aseismic deformation along the CRCV is minor.

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