Seismotectonics of the Coast Ranges in the vicinity of Lake Berryessa, northern California

Abstract

Tectonic deformation as manifested by seismicity and active faulting in the Coast Ranges in the vicinity of Lake Berryessa appears to be concentrated along the Green Valley-Cedar Roughs fault trend and to a lesser degree, the Coast Ranges-Sierran block (CRSB) boundary zone. The Green Valley-Cedar Roughs fault trend appears to be a complex, discontinuous or segmented shear zone consisting of multiple faults. Geomorphic evidence of Quaternary displacement exists along many portions of the fault trend with Holocene features and evidence for fault creep occurring along the Green Valley fault. In contrast, no historic earthquakes greater than ML 5 are known to have occurred along the fault trend. Earthquakes relocated in this study from 1970 to 1988 occur in the vicinity of mapped faults; however, the seismicity is discontinuous with a strong tendency to occur in clusters. Also superimposed on the general north-northwest fault trend are north-south epicentral trends not associated with any mapped faults. Focal mechanisms suggest predominantly right-lateral, strike-slip faulting on both north and north-northwest-trending planes. The existence of reverse and normal focal mechanisms also attests to the complex faulting occurring along the Green Valley-Cedar Roughs trend. In general, the geologic and seismogenic characteristics are consistent with the hypothesis that the Green Valley-Cedar Roughs fault trend represents young faulting, possibly related to a newly developing plate boundary east of the San Andreas fault. Earthquakes are confined to the top 12 km of the upper crust. Seismicity within the CRSB boundary zone since 1970 has been generally confined to a sequence in 1978 near Madison with a mainshock of ML 4.2. Thrust/reverse faulting appears to be the source of this activity, consistent with the seismogenic processes observed along other segments of the boundary zone. Only one earthquake has occurred since 1970 in the epicentral area of the 1892 ML 6¾ Vacaville-Winters earthquake. The state of stress in the Lake Berryessa region as exhibited by focal mechanisms appears to be characterized by NE-directed tectonic compression as observed elsewhere away from the San Andreas fault system in northern and central California.