Abstract

The Sacramento Valley, which occupies the northwestern portion of the Sierran block, appears to have undergone a moderate level of crustal deformation, at least in Quaternary times. Seismicity is broadly distributed throughout the valley and along its margins. Areas which have exhibited a moderate level of earthquake activity include: along the southwestern margin in the Coast Ranges near Lake Berryessa and within the Coast Ranges-Sierran block (CRSB) boundary zone, source of the 1892 M > 6-1/2 Vacaville-Winters earthquakes; near Williams where three earthquake swarms occurred from 1980 to 1985; in the vicinity of Willows and northward to Corning; near Oroville in the Sierran foothills, site of a M_L 5.7 earthquake and numerous aftershocks in 1975; in the foothills near Chico; in the stretch of the foothills east of Red Bluff to Redding; and in the Coast Ranges west and northwest of Black Butte Reservoir. An area of particular seismic quiescence is in the valley around and south of Sacramento.

Detailed analyses of this seismicity and focal mechanisms indicate that active geologic structures include blind thrust and reverse faults and associated folds (e.g., Dunnigan Hills) within the CRSB boundary zone on the western margin of the Sacramento Valley, the Willows and Corning faults in the valley interior, and reactivated portions of the Foothill fault system. Other possibly seismogenic faults include the Chico monocline fault in the Sierran foothills and the Paskenta, Elder Creek and Cold Fork faults on the northwestern margin of the Sacramento Valley.

The contemporary state of tectonic stress within the Sacramento Valley appears to be transitional, based on a limited number of focal mechanisms. East-west compressive stresses along the CRSB boundary zone accommodated by thrust and reverse faulting extend into the valley interior where a transition occurs to an extensional or possibly strike-slip stress field characterized by an approximate north-south maximum principal stress and/or an east-west minimum principal stress. In the Sierran foothills, this stress field is manifested by both normal faulting on northerly-striking faults, as exemplified by the 1975 Oroville earthquakes, and strike-slip faulting on northwesttrending faults, as observed in the 1966 M_L 5.7 Chico earthquake.