Beneath the physiographic boundary between the Coast Ranges and the Great Valley, a fundamental tectonic boundary exists between the Coast Ranges province and the Sierran block. Recent geophysical studies have revealed the Coast Ranges–Sierran block (CRSB) boundary zone to be a complex region of compressional tectonics wherein wedges of comparatively ductile Franciscan Complex of the Coast Ranges have overridden the competent Sierran block basement and peeled up the overlying sediments of the Great Valley Sequence. The boundary zone was the probable source of the two 1892 Winters earthquakes ( M L 6–7) and the 1983 Coalinga earthquake ( M L 6.7). We evaluated the seismicity within the northern and central portions of the zone from Red Bluff south to San Luis reservoir for the period 1969-1985. Seismicity was spatially diffuse; most of the earthquakes occurred in somewhat isolated concentrations within the upper crust. A low activity level, temporal clustering, and episodic behavior also characterized this seismicity. The largest earthquake in the boundary zone was a M L 4.2 event near Madison; 20 events exceeded M L 3.0. In a few cases the seismogenic sources appear to be eastward dipping, high-angle reverse faults, although some right-slip, north trending faults also appear to be seismogenic. In comparison, the Coalinga and 1985 Kettleman Hills main shocks in the southern portion of the zone appeared to involve gently westward dipping thrust faults, although aftershocks also occurred on moderately to steeply dipping reverse faults. Fault plane solutions along the zone exhibited a tectonic stress field of NE to E trending compression, normal to the boundary. Such compression may be a consequence of one or a combination of several influences: (1) the westward movement of the Sierran block against the Coast Ranges due to late Cenozoic crustal extension in the Great Basin, (2) slightly convergent motion between the Pacific and North American plates, and (3) fault-normal crustal compression due to the low shear strength of the San Andreas fault. The tectonic deformation manifested by the Coalinga and recent moderate-sized earthquakes in the southern zone appears to be occurring along the full extent of the boundary albeit at varying rates for different segments. Thus the potential for large earthquakes may exist along segments of the CRSB boundary and the 1892 main shock may represent a characteristic earthquake for that portion of the zone.

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 northwest-trending faults, as observed in the 1966 M L 5.7 Chico earthquake.