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 (ML 6--7) and the 1983 Coalinga earthquake (ML 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 ML 4.2 event near Madison; 20 events exceeded ML 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 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 represent a characteristic earthquake for that portion of the zone. ¿ American Geophysical Union 1988