A microearthquake monitoring program was conducted from 1976 to 1979 in the eastern San Joaquin Valley and Sierran foothills of central California. This program revealed one of the deepest occurrences of intraplate earthquakes observed in the conterminous United States. Earthquakes in the ML range of approximately 0 to 3.2 were confined to the lower crust of the Sierran block at depths from 12 to 38 km. The distribution of epicenters was diffuse and widespread with some clustering in space and time. Two particulary active areas that were consistent with the historical seismicity record were recognized: an area 15 km south of the town of Madera and an area 15 km south of Mariposa. No association of the seismicity with faults or other geologic structures expressed at the surface was apparent. Based on regional seismic monitoring by the U.S. Geological Survey and the detailed monitoring performed in this study, this occurrence of deep seismicity does not appear to extend farther north along the foothills than Jamestown, although two deep events have been observed near Oroville. The southern limit is not as well defined due to the lack of seismographic coverage.
Focal mechanisms of these deep crustal earthquakes exhibited: (1) strike-slip faulting combined with some reverse and normal components; and (2) a regional tectonic stress field characterized by a maximum compressive stress generally trending north-south and a minimum compressive stress trending east-west. These observations support the view that (1) the Sierra Nevada represents a transition between a region of primarily shear deformation along the San Andreas fault zone and extensional tectonics of the Basin and Range province; and (2) the orientation of the tectonic stress field is generally uniform throughout the crust of the Sierran block.