M 5.1 - 15km ESE of Alum Rock, CA

Tectonic Summary

Based on the earthquake’s epicenter location, moment tensor solution, and alignment of aftershocks, the earthquake appears to have ruptured a patch at depth on the near-vertical Calaveras Fault. The Calaveras Fault, similar to the better-known Hayward Fault and San Andreas Fault to the west, is a right-lateral strike-slip fault that is part of the San Andreas Fault System in the San Francisco Bay region of California. The San Andreas fault system forms the plate tectonic boundary between the North American Plate and the Pacific Plate. Geodetic monitoring of the Calaveras fault near this earthquake’s location indicates about 6 mm/yr (0.24 inches/year) of long-term fault slip, but further south near Hollister the fault’s long-term slip rate is about 17 mm/year (0.66 inches/year). Historically, this section of the Calaveras fault has been very seismically active, producing earthquakes of magnitude 5 to 6.5 on the time scale of decades (four within 20 km of this event in the last 50 years). Larger earthquakes may have occurred prehistorically. The fault is also known to be slowly slipping (creeping) along much of its length, which contributes to the release of tectonic strain and may modulate where the larger earthquakes on this fault occur.