Calaveras Fault

The Calaveras Fault is a mostly right-lateral, strike-slip fault. The 2003 Working Group Report divided the fault into three segments. The northern segment of the Calaveras Fault is virtually locked and this segment of the fault creeps at a rate between 2 and 3 mm/yr. At the southern end of its central stretch it creeps at about 14 mm/yr dropping northward to about 6 mm/yr (0.24 inch/yr) of creep at its intersection with the Hayward Fault.

The total slip rate on the Calaveras Fault is approximately 6 mm/yr north of its intersection with the Hayward Fault and is approximately 15 mm/yr to the south.

The northern end of the central segment of the Calaveras Fault last produced a notable earthquake in October 2007, the M5.4 Alum Rock earthquake, which ruptured to the south.

Historically, the southern half of the central segment of the Calaveras Fault has been the most seismically active segment of the fault. It produced the M6.2 Morgan Hill earthquake in 1984 and a M6.2 earthquake in 1911. The M5.9 Coyote Lake earthquake in 1979 ruptured slightly to the south of these other earthquakes. Because its rate of creep nearly matches the total fault slip rate, it is widely believed that this segment of the fault is not capable of an earthquake having a magnitude much larger than the 1984 Morgan Hill earthquake.

The northern segment of the Calaveras Fault has little microseismicity, consistent with its low creep rate of 2 to 3 mm/yr and nearly locked behavior. The 2007 Working Group Report established a recurrence interval of surface rupturing earthquakes on this northern segment of 465 ± 130 years. However, all that we know about the date of the last surface rupturing earthquake is that it predates 1776.

The 2003 Working Group for California Earthquake Probability assigned an 11% probability that the Calaveras Fault would produce a magnitude 6.7 or larger earthquake in the next 30 years.