Most folks appear to have slept through it but a low, 3.3 Magnitude earthquake rumbled under Alamo at 4:55 a.m. Monday – most significant of a flurry of three shakers and apparently leaving no significant damage in its wake.
USGS scientists attributed the shaker to the presence of the Mt. Diablo Thrust Fault, a fault deep in the crust that does not rupture to the surface but instead branches into a number of faults and folds in the upper few kilometers of the crust.
From the USGS:
This fault may accommodate the transfer of slip from the Greenville Fault to the Concord Fault, and is thought to be responsible for the ongoing uplift of Mount Diablo.
The total slip rate for the fault, approximately 3 to 4 mm/yr (1/8 inch/year), represents long-term averages derived from balanced cross sections and analysis of stratigraphic relations to determine when the shortening of the Mount Diablo region began.
The thrust fault appears to result from a restraining transfer of slip on the Greenville Fault to the slip on the Concord-Green Valley Fault.
The 2003 Working Group for California Earthquake Probability assigned a 3% probability that the Mount Diablo Thrust Fault would produce a magnitude 6.7 or larger earthquake in the next 30 years.