Magnitude 6.0 Earthquake, Napa, California, August 24, 2014
Major earthquake this morning on the West Napa fault. Earthquake occurred at 3:20 am this morning. Here in Davis, it was felt by many. It is reported that there are at least 89 injuries in Napa, with widespread damage in the downtown area. The fault crossed at least one road in the area, and showed evidence of right-lateral slip. Many water mains broke, and there are reports of 50 gas line breaks and fires as a result.
I used the hazard viewer on this web site to check the forecast as of last night for the area. I selected a circle of 100 km radius around Napa. Results are shown below. There was a high probability (83%) of a M>5 within 3 years and 100 km, and a moderate probability (24%) of a M>6 within 3 years of last night.
(Update as of 11:00 am: Probabilties on site are being updated to reflect the Napa earthquake and aftershocks)
In terms of earthquake history, there was a major earthquake in 1838 in the area where the M7 1989 Loma Prieta occurred. In 1868, the Hayward fault sustained a M7 earthquake. These events were part of the run-up to the 1906 San Francisco earthquake. There were many M>6 events during that time period.
It has been 25 years since Loma Prieta, so the possibility exists that we are in the early stages of the run-up to more significant earthquakes. This is what our models indicate.
With respect to the local area, the earthquake follows a reasonably active earthquake history within 100 km of the epicenter. The last significant event occurred on September 3, 2000 a magnitude 4.9.
Aftershocks can be expected to continue for at least 1 week. Bath's law would suggest that we should expect an aftershock of as much as M~5.
The figure below shows the selection circle: