|Building destroyed in Napa. Photo by Justin Sullivan,|
Getty Images as published on www.sfgate.com here
Headlines this morning announced that a M6.0 (or 6.1, conflicting reports) earthquake at 3:20 a.m. awoke people around the area of Napa, California, north of San Francisco. Dozens of people are injured, four homes in a mobile park burned, and damage to buildings in downtown Napa appears extensive. The quake is the largest in the Bay Area since the 1989 Loma Prieta earthquake.
Here's a bit of context that I found in an on-line technical report authored by John R. Wesling and Kathryn L. Hanson, 2008 (reference at the end of this post). Here is also a link to the USGS earthquake event page.
|Map of the five sections of the fault defined by Wesling|
and Hanson (Figure 3 from the cited report)
Wesling and Hanson divided the fault into five reaches based on geomorphic expression, terrain traversed, and availability and quality of data. These branches are: St. Helena-Dry Creek; Yountville-North Napa; North Napa-Napa River; Napa River-American Canyon; and American Canyon-Carquinez Strait. The USGS is reporting that the earthquake struck 3 miles northwest of American Canyon, and placed the epicenter between 6 miles southwest of Napa, toward Vallejo (see adjacent map). According to the map above, this would place the epicenter on the Napa-River-American Canyon fault toward the northern end or, possibly, the southern end of the North Napa-Napa River branch, depending on where the reference point within Napa city is located.
No historical earthquakes larger than M6.0 have been associated with the West Napa fault, although the M5.0 Mount Veeder earthquake ("Yountville earthquake") in 2000 may have been linked to it. This earlier earthquake was centered about 5 km west of the West Napa fault, and caused considerable damage in Napa.
Reference: "Mapping of the West Napa Fault Zone for Input into the Northern California Quaternary Fault Database," by John R. Wesling and Kathryn L. Hanson, 2008.