South Napa Earthquake today, M 6.0-6.1--geologic context

Building destroyed in Napa. Photo by Justin Sullivan,
Getty Images as published on www.sfgate.com here

UPDATE AUGUST 25: Greg Braswell, as noted in his comment, has published images of the damage and an iso-damage map. They can be found at:

https://plus.google.com/photos/111007793583080982677/albums/6051216644873549681

and

https://mapsengine.google.com/map/edit?mid=zv4bNVwln6qk.kOPvmSymk_d8

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)

        Napa Valley is a large valley that trends to the northwest. It extends from Calistoga to the southern part of Napa and includes much of the core of the city. The valley is filled with Quaternary alluvial and fluvial deposits from the Napa River system, and earthquake damage in Napa can be especially severe during earthquakes because of shaking of these deposits. The West Napa Fault and its branches were first mapped by Weaver (1949), and subsequent work extended from the 1970's into the 1990's (references in the article cited). This early work reported that the fault and its branches extend 30-35 km along the western margin of the Valley; the Wesling and Hanson report suggests that the fault is 57 km long, extending from Carquinez Strait northwest toward St. Helena. The orientation and geomorphic expression are consistent with the West Napa fault being dominantly a right-lateral slip fault, with some compression that allows development of the nearby mountain ranges. The West Napa fault is one of a series of fairly short faults that include the Franklin and Sothampton faults. These faults lie between the Hayward-Rodgers Creek Fault zone (west) and the Calavaras-Concord-Green Valley fault zone (east).

          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.