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This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

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Susan Kieffer can be contacted at s1kieffer at gmail.com


Sunday, August 24, 2014

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:


and



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.

3 comments:

Jack Duggan said...

Thanks, that was a nice blog to find, good information on the earthquake I felt this morning. I look forward to reading more.

Jack Duggan
Richmond, California
about 20 miles south of the epicenter

Astrosymm said...

Global predictions for localized events were posted on Astrosymm.com on 6/14/14. Predictions included broad spectrum events for 8/25/14 at 1413 UTC. Both the Napa and Peru earthquakes struck within about one day of this prediction. Hurricane Marie peaked on 8/24 at 21 UTC - about 2 hours from the Peru earthquake.

Greg Braswell said...

Good work. The Hanson and Wesling reference was very helpful. I've been out mapping surface cracks showing the right lateral fault movement. Map at:
https://mapsengine.google.com/map/edit?mid=zv4bNVwln6qk.kOPvmSymk_d8

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