Uncharted Faults: South Napa 2014 Earthquake

Hello readers! Today we are taking a look at the magnitude 6.0 earthquake that hit South Napa, California on August 24th, 2014. The earthquake originated in the West Napa Fault Zone (WNFZ), a series of faults over 46 kilometers (28.6 miles), and possibly more, marked as potentially dangerous. The last damaging earthquake from the WNFZ was the magnitude 5.0 Yountville earthquake fourteen years prior.  It occurred at a depth of 8.8 kilometers (about 5.5 miles) and caused widespread damage to infrastructure, buildings, and roads. While scientists will study any earthquake, this one happened in the populous San Francisco Bay area and thus affected numerous people, drawing more attention than usual.
So, what did we learn from all this attention? Let me introduce today’s article, Tearing the terroir: Details and implications of surface rupture and deformation from the 24 August 201M6.0 South Napa earthquake, California, written by Stephen B. DeLong et al. Published in 2016 in the American Geophysical Union’s publication Earth and Space Science, this article takes us through what was learned from this earthquake.
Damage to a South Napa road after the earthquake. Image credit: Dan Ponti, USGS. Image is in the public domain.
Damage to a South Napa road after the earthquake. Image credit: Dan Ponti, USGS. Image is in the public domain.
The data used in this paper came from a mixture of radar, helicopter photography, and other instruments, as well as measurements from in-field observations. While occasionally there were discrepancies, such as one type of data not matching the other, there is general agreement. In the morning after the earthquake, two fault traces (where the fault line meets the ground) were found, having about 0.1 to 0.2 meters (about 4 to 8 inches) of where the ground slipped. The exception was one section 2 kilometers long (1.2 miles) where slip was up to 0.46 meters (18 inches). One trace showed post seismic (after the earthquake) slip, most of which was 3 to 9 kilometers (1.7 to 5.6 miles) north of the epicenter. There are a couple of possible reasons why it occurred on one trace and not others, but there is not enough clear data to determine the root cause in this case. Some data from instruments showed that one trace may extend up to 10 kilometers (6.2 miles) southeast of the epicenter, and that deformation extends 23 kilometers (14.2 miles) north of the epicenter.
Ruptured fault from the South Napa earthquake. Image credit: Dan Ponti, USGS. Image is in the public domain.
Ruptured fault from the South Napa earthquake. Image credit: Dan Ponti, USGS. Image is in the public domain.
This earthquake was unusually complex both spatially and how it acted over time given its magnitude. Before the earthquake, the main rupture trace for this earthquake had been only partially mapped; the secondaries were mostly unknown; and the trace best expressed on the surface was thought to have its north end near the Napa Airport after around 8 kilometers (5 miles). Some of the 2014 ruptures were one kilometer (0.6 miles) or more from those previously mapped, though some occurred on those already mapped. Rupturing also happened 2 kilometers (1.2 miles) west of the parts thought to be most recently active, on a fault only partially mapped. This is not where one would have anticipated an earthquake after studying the old maps. The earthquake made apparent various traces not previously mapped; these may have the potential for future events, though events on previously mapped faults are of course still possible. At the same time, some faults that were previously mapped did not rupture.
Clearly, the West Napa Fault Zone is more complex than we had thought. There is now more to be done in the future for mapping these faults and determining when they might be active again. This is part of why scientists study everything–there is always something more to be learned.

 

Bibliography
DeLong, S. B., et al. (2016), Tearing the terroir: Details and implications of surface rupture and deformation from the 24 August 2014 M6.0 South Napa earthquake, California, Earth and Space Science, 3, 416430, doi:10.1002/2016EA000176 .
Today’s main article.
Marshak, Stephen. Earth: Portrait of a Planet3rd ed. W.W. Norton & Company. Print.
My geology textbook, used for general reference about earthquakes and faults.
United States Geological Survey. 2014 South Napa CA M6 Earthquake-August 24. USGS23 August 2014. Web.
The source of the image of the damaged sidewalk and of the slip in the field.
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