By David Jacobson, Temblor
At 3:45 p.m. local time yesterday, a M=3.7 earthquake struck the East Carson Valley Fault Zone in southwestern Nevada, and shaking was felt in South Lake Tahoe, California. According to the USGS, this quake occurred at a depth of 9 km, with weak shaking felt close to the epicenter. This M=3.7 earthquake was followed by at least 20 aftershocks. At 8 a.m. this morning (June 7), more than 130 people had reported feeling the quake on the USGS website, with the majority of them coming in from South Lake Tahoe and Gardnerville, which combined are home to over 25,000 people.
While this small earthquake occurred along the East Carson Valley Fault Zone, the broader region is known as the Walker Lane Fault System. This system runs nearly parallel to California’s San Andreas Fault and is composed primarily of discontinuous right-lateral strike-slip faults. Having said that, based on the location of yesterday’s event, the USGS focal mechanism, and surrounding faults, this earthquake likely occurred on a small left-lateral strike-slip fault.
Even though the Walker Lane and San Andreas fault systems run nearly parallel to one another and together accommodate the majority of stress across the Western United States they have different patterns of faulting. The San Andreas is made up of relatively linear and joined faults. Walker Lane on the other hand is primarily composed of discontinuous faults. This distinct difference could be due to the fact that the Walker Lane is younger than the San Andreas, meaning it hasn’t had enough time to mature. Because displacements on the San Andreas are 3-4 times greater than those seen on Walker Lane faults, Western Nevada has a much more complex array of faults. Additionally, the eastern flank of the Sierra Nevada Mountains is undergoing a small component of extension in addition to being sheared. These differences help explain why the Walker Lane Fault System is much more disjointed than the San Andreas.
These smaller, disjointed faults that make up the Walker Lane Fault System also mean the region is not susceptible to earthquakes as large as those that can occur on the San Andreas. Having said that, the larger, more continuous faults in Walker Lane are still capable of rupturing in M=7+ earthquakes. Additionally, the 1872 M=7.6 Owens Valley earthquake occurred in what can be considered either part of Walker Lane or the Eastern California Fault Zone. Regardless, the area is not immune from damaging quakes. Based on the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, the area just to the southeast of Lake Tahoe is susceptible to M=6.25+ earthquakes. This model uses global strain rates and seismicity since 1977 to forecast what the likely earthquake magnitude in your lifetime is for any location on earth. So, even though yesterday’s M=3.7 was by no means damaging, it highlights a tectonically active region that could undergo a moderate-sized earthquake which could cause extensive damage.
Nevada Seismic Laboratory at University of Nevada Reno
Wesnousky, Steven, The San Andreas and Walker Lane fault systems, western North America: transpression, transtension, cumulative slip and the structural evolution of a major transform plate boundary, Journal of Structural Geology 27 (2005) 1505-1512