M5.5 earthquake in California occurred at northwestern edge of Walker Lane Shear Zone

Similar earthquake occurred in 2013

Citation: Hubbard, J. and Bradley, K., 2023. M5.5 earthquake in California occurred at northwestern edge of Walker Lane Shear Zone. Earthquake Insights, https://doi.org/10.62481/81f43bc2

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A M5.5 earthquake occurred in northern California on May 11 2023 at 23:19 UTC. The earthquake did not occur on the San Andreas or related strike-slip faults along the coast, but at the northern edge of the Walker Lane Shear Zone. This region is a wide area that experiences transtension - a combination of strike-slip and normal faulting - on the eastern side of the Sierra Nevada. The Walker Lane Shear Zone accommodates ~15-25% of the total relative movement between the Pacific and North American Plates (the remainder is largely on the San Andreas and associated faults to the west).

Figure 1: Tectonic map of the region, highlighting the location of the recent M5.5 earthquake (yellow star). Source: Sawyer et al. (2013).

A number of earthquakes have occurred along this fault zone: seismicity aligns to match the mapped faults.

Figure 2: Recorded seismicity, with bulls-eye highlighting the May 11 M5.5. Earthquakes align along the major faults.

The earthquake was a normal mechanism event, representing NE-SW extension on a fault oriented NW-SE.

Figure 3: Focal mechanism of the May 11 M5.5 earthquake, representing normal faulting. Source: USGS, https://earthquake.usgs.gov/earthquakes/eventpage/nc73886731/moment-tensor

That orientation aligns well with the mapped fault zones and the mechanism matches the trans-tensional faults in the region. Other earthquakes in the area exhibit similar behavior.

Figure 4: Focal mechanisms of earthquakes in the region, illustrating mix of strike-slip and normal earthquakes along the eastern Sierra.

What seismicity has been observed in this region in the past? It turns out that the recent M5.5 occurred just northwest of a larger event in 2013 - the M5.7 Canyondam earthquake. That earthquake was a combination strike-slip/normal earthquake, on a fault of the same orientation as today’s. Between 2013 and 2023 there have been a number of minor quakes, but it has been about 8 years since there was an earthquake here approaching even M4.

Figure 5: Earthquakes around the May 11 M5.5, colored by time. Earthquakes are projected onto a timeline on the right and compiled into a time series below.

Too small, too remote to cause significant damage

A seismometer very close to the May 11 M5.5 recorded intensity VI shaking (strong). However, because the area is sparsely populated, most reported shaking is intensity III (weak), from locations at least 50 km from the epicenter.

Figure 6: Shaking intensity estimated for the May 11 M5.5 earthquake. Source: USGS, https://earthquake.usgs.gov/earthquakes/eventpage/nc73886731/shakemap/intensity

Although shaking does not initially appear heavily damaging, it is informative to consider the damage caused by the 2013 Canyondam earthquake. Although it was slightly larger, the acceleration at the nearby seismometer was similar. Although the damage was small by the standards of large quakes, it was not insignificant: several homes were damaged, gas pipes in five houses were ruptured, and dozens of chimneys cracked, broken, or collapsed. A full survey of the nearby communities will be needed to fully assess the damage from today’s event.

References:

Sawyer, T.L., Briggs, R.W., and Ramelli, A.R., 2013. Paleoseismic investigation of the Mohawk Valley fault zone, Sierra Nevada, northeastern California. Final Technical Report, USGS, https://earthquake.usgs.gov/cfusion/external_grants/reports/04HQGR0089.pdf