Southern California: Magnitude 4.3 earthquake hit Eastern California Shear Zone

The Mojave Desert was hit by a magnitude 4.3 earthquake at 10:31 this morning (27 December 2015). Even though the epicenter was located in the sparsely populated Southern Californian high desert, the USGS PAGER system estimates that 700,000 people should have felt the earthquake. The communities of Barstow, Ridgecrest, Boron, Mojave, Lancaster, Palmdale, Edwards Air Force Base, NASA Dryden Flight Research Center and Ft. Irwin, were likely experienced shaking.

Today’s quake struck on a small, heretofore unknown fault that straddles active faults

Based on its USGS focal mechanism, the earthquake most likely resulted from sudden right-lateral slip (meaning that whatever side of the fault you are on, the other side moves to the right) on a minor, unnamed near-vertical fault striking (or oriented) northwest-southeast, parallel to the Harper fault zone (locally called the Gravel Hills fault) 7 miles to the southwest, and the Blackwater Fault zone 5 miles to the northeast.

ECSZ map with Reno and Barstow shocks
This event struck within what is known as the Eastern California Shear Zone (ECSZ in the map from Lee et al., Geol. Soc. Bull., 2007 above), which cuts through the Mojave Desert, continues along the east flank of the Sierra Nevada range, and extends to the eastern Tahoe basin and into westernmost Nevada, where it is called the Walker Lane (WLB above). While most of the Pacific-North America ‘transform’ or right-lateral plate motion is accommodated by the San Andreas system along coastal California, about one-quarter is splayed onto the Eastern California Shear Zone-Walker Lane, which also accommodates some Basin and Range stretching. Today’s shock struck the southern part of the Shear Zone, whereas the 23 December 2015 M=4.4 Reno shock struck near its northern end. These two moderate shocks are almost certainly unrelated, but they are evidence of the active nature of this boundary, which ruptured in the 1872 M=7.6 Owens Valley, CA, earthquake, the third largest historical California shock.

Have we underestimated the seismic hazard at the site of today’s quake?

Given that at a M=7.3 quake struck to the south of today’s M=4.3 in 1992, and a M=7.6 struck to the north in 1872, why is the Temblor hazard rank for this location only 16 out of 100? The answer is that while the Eastern California Shear Zone is laced with active faults, their slip rates are very low and uncertain, and the shear across the region measured by GPS receivers is also moderate, and so the expected quake rate and therefore frequency of shaking is much lower than along the San Andreas system. The seismic hazard rank is based on the annual probability of strong (0.4 times the acceleration of gravity) shaking, based exclusively on the USGS 2014 hazard model; it could underestimate the hazard here if the local faults slip faster than assumed.

The M=4.3 quake may have been brought closer to failure by the 1992 Landers shock

King et al (Bull. Seismol. Soc. Amer., 1994) argued that more than three-quarters of Landers aftershocks struck in regions brought closer to Coulomb failure by the M=7.3 shock. This assumes that failure was promoted on faults in which the shear stress increased, or were unclamped, by the M=7.3 quake. In their figure below, the warm colors between the white lines represent the Coulomb increases; the cool colors are where the Coulomb stress decreased, and so where the rate of shocks should have—and was largely observed to—decrease. One can see that the M=4.3 shock, along with the 1992 Barstow streak, lie in the Coulomb increase zone.


Swarm-like nature of today’s shock and nearby aftershocks of the M=7.3 Landers quake

The 27 December 2015 M=4.3 mainshock was preceded over the past 1-2 days by several smaller shocks in almost the same location, which is most characteristic of seismic swarms. The event also lies about 20 miles northwest of an enigmatic streak or cluster of isolated aftershocks of the 1992 M=7.3 Landers earthquake (see the events above the word ‘Barstow’ in the map below, from Hauksson et al. (1993). Based on GPS displacement data from the 1992 quake, these aftershocks appear to accompany perhaps 20 cm (8”) of creep on an unnamed fault that was triggered by the mainshock. The 1992 streak also had a swarm-like character, with a sustained high rate of small shocks.
Ross Stein & Volkan Sevilgen, Temblor, Inc.

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Data from Caltech/USGS Southern California Seismic Network, California Geological Survey, Hauksson et al (1993), King et al (1994), and Lee et al (2007).