Garlock fault lights up in a Magnitude-3.5 seismic swarm in the high desert of southern California

Temblor Insight | Jan 13, 2016

A M=3.5 shock at a depth of 5 km struck on the active Garlock fault.

The mainshock has a focal mechanism consistent with the left-lateral motion of this 250-km (150-mi) long and major fault that bounds the Mojave desert to the south and the Tehachapi mountains to the north. The swarm occurs where the fault bisects into two branches with a damage zone of secondary faults between them. The lack of vegetation makes the fault crystal clear in satellite imagery.

Extensive work by Sally McGill (Cal State University San Bernardino) and Kerry Sieh (Earth Observatory of Singapore) reveals a Garlock fault slip rate of 4-7 mm/yr. Given its slip rate and length, the fault should be capable of a M~7.5 earthquake every few thousand years, and smaller quakes much more frequently. Paleoseismic trenching by McGill and others bears out a 1,000-3,000 recurrence interval for larger (M7) quakes.

But the Garlock fault is also a strange beast. It is highly misaligned to accommodate the shear stress that loads the entire San Andreas system, and the misalignment increases to the east. In fact, unless the fault were extremely slippery, it would be difficult to get it to rupture at all. Think of a water melon seed: If it is wet on a table, you can push your thumb straight down on it and it will fly sideways off the table; if dry, it won’t budge. In all likelihood, the Garlock fault has rotated counterclockwise 30-40° since it formed, but is long, straight and smooth enough to keep knocking off quakes despite being a misfit by comparison to California’s great faults.

Ross Stein and Volkan Sevilgen, Temblor

Data from USGS, Caltech/USGS Southern California Seismic Network, California Geological Survey, and McGill et al (2008), McGill and Sieh (1992, 1993)

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