Dec 29, 2015 6:45pm
The most likely source of the earthquake is slip on the San Jacinto fault
The M=4.4 has been followed by 10 aftershocks in the first 30 min, the largest of which is magnitude 3.8. The mainshock was relatively shallow, at 4-8 km (2.5-5.0 mi) depth. The mechanism is consistent with right-lateral slip on the San Jacinto fault, which it straddles. A month ago, a preceding burst of smaller shocks struck only 5 km (3 mi) to the south, as discussed in our 17 November 2015 blog post.
After 159 years of quiet, the southern San Andreas is capable of a great earthquake
The quake struck where three major fault systems join: The San Andreas, San Jacinto—both right-lateral strike slip faults—and the Sierra Madre thrust fault that has uplifted the Sierra Madre Range west of the mainshock. This section of the Sierra Madre is called the Cucamonga fault. This portion of the San Andreas has not suffered a large shock since the 1857 M=7.9 Fort Tejon earthquake. In the intervening 159 years, the San Andreas has slipped about 10 feet at depths below about 15 km (10 mi), but the upper portion of the fault is frictionally locked. When stress overcomes that friction, an earthquake with about 10 feet of slip would not be unexpected, which could yield a magnitude between 7.4 and 8.0.
Earthquakes at fault junctions can sometimes be harbingers of larger quakes
Typical of fault junctions, the faults break up into many strands, and so the San Jacinto is very broad at this location. All three faults are capable of Magnitude-7 or larger earthquakes, and have relatively high slip rates. For all of these reasons, the Temblor Seismic Hazard Rank of this location is 83 out of 100.
Sometimes, but not always, large earthquakes nucleate at fault bends and junctions, as these points can probably build up greater stress than along straight and smooth fault sections. For this reason, the pattern of aftershocks and strain measured by GPS receivers will be closely monitored by the USGS and academic groups.
Ross Stein and Volkan Sevilgen, Temblor
Data from USGS, Caltech/USGS Southern California Seismic Network, and California Geological Survey.
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