Wednesday, February 24, 2016

Another Moderate Earthquake in Central California: 4.9 the Great Valley???

The Wasco 4.9 earthquake, as recorded at Modesto Junior College

California has been shaken by another near-magnitude 5 earthquake, and this one is a bit odd. It hit in the Great Valley, one of the few places in California NOT known for having faults and earthquakes. It occurred near the south valley town of Wasco, and had a magnitude of 4.9, along with a couple of aftershocks as high as magnitude 2.6. The first motion solution suggests right lateral strike-slip motion in the same orientation as the San Andreas fault, which lies to the west in Coast Ranges.

Not the kind of place one expects to see fault zones...
The Great Valley indeed has few active faults, but the Bakersfield region is a definite exception. The south end of the valley is a structural nexus in California where five geologic provinces come together. The region is riddled with structures related to movement along faults like the San Andreas, the Garlock, and the White Wolf, which generated a magnitude 7.6 magnitude earthquake in 1952 that killed a dozen people. A few faults are mapped close to Wasco, including the Pond-Poso Creek fault (see the California State interactive fault map at
I have to admit that my first thought upon seeing quake reports was that it resulted from waste disposal related to fracking . The more I've looked though, the less I am concerned about this possible cause. The quake was deep, around ten miles, and has motions consistent with a tectonic origin. There are no reports of damage that I have seen.

The U.S. Geological Survey event page for the earthquake can be seen at
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