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


John Bellar said...

I live about 40 miles NE of the epicenter in a Town called Porterville, the area that was struck is heavily populated with oil pumps though to my knowledge very little if any fracking goes on there. A fault line seems more plausible. We felt a strong initial jolt followed by a 1 to 2 second pause then about 6 seconds of rattling and shaking. No damage but definitely got our attention.
Thanks for the report on this.

Frank Moore said...

The area in and around the epicenter IS NOT heavily populated with oil pumps. The epicenter of the quake occurred in an area that is exclusively agricultural. The epicenter is in farm field with no nearby oil production facilities. The Lost Hills and Belridge oilfields are a good 20 miles or more to the West of the epicenter of the quake. While "fracking" per se is not necessarily common in the Lost Hills area, other enhanced recovery methods are.

If you take the USGS event map of the quake, enlarge it, and switch to satellite view you can see the land uses in the area.

John Bellar said...

I stand corrected, perhaps it's further south that I was thinking of. Though you are correct regarding the agricultural diversity of the area Google maps still show pumping operations in the area. The point I was attempting to make in my comment was an agreement with the author that a fault and not fracking operations such as those in Oklahoma seemed more reasonable as the cause. Several sources on social media are trying to say its related to fracking simply because of the oil production process in the southern end of Valley.