Earthquake swarm: Why 600 mini-quakes hit Mammoth Lakes, Calif.

Earthquake swarm: Hundreds of small earthquakes over two days near Mammoth Lakes do not appear to be a precursor to a big quake or a volcanic eruption.

A swarm of more than 600 small earthquakes have hit the Mammoth Lakes area in northern California.

Most of the quakes are too small to be even felt by people. It usually takes a magnitude 3.0 quake to be felt.

Since about 4 a.m on Thursday until 11 a.m. on Friday, the US Geological Survey (USGS) recorded more than 500 earthquakes of magnitude M1.0 and above, including 8 earthquakes between M3.0 and M3.8, which were felt locally.

Is an earthquake swarm an indication of a major quake coming? While the physics of earthquake swarms are not well understood, this appears to be a case of geological indigestion.

David Shelly, a seismologist and geophysicist with the USGS told the Mammoth Times that the quakes appear to come from the release of some carbon dioxide gas and water deep in the earth into existing cracks or faults in the ground under the Eastern Sierra.

“This fluid moves episodically into cracks or faults in the crust,” he said. “We think these quakes were triggered by this movement but driven by existing tectonics.”

The USGS California Volcanic Observatory states that these quakes do not seem to be the result of magma movement below the surface, so there's no concern that the swarm is a precursor to a volcanic eruption.

The quakes are occurring beneath the Long Valley Caldera, about a 20-mile wide depression in the earth next to Mammoth Mountain. The USGS reports that this is the latest of several earthquake swarms this year under the caldera, which is slowly rising.

 Despite the several felt earthquakes, this is still rather modest activity compared with the much more energetic swarms occurring in the 1980s and 1990s. We do not see any evidence for anomalous ground deformation associated with the swarm at this time. Part of the Long Valley Caldera, known as the "resurgent dome," has been uplifting at a rate of about an inch per year since late 2011, and this remains unchanged. Caldera uplift has occurred sporadically for the last few decades. The uplift rate observed since 2011 is small compared to rates observed in the 1980s and 1990s.

Two years ago, an earthquake swarm in Southern California, in Imperial County, a rural area east of San Diego, made headlines.

"Compared to the rest of California, it's thinner crust, and hotter, and [has] more fluids," Lucy Jones, a seismologist at the US Geological Survey in Pasadena, Calif., told The Christian Science Monitor at the time. She described the imperial Valley as "one big bowl of sediment."

"Those would likely be reasons that contribute to this," Ms. Jones said. "But we don't yet have the details of the physics."

The earth's thin crust in the region, and the proximity of hot magma beneath it, relates to the Imperial Valley's location at the top end of a fault in which tectonic plates are moving apart from one another to form the Gulf of California. The Brawley area, 100 miles east of San Diego, is "like the very end of a ridge that's trying to spread open. It's actually probably giving up on spreading open," Jones said.

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