After more than 700 earthquakes, Mammoth Lakes swarm slows

After more than 700 small earthquakes in four days, the Mammoth Lakes quake swarm slowed considerably Monday.

Only 12 quakes have been recorded so far on Monday. The intensity of the quakes also declined over the weekend. No damage has been reported from the shaking.

Earthquake swarms are not uncommon in this region in California’s Eastern Sierra. Countless small faults crisscross an area known as the Long Valley Caldera, a roughly 20-mile-wide crater-like depression adjacent to Mammoth Mountain that was formed from ash and pumice deposits during a volcanic “super eruption” about 760,000 years ago.

At 11,053 feet, Mammoth Mountain is a lava dome complex on the southwest rim of the caldera and last erupted about 57,000 years ago. The volcanic region is one of the most seismically active in a mostly quiet network of 17 volcanoes throughout California.

The central part of the caldera has been rising slowly in recent decades, and earthquake swarms occur periodically as part of the volcanic and tectonic interactions in the area. The earthquakes are usually triggered when water and carbon dioxide above the magma move up into higher layers of the Earth’s crust and into the cracks of the small faults. The increase in pressure sets off the movement. In 1980, it produced four quakes greater than 6.0 magnitude.