California earthquake swarm continues, but slows

The earthquake swarm in Mammoth Lakes continued into Saturday, but it showed signs of slowing.

More than 700 small quakes have been recorded in the Mammoth area since Thursday, but the last one of 3.0 magnitude or greater occurred Friday afternoon. And the frequency of the quakes has declined significantly.

Experts at the U.S. Geological Survey said the quake swarm is one of the largest in the past decade.

Earthquake swarms are not uncommon to this region in California’s Eastern Sierra. Countless small faults crisscross the area known as the Long Valley Caldera, Shelly said. This roughly 20-mile-wide crater-like depression, adjacent to Mammoth Mountain, 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 uplifting slowly in recent decades, and earthquake swarms occur episodically 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 in higher layers of the Earth’s crust and into the cracks of the small faults. The increase in fluid pressure sets off the movements.