More than 700 earthquakes swarm Mammoth Lakes: Map tells the story

Mammoth Lakes continued to rumble Sunday with small earthquakes, but the swarm that has produced more than 700 temblors in the last four days appears to be bottoming out.

The number and intensity of the quakes declined over the weekend. No damage has been reported from the shaking.

“This is one of the largest earthquake swarms we’ve seen in the past decade or so,” David Shelly, a USGS research seismologist who has been studying the volcanic system near Mammoth Lakes told The Times on Friday. “We’ll be tracking it closely.”

So what do hundreds of quakes over a small area look like? Here are a map and a chart:

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, 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 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.