Mammoth earthquake swarm is the largest in nearly a decade

More than 600 small earthquakes have rattled the Mammoth Lakes region in less than 36 hours as ripple effects continued across one of the most seismically active volcanic regions in California, according to the U.S. Geological Survey.

The swarm of quakes — ranging from magnitude 1.0 to 3.8 — began just before 5 a.m. Thursday, according to the USGS.

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

Residents reported periodic rattles through the day but said they were used to the shaking given that Mammoth is a seismically active area.

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 these earthquake swarms happen episodically as part of the volcanic and tectonic interactions in the area, Shelly said.

Deep down in the earth, there is magma, but the magma is not what’s moving, Shelly said. 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 fluid pressure sets off the movements.

“It doesn’t mean that the volcano is any more active,” he said. “It’s an ongoing process in an volcanic system.”

The latest earthquakes seem to be occurring in the same location as a swarm in July, when about 200 quakes of magnitude 2.7 or smaller rocked the area.

The size of the most recent swarm was notable, but was not nearly the size of some swarms in the 1980s and 1990s.

In the 1980s, the area was hit with multiple 6.0-magnitude temblors, which were likely overshadowed by the Mount St. Helens eruption in Washington state, Shelly said.

The last larger swarm occurred in 1997, when temblors as high as magnitude 4.9 shook the region. Thousands of earthquakes were part of that sequence, which lasted several months, Shelly said.

There has been no indication that this week’s earthquakes will turn into anything like what happened in 1997. About 109 quakes of magnitude 2.0 or greater have been recorded since Thursday morning, while hundreds of smaller 1.0-magnitude quakes made up a bulk of the activity. At least six were greater than 3.0 magnitude.

By midday Friday, the swarm seemed to be slowing down, but Shelly said scientists would continue to monitor the area closely.

“At this point, we don’t know if it would continue to die down, or if there’d be another stage to this swarm,” Shelly said. “This is certainly an interesting scientific opportunity to better understand the processes that are driving this activity.”

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