USGS: Swarm of small earthquakes shake area near Mammoth, no danger at this time

A swarm of small earthquakes, including a 3.9 temblor and a 3.8 shaker, shook an area six miles east of Mammoth Thursday, Sept. 25, and continued into Friday at lesser magnitudes and frequency, according to authorities.

The quakes were triggered by gas and water moving deep within the earth’s crust and are not considered to be a sign of danger at this time, according to the U.S. Geological Survey.

The strongest quake occurred Thursday evening at about 8:30 p.m. and registered at 3.9 on the Richter scale.

This ”swarm” of several hundred quakes (see below) were all located in the Long Valley area and centered about one mile north of the Mammoth Yosemite Airport and six miles east of Mammoth. They were measured at a depth of five kilometers, officals with the USGS said.

David Shelly, a seismologist and geophysicist with the USGS said 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 swarm of quakes began in the morning of Sept. 25 and were reported by some people in the Town of Mammoth Lakes, Crowley Lake, and other communities later in the day and evening, when the stronger quakes struck.

According to Shelly, as of Friday, Sept. 26 at 12:30 p.m., there had been 100 quakes at 2.0 or greater on the Richter scale, 500 quakes that were at 1.0 or greater and eight quakes that were at 3.0 or greater on the scale since the earthquake swarm began Thursday morning.

“Things are quieting down today,” Shelly said Friday, noting the quakes are still going on, but are mostly not strong enough to be felt by most people.

However, it is impossible to know if the quakes will continue to die down for good.

“It’s possible it could ramp up again,” he said.

The biggest quake, a 3.9 quake, occurred at 8:32 p.m., the USGS reported. A 3.8 quake, also strong enough for people to feel, occurred at 9:21 p.m.

One of the 3.2 quakes occurred at 7:54 p.m.

As a general rule, according to the USGS, most earthquakes measuring above 3.0 are strong enough for people to feel.

The USGS monitors the Eastern Sierra’s Long Valley Caldera 24/7 and any earthquake above a 3.0 on the Richter scale trigger a beeper to go off in their Menlo Park office, or wherever the scientist on call might be, he said.

A series of more than 20 quakes within an hour, regardless of the magnitude of the quakes, also triggers an alert, he said.

Any quake above 4.0 on the Richter scale triggers the USGS to contact local authorities through the “Unified Command” emergency response system, he said.

The Mammoth Times will provide updates as events occur.