Central California Rocked By Swarm Of More Than 600 Earthquakes In 24 Hours

Chuck Bednar for redOrbit.com – Your Universe Online
In what is being called one of the worst spells of seismic activity in a decade, residents of the Mammoth Lakes region of central California experienced more than 600 earthquakes in the span of 24 hours late last week.
According to Willis Robinson of the Daily Mail, the so-called “earthquake swarm” started just before 5am on Thursday and the earthquakes themselves ranged in magnitude from 1.0 to 3.8. At least 109 of them were magnitude 2.0 or greater, and at least six were greater than magnitude 3.0, he added.

David Shelly, a seismologist at the US Geological Survey’s Volcano Science Center, told the Los Angeles Times that the activity originated in the Long Valley caldera east of the central Sierra Nevada Range, and that the strongest was a 3.8 magnitude earthquake which occurred six miles from Mammoth Lakes at 9:21pm on Thursday.
“This is one of the largest earthquake swarms we’ve seen in the past decade or so,” he said, adding that the USGS was closely monitoring the region, but did not believe it was linked to any volcanic activity in the area.
Shelly also told the Associated Press (AP) that the earthquake could have been caused by water pressure from nearby hot springs, which would have shifted through the ground surface causing the tectonic plates to become stressed.
Robinson said the region in which the earthquakes occurred is one of the most seismically active in the country, and is also home to 17 volcanoes, most of which are dormant. There are also a vast number of fault lines in the Long Valley Caldera, a crater formed 760,000 years ago during a “super-eruption” that covered much of what is now the Western US in a blanket of hot ash.

Shelly told Los Angeles Times writer Veronica Rocha that this recent span of activity was not nearly equal in size or scale to some of the activity measured in the 1980s and 1990s. As Rocha explained, the region was hit with a swarm of multiple 6.0-magnitude temblors in the 1980s, and in 1998, the region experienced a series of primarily 4.9-magnitude earthquakes that took place over the span of several months.
Shelly confirmed that the central part of the Long Valley Caldera has been slowly uplifting over the past few decades, and earthquake swarms such as the one that happened last week occasionally occur due to the volcanic and tectonic interactions in the area. While there is magma deep in the Earth, it is not what’s moving.
“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,” Rocha said. Shelly emphasized that this does not indicate the volcano is becoming more active, only that this is an “ongoing process in the volcanic system.”
In addition, Lori Sandoval of Tech Times reports that the USGS described the earthquakes as “small, brittle-failure (rock breaking) events,” and emphasized that they would not result in magma movement. The agency said they are able to “distinguish between brittle-failure earthquakes and those resulting from magma movement by the characteristics of the seismic waveforms,” and added that the earthquake swarm posed no immediate threat.
“At the beginning of this year, residents commemorated the 20th anniversary of the North Ridge earthquake,” Robinson noted. “It centered in Reseda, a neighborhood in the north-central San Fernando Valley region of Los Angeles, killing 57 people and injuring more than 5,000.”
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