Monthlong ‘Swarm’ of Quakes Under Volcano Drops Sharply
MENLO PARK, Calif. — The number of earthquakes beneath a California volcano sharply diminished last week, ending the latest “swarm” of jolts to rattle the Sierra ski resort town of Mammoth Lakes.
“The increased seismicity . . . that began over a month ago has pretty much come to an end, at least for the time being,” the U.S. Geological Survey said in a weekly report on quakes in Central and Northern California.
The Long Valley caldera, a 19-mile-long, 9-mile-wide volcanic crater, was shaken by as many as 60 small quakes each hour when the swarm started March 23.
By mid-April, there were about five quakes a day. Last week, the number of quakes averaged less than two a day.
The largest jolts in the swarm of almost 1,200 quakes measured only 3.6 in magnitude.
The resort town of Mammoth Lakes, about 250 miles north of Los Angeles, sits inside the Long Valley caldera. The giant crater was created 730,000 years ago by a massive eruption that spewed 600 times more material than the May 18, 1980, eruption of Mt. St. Helens in Washington state.
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