Mystery of small quakes solved

Cause of small quakes at Lake Mead may be snow runoff

LAS VEGAS | January 6, 2012

A series of small earthquakes at Lake Mead may have been caused by Colorado snow runoff that rapidly filled the lake last spring and summer, a geologist says.

Lee Allison, Arizona state geologist and director of the state's geological survey, said he began to notice the small quakes, ranging in magnitude from 1.5 to 2.2 on the Richter scale, in late April around the 112-mile-long reservoir behind Hoover Dam.

He recorded about two dozen jolts in the months that followed, he said.

"I saw more (seismic activity) there than in the past five years," Allison told the Las Vegas Sun, citing about one a week with three small quakes occurring on one day in July.

"At that point we were going, 'Ha, interesting,'" he said.

Researchers have previously documented a relationship between lake fluctuations and movements in the Earth's crust.

"When Lake Mead was filled in the 1930s, there were a number of magnitude-5 earthquakes, most likely directly associated with the filling of the lake," said Ken Smith, seismic network manager at the University of Nevada, Reno.

Still, Smith says he's not quite ready to buy into Allison's theory.

"It's not unusual to have magnitude-4 type events in that area," Smith said. "It's not surprising to see earthquakes anywhere in Nevada."

2012 United Press International, Inc. All Rights Reserved

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