Magma moving deep below Lake Tahoe apparently triggered an unprecedented swarm of 1,600 tiny earthquakes during a seven-month period but they stopped in February and there's no cause for alarm, experts said Thursday.
The migration of the molten rock material 20 miles beneath the surface of the Sierra Nevada also likely caused the mountain beneath the Mt. Rose Ski resort southwest of Reno to rise 8 millimeters, or about 3/8 of an inch, researchers said.
"We've been watching earthquakes for 30 years in the Tahoe area and have never witnessed an earthquake swarm anything like this," said Ken Smith, a research seismologist for the Nevada Seismological Laboratory at the University of Nevada, Reno.
"The magma itself is the force breaking the rock and that's what we see the earthquakes associated with," he told The Associated Press on Thursday.
"This is going to help us better understand how earthquakes develop in the first place," he said.
The deep earthquakes beneath the lake about 25 miles southwest of Reno forced several miles of rock to spread apart by about 1 meter but were of no greater magnitude than 2.2. They occurred from Aug. 12, 2003 to Feb. 19, 2004, then stopped.
Since then, no deep quakes have been recorded and there has been no notable movement of Slide Mountain at the Global Positioning System station atop the mountain, home of the Mt. Rose Ski Resort on the main highway from Reno to Lake Tahoe, Smith said.
Smith is the lead author of an article on the findings that will be published in the August edition of the journal, Science. The article, "Evidence for Deep Magma Injection Beneath Lake Tahoe, Nevada-California," was posted on the journal's Science Express Web site on Thursday.
Co-authors include David von Seggern, seismic network manager of the Nevada Seismological Laboratory, and Geoffery Blewitt, a research geophysicist at the Nevada Bureau of Mines and Geology who has written GPS computer software and won several awards from NASA. Others are based at the California Institute of Technology and Harvard Smithsonian Center for Astrophysics.
The researchers emphasized the findings speak more to emerging technology that enables scientists to detect such minute changes in the earth than any increased threat of earthquakes along the Sierra's eastern front.
"We don't see any concern from this. It's so deep, about 20 miles beneath the surface," Smith said. That's about twice as deep as the deepest earthquakes that have been recorded previously in the area since measurements began 30 years ago, he said.
The study marks the first time that techniques used to measure seismology have been teamed up with satellite technology to pinpoint such activity in the Sierra. A similar effort was carried out in recent years in the northern Cascade range in the Pacific Northwest, but the practice remains rare, the researchers said.
"This event was startling to us. It was unprecedented here to bring them together and we hope it spurs similar work in other places," von Seggern said.
Blewitt said the findings should provide a boost to a new federal program aimed at installing hundreds of GPS stations and seismic recording stations across the United States.
"We are just now at the point we can detect signals this small and be confident about it," Blewitt said.
"We sort of demonstrated that combining these two types of data does work," he said.
The northern Sierra's volcanic history dates back about 30 million years, but that period ended about 3 million years ago, Smith said. Since then, the uplifting of tectonic plates has dominated the region's geology, so much so that the Sierra range is moving to the northwest at a rate of 12 to 14 millimeters a year, he said.
"The oldest volcanoes in this area are 1 to 2 million years old. Right now, the Sierras are essentially popping up like a cork," Smith said.
Michael Reichle, acting California state geologist, said that as a result of the new information state experts will be "keeping a close eye on future seismic activity in the Tahoe area."
"This is a very interesting scientific discovery, but there's no cause for the public to be alarmed," he said.
The most recent instances of magma reaching the surface in the Lake Tahoe area occurred about 1 million years ago.
"The chances of us seeing a volcanic eruption in the Tahoe region in our lifetime are practically nil," said Darryl Young, director of the California Department of Conservation.
On the Net:
Nevada Seismological Laboratory: www.seismo.unr.edu/
Nevada Bureau of Mines and Geology: www.nbmg.unr.edu
Science journal's Science Express: www.sciencexpress.org