RENO, Nev., Aug. 11 (UPI) -- U.S. researchers recorded nearly 1,600 small earthquakes 20 miles beneath Lake Tahoe, which coincided with the uplifting of a mountain in the Sierra Nevada.
University of Nevada researchers -- based on observations from the school's Nevada Seismic Network and the global positioning system -- found Slide Mountain uplifted nearly 8 millimeters and caused earthquake activity from Aug. 12, 2003, through Feb. 19, 2004.
"We've been watching earthquakes for 30 years in the Tahoe area and have never witnessed an earthquake 'swarm' anything like this," said study author Ken Smith, a research seismologist at the university's Nevada Seismological Laboratory.
The series of quakes never rose above magnitude 2.2, the researchers said, and were caused when the movement of magma beneath the ground forced several miles of rock apart by about 1 meter and resulting in Slide Mountain uplifting.
The eastern side of the Sierra Nevada is the third most seismically active area in the western United States, after Alaska and the San Andreas fault in California.