A swarm of more than 700 earthquakes have struck near the small Sierra County, Calif., community of Sierraville since August, although there's a good chance few if any of the quakes have been felt.
There have been more than 30 quakes of a magnitude 1 or more in the last week, the largest being about 1.8, said Ken Smith, seismic network manager for the Nevada Seismological Lab.
But the quakes are so deep - most are 18 miles below the surface or more - and so minor Smith thinks they have little chance of being felt at the surface. They are centered about 2 miles west of Sierraville and 31 miles west of Reno.
Seismologists can't say with certainty yet what is happening, but appears the quakes are being caused by moving magma. The earth is basically divided into layers of the crust, the mantle and the core with the temperature getting hotter the deeper the depth. Smith said the ground in this area is constantly in motion, moving about 14 millimeters a year. Because of that motion, it appears magma found a way to flow from the mantle, the middle area, to the crust, the upper area.
"The upper mantle in this region has a lot of magma in it," Smith said. "Sometimes it finds a way to work its way into the lower crust or the crustal-mantle plate. ... As everything is moving around, they have an opportunity to inject magma into places where it can."
These quakes appear similar to the swarm of quakes that struck under Lake Tahoe in 2003, which were later determined likely to be deep magma injection, Smith said.
This magma - called lava as soon as it reaches the earth's surface - is not associated with the Lassen Volcano to the north, he said. While it's not going to end up as a volcanic eruption, it could deform the earth's crust and set the stage for an earthquake - but not any time soon.
The first quake in this swarm happened Aug. 9. The lab put extra equipment out there to monitor it in the middle of September. The number of quakes increased significantly in the past two weeks, Smith said.
"The observations we have now are the result of better observation and better monitoring of these processes," Smith said. Seismologists may not have been able to detect this swarm as recently as 15 years to 20 years ago, but the equipment is better now, he said.