© n/a
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.
