Seismic monitoring at Clear Lake Volcanic Field
As magma moves through the earth, it displaces and fractures rock along the way. This movement causes earthquakes that can be recorded with seismometers at the surface of the earth. As of 2008, seismic monitoring is the most used technique for volcano surveillance.
Volcanic earthquakes often provide the initial sign of volcanic unrest. Their signals differ from typical, tectonic, earthquakes because they tend to be found at depths shallower than 10 km, are small in magnitude (< 3), occur in swarms, and are restricted to the area beneath a volcano. Harmonic tremor, or volcanic tremor, is the name for the continuous, rhythmic seismic energy associated with underground magma movement.
Although CLVF has not erupted for several millennia, sporadic volcanic-type earthquakes do occur in addition to the regular earthquakes associated with The Geysers geothermal field. The first of the 25 seismometers that makes up the network array was installed in 1975. Many new instruments were added between 2003 and 2005. Monitoring by the USGS and Calpine Corporation provides real-time tracking of earthquake activity.