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 and are measured with seismometers. 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.
There are currently 13 seismometers installed around Lassen Volcanic Center. The network was installed in 1976 with several additional instruments added in each decade since. Volcanic earthquakes are common, although most are too small to be felt. Non-volcanic earthquakes along regional faults also occur— earthquake swarms in 1936, 1945-1947, and 1950 included several events above magnitude 4.0, with the two largest registering 5.0 and 5.5.