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.
The USGS and UNAVCO seismic network contains 12 seismometers and provide real-time volcano monitoring data. The first instrument was installed in 1976, and several were added throughout each decade since that time. Earthquake activity has been low for the last few decades at Mount Shasta.