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 seismometers located near the Mono-Inyo Chain are part of the greater Long Valley Caldera seismic network array. Data from group of 61 seismometers help to determine earthquake location, energy, waveform and evolution of movement with time. The first instrument was installed in 1974 and additional instruments were added throughout the 1980's and 1990's. Between 2000 and 2003, the seismic network was updated to include modern instruments.