Volcano Seismology - John Seach

Seismology is the study of earthquakes.
Earthquakes are classified by their appearance on a seismograph.

Earthquake Type Comments
High-Frequency Dominant frequencies from 5-15 Hz. Caused by shear failure or slip on faults. Volcanic earthquakes = Rock breaking or explosion events.
Low-Frequency Dominant frequencies between 1-5 Hz. Caused by fluid pressurisation processes.
Volcanic earthquakes = movements of fluids, steam or gas (rarely liquid magma) imparting a slower shaking or rocking to the ground).
Hybrid Contain both high and low frequency signals. Represent a mixture of processes.
Explosion Caused by explosive eruptions. Signal contains an air-shock phase as well as an ground wave.
Tremor Tremor is a continuous signal with a duration of minutes to days. Dominant frequencies are 1-5 Hz.
a) Harmonic Tremor has a smooth amplitude.
b) Spasmodic Tremor has irregular signals.
Very-Long Period Periods of 3 to 20 seconds. Associated with eruptions or vigorous fumarolic activity.
Superficial Non-volcanic processes can produce seismic signals.
Examples: glacial movement, shore ice, landslides, rockfalls, and floods.