A volcanic tremor is a type of volcanic earthquake which is directly associated with the movement of magma preceding an eruption. They are low-frequency tremors of long duration, and are generally the best indicator that an eruption is imminent.
Tremors and quakes of many kinds surround volcanic eruptions. Some of these are the tectonic earthquakes that we are familiar with, when solid rock binds and snaps along a fault line. Some are seismic swarms, a group of tremors of approximately equal intensity, which may be caused by a number of different geological processes, including the expansion and inflation of underlying magma chambers. But of the various types of tremors to occur in volcanic areas, the volcanic tremor is the most indicative of an impending eruption. Even so, it is not particularly reliable, as in some cases they may not occur at all, or may occur only after the initial eruption.
Volcanic tremors are caused by sustained mass movements of high-pressure magma and volcanic gases moving trough the surface of the Earth. These tremors are low-frequency (~1-5 Hz), often have a harmonic tremor (showing a harmonic component), and last for longer than the transient shock of tectonic earthquakes, with a duration from minutes to multiple days. Volcanic tremors are continuous, but they are not always of consistent intensity; volcanic tremors with an pulsating or irregular intensity or frequency may be referred to as a spasmodic tremor.
Volcanic tremors may also be called long period earthquakes or simply volcanic earthquakes.