What is "harmonic tremor"

n. (context geology English) A rhythmic shaking of the ground detectable by a seismograph, and associated with volcanic activity. Often a symptomatic warning of an eruption.

thumb|right300px|Four major types of seismograms, or seismic signatures. A harmonic tremor is a sustained release of seismic and infrasonic energy typically associated with the underground movement of magma, the venting of volcanic gases from magma, or both. It is a long-duration release of seismic energy, with distinct spectral lines, that often precedes or accompanies a volcanic eruption. More generally, a volcanic tremor is a sustained signal that may or may not possess these harmonic spectral features. Being a long-duration continuous signal from a temporally extended source, a volcanic tremor contrasts distinctly with transient sources of seismic radiation, such as tremors that are typically associated with earthquakes and explosions.

The relation between long-period events and an imminent eruption was first observed by Bernard Chouet, a volcanologist who was working at the United States Geological Survey.