Harmonic tremor describes a long-duration release of seismic energy, with distinct spectral (harmonic) 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.
A harmonic tremor is a sustained release of seismic and/or infrasonic energy typically associated with the underground movement of magma and/or venting of volcanic gases from magma. 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 earthquake and explosion.
For more info, see the work of Bernard Chouet, a USGS volcanologist who was working at the United States Geological Survey and who first observed a relation between long-period events and an imminent eruption.
- Chouet, Bernard (1996). "Long-period volcano seismicity: its sources and use in eruption forecasting". Nature 380 (6572): 309–316. Bibcode:1996Natur.380..309C. doi:10.1038/380309a0.
- Interview with Bernard Chouet regarding his research into long-period events and volcanic eruptions: http://www.esi-topics.com/volcanoes/interviews/BernardChouet.html .
- U.S. TV program on use of long-period events to predict volcanic eruptions: "Nova: Volcano's Deadly Warning": http://www.pbs.org/wgbh/nova/volcano/ . See also "Volcano Hell" episode of BBC TV series "Horizon" on same subject: http://www.bbc.co.uk/science/horizon/2001/volcanohell.shtml .
- Seismicity, low frequency events and tremor at the Katla subglacial volcano, Iceland
- The harmonic tremors of a volcanic eruption
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