Harmonic tremor is defined as seismicity consisting of a continuous low frequency, often single frequency, sine wave with smoothly varying amplitude, and is caused by complex interactions between magma (see Magma), exsolved gases, and bedrock at volcanoes (McNutt, 2000).
Harmonic tremor comprises a sequence of seismic waves whose dominant frequencies typically range from 1 to 5 Hz. Harmonic tremor and spasmodic bursts (also called spasmodic tremor) are specific variants of more general volcanic tremor. However, harmonic bursts have a relatively uniform signal, whereas spasmodic bursts have a higher frequency, pulsating, irregular signal.
Volcanic tremor, including harmonic tremor, may persist for minutes to days or longer, may lack a distinct beginning or end, frequently precedes and almost always accompanies eruptions, and was the most consistent short-term indicator of impending eruptions in a study of 200 calderas worldwide (McNutt, 2000) (see also Volcanoes and...
- McNutt, S. R., 2000. Seismic monitoring. In Sigurdsson, H. (ed.),Encyclopedia of Volcanoes. San Diego: Academic Press, pp. 1095–1119.Google Scholar