Harmonic tremor

Harmonic tremor describes a continuous rhythmic earthquake in the Earth's upper lithosphere that can be detected by a seismograph and often precedes or accompanies volcanic eruptions.

A harmonic tremor is a continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.

After twenty years of seismic observations on Mt. Erebus with only rare incidences of sustained tremor-like signals being recorded, tremor activity became conspicuous around June 2000. More than 310 tremor episodes have been recorded through 2004, with three main periods of activity (February 2001, May–June 2002, and January–June 2003). These tremor episodes show a variety of waveforms, from chaotic to highly harmonic, showing up to 28 harmonics of the fundamental frequency. Episodes showing gliding (proportional shifting of all spectral peaks) are commonly seen. Rapid-fire tremors, a sequence of many equally spaced short tremor events, have also been recorded. This increase of tremor activity has not accompanied by increased activity at the crater of Erebus, where an open lava lake and persistent low-level strombolian activity have long been observed. The onset of tremor has corresponded with the calving of several megaicebergs from the Ross Ice Shelf. The association of these regional tremor signals with megaiceberg collisions and other activity is currently a topic of interdisciplinary research between Erebus researchers and several other groups.