12 - Volcanic Tremor

Abstract

Volcanic tremor is the very popular seismic signal recorded at volcanoes. The general characteristics of volcanic tremor and its relationship with volcanic processes are discussed. Volcanic tremor represents a continuous record of monotonic (harmonic) or non-harmonic vibrations that may continue for minutes to months. It is seen that the short-period spectrum for monotonic tremor is characterized by peaks within a narrow frequency range at about 2.5 Hz and its long-period spectrum is characterized by the absence of any predominant peaks. For nonmonotonic tremor, the opposite situation is observed: no unique peaks for the short-period spectrum but a narrow frequency range of peaks at about 0.2 Hz for the long-period spectrum. Volcanic tremor may be represented by surface waves or body waves. The sources of volcanic tremor are located at shallow depths beneath the crater floor. Volcanic tremor reflects many important variations in eruptive activity. A particular kind of volcanic tremor characterized by regular cyclic increases of amplitude was observed in several volcanic and hydrothermal areas. Because of its peculiar seismic signature, forming evident stripes on seismograms, this kind of seismic signal is called a “banded” tremor. A number of studies associate the deep tremor as a manifestation of the process on the transition zone between the seismogenic coupled and deep free-slipping segments of the subduction interface. Therefore, it is possible that the deep tremor beneath Hawaii is also of the same nature but not of volcanic origin.