Observations of two special kinds of tremor at Galeras volcano, Colombia(1989-1991)

Since the reactivation of Galeras volcano in 1988 its seismic activity has been dominated by a variety of LP waveforms and tremor events. Some of these signals occurred as a response to volcanic activity. Among them, two kinds of tremor deserve special attention, Flute tremor and Spasmodic tremor. Flute tremor has a spectrum of equally spaced peaks and is associated with a quasi-steady degassing process at the top of the lava dome. It is accompanied by a flute-like sound. Its spectral features and the correlation with field observations are consistent with a model generation indicating that a crack or set of cracks are excited to resonance by the release and flow of gas through the lava dome. Spasmodic tremor is composed of several distinct LP-like events joined together by a continuous signal with lower amplitudes. Two types of spasmodic tremor may be distinguished on the basis of their spectral characteristics and field observations. Spasmodic tremor type I is apparently dominated by a mix of P, SH and Rayleigh waves as determined from preliminary polarization analysis. The source appears to be located, in a region west of the active crater. As a first approximation, Spasmodic tremor type I could be associated with magmatic intrusion process occurred in 1989-1991.