A warning bell? Tornillo events at Galeras Volcano, Colombia

In 1993, five of the six ash eruptions at Galeras Volcano, Colombia were preceded by distinctive seismic events, called tornillos. These unusual tremor wavelets have quasi-sinusoidal waveforms with screw-like envelope profiles and can last up to several minutes. Since December 1990, more than 60 of these events have been recorded at Galeras Volcano. As a class, they appear to be more complex than those previously recorded with the broadband instruments or with the shortperiod network of the Observatorio Vulcanológico y Sismológico in Pasto. They are multichromatic with a varying number of spectral peaks between 1 and up to 50 Hz. The peaks for frequencies which extend into the coda are extremely narrow, while those present only during the initial excitation are relatively broad. We parameterize the tornillo signals in the time and frequency domains, examining differences in the two classes of spectral peaks, and derive distribution and correlation functions for signal parameters such as frequency, Q, energy, and polarization. We investigate variations in these parameters with regard to ash eruptions during the past 3 years. In addition, we use the parameters along with the signal signature to derive qualitative conclusions about possible underlying processes and excitation mechanisms and to provide constraints for modelling variations of the source process, as, for example, a cavity resonator. For example, the distribution of frequencies for the suite of tornillos appears to be discrete below 6 Hz and a continuum above, as would be expected for a cavity.