Parameterization of multichromatic tornillo signals observed at Galeras Volcano (Colombia)

In the past decade several of the ash eruptions at Galeras Volcano, Colombia, have been preceded by tornillos. These unusual tremor wavelets have quasi-sinusoidal waveforms with screw-like envelope profiles and can last up to several minutes. A swarm of tornillos occurred at Galeras Volcano between 8 December 1999 and 12 February 2000. These tornillos appear to be more complex than those previously recorded with the broadband instruments or with the short-period network of the Observatorio Vulcanolَgico y Sismolَgico in Pasto. They are multichromatic with a varying number of narrow spectral peaks between 1 and 20 Hz. We describe a procedure for parameterizing the tornillo signals in the time and frequency domains to determine signal parameters. In addition to wavelets like the tornillos, the procedure can be applied to random signals such as volcanic tremor. We derive distribution and correlation functions for the signal parameters determined from the swarm. These provide, along with the signal signature, constraints for modelling variations of the source process. From these observations we derive qualitative conclusions about the characteristics of differential equations which describe the underlying processes and excitation mechanisms as forced or self-excited oscillators.