Seismic signals from Lascar Volcano

Lascar, the most active volcano in northern Chile, lies near the center of the region studied during the Proyecto de Investigación Sismológica de la Cordillera Occidental 94 (PISCO ’94). Its largest historical eruption occurred on 19 April 1993. By the time of the PISCO ’94 deployment, its activity consisted mainly of a plume of water vapor and SO2. In April and May 1994, three short-period, three-component seismometers were placed on the flanks of the volcano, augmenting the broadband seismometer located on the NW flank of the volcano during the entire deployment. In addition to the usual seismic signals recorded at volcanoes, Lascar produced two unique tremor types: Rapid-fire tremor and harmonic tremor. Rapid-fire tremor appears to be a sequence of very similar, but independent, “impulsive” events with a large range of amplitudes. Harmonic tremor, on the other hand, is a continuous, cyclic signal lasting several hours. It is characterized by a spectrum with peaks at a fundamental frequency and its integer multiples. Both types of tremor seem to be generated by movement of fluids in the volcano, most probably water, steam or gas.