Listening carefully: unique observations of harmonic tremor at Lascar volcano, Chile

Abstract

During the deployment of Proyecto de Investigación Sismológica de la Cordillera Occidental 94 (PISCO'94) in the Atacama Desert of Northern Chile, a broadband seismic station and a network of three short-period three-component stations were installed around the active volcano Lascar. The resulting data set includes a sequence of harmonic tremor with a fundamental at about 0.63 Hz and up to 30 overtones lasting 18 h. Power spectra and spectrograms of Lascar's harmonic tremor from the various stations demonstrate that the frequencies recorded cannot be explained as path effects, and must therefore be attributed to mechanisms at or near the source. The polarization of the wavefield cannot simply be explained as the propagation of any of the classical types of seismic waves, thus we apply new methods to the data to investigate the narrowband signals of the harmonic peaks. While the amplitude characteristics of these signals cannot be correlated across the network, frequency characteristics of the harmonic wavefield are consistent across stations and components. The tremor's fundamental frequency changes at the same time at all stations, indicating that such changes must be caused at the source. In addition, a change in the frequency of the fundamental, f1, is reflected exactly in the frequencies of the overtones, nf1 and peak-broadening in the power spectrum is the result of shifts in the fundamental frequency. It is therefore unlikely that the overtones are produced as resonances. This spectral behavior indicates rather that the source is some resonance at a single frequency within the magma, magma/gas or gas parts of the volcano whose amplitude exceeds the range for which the assumptions of linear acoustics are valid.