Physical models for the source of Lascarʹs harmonic tremor

Over an 18 h interval in April 1994, the tremor at Lascar volcano, Chile, was characterized by a spectrum with narrow peaks at a fundamental freqency of about 0.63 Hz and more than 25 overtones at exact integer multiples. This harmonic tremor was recorded at four three-component, high-dynamic range stations during the deployment of the Proyecto de Investigación Sismológica de la Cordillera Occidental 94 (PISCOʹ94). Usually this tremorʹs source is modeled as the resonance of a fluid-filled crack or organ pipe-like structure in the volcano. sonance of a real, physical structure, however, can produce neither as many overtones nor such exact multiples as those observed in the harmonic tremor at Lascar. Harmonics also occur in a spectrum if the source signal is repetitive but nonsinusoidal. Fluid dynamics offers at least three realistic source models for harmonic tremor which produce repetitive, nonsinusoidal waveforms: the release of gas through a very small outlet (the soda bottle model), slug flow in a narrow conduit, and von Kármán vortices produced at obstacles. These models represent different flow regimes, each with its own characteristic range of Reynolds numbers. For each model the fundamental frequency of the tremor is related to the Reynolds number for the flow. Combining the Reynolds numbers for each model with typical kinematic viscosities for the possible fluids present in a volcano—magma, water, steam, air or some combination, at appropriate temperatures and pressures—provides limits on such physical parameters of the volcano as the dimensions of the flow conduit and the flow velocity of the fluid generating the tremor. If any single one of these three models is actually the process in the volcano which generates harmonic tremor, then the tremor is caused by movements of water or gases in the hydrothermal system near the volcanoʹs surface.