The origin of harmonic tremor at Old Faithful geyser

Citation

Kedar, Sharon (1996) The origin of harmonic tremor at Old Faithful geyser. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/X1RD-ZV73. https://resolver.caltech.edu/CaltechETD:etd-01262009-133713

Abstract

Volcanic tremor, the seismic signal which is often associated with eruptions, is among the least understood phenomena in seismology. Large variations in the physical properties of volcanic rocks and fluids along with the complex geological structure make it difficult to obtain reliable source models. Old Faithful geyser, Yellowstone National Park, was chosen as a laboratory for studying fluid-flow-induced seismicity. The geyser's cyclic behavior on an hourly time scale, and its accessibility, make it suitable for a detailed study of seismic behavior between eruptions simultaneously with underwater pressure measurements inside the geyser.

We observe that sharp pressure pulses inside the water-column attributed to steam bubble collapse are followed by distinct seismic events, with a sharp onset and a harmonic horizontal motion whose frequency varies spatially but not temporally. A superposition of these events creates the appearance of continuous harmonic tremor. Since the pressure in the water column exhibits no sign of resonance, the harmonic motion must be caused by elastic waves reverberating in the solid medium. A near-surface soft layer is demonstrated to be a possible elastic model.

The harmonic tremor observed at Old faithful seems analogous to some examples of shallow volcanic tremor.

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