Seismic Properties of Rocks from an Active Volcanic System: Long Valley Caldera, California

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EAR-9980548: David M Fountain and Barbara E John Advances in high-resolution seismic techniques indicate that it may be possible to image plumbing systems of active or dormant explosive volcanic systems. In order to interpret P- and S- wave velocity data obtained from these studies, it is important to thoroughly understand the variables that control seismic velocities (magma, hydrothermal alteration, fractures, rock porosity, water and/or steam saturation, elevated pore pressure of water and/or steam, and variations rock composition). Toward this end, this research project will investigate the physical properties of rocks comprising the large Long Valley Caldera, California. Intensive geophysical experiments, geological studies, and drilling in the hydrothermal system provide an unparalleled view of the interior of a large caldera system. Samples from the numerous intracaldera drill-holes, including the Long Valley Exploratory Well, afford an opportunity to examine variations in seismic velocities of rocks within the upper three kilometers of the caldera. These drill-holes penetrate the main intracaldera components of this giant volcanic system: various post-caldera rhyolites and basalts, the Bishop Tuff, and Sierran basement rocks. Laboratory measurements will be conducted on core recovered from intracaldera drill-holes and outflow deposits and include P- and S- wave velocities as a function of confining pressure, pore fluid pressure, and temperature. Companion analysis of mineralogical and chemical composition, pore geometry, pore volume, density, nature and degree of alteration, and other textural parameters will allow quantification of the controls on seismic velocity. This study will be the first systematic study of the seismic properties of the constituents of large explosive silicic caldera systems measured at elevated confining pressure, high temperature, and pore pressure.