HR: 14:10h
AN: V13D-03    [Abstracts]
TI: Source Mechanism of Vulcanian Degassing at Popocat\'{e}petl Volcano, Mexico, Determined From Moment-Tensor Inversion of Very-long-period Seismic Waveforms
AU: * Chouet, B
EM: chouet@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS-910, Menlo Park, CA 94025 United States
AU: Dawson, P
EM: dawson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS-910, Menlo Park, CA 94025 United States
AU: Arciniega, A
EM: maac@tonatiuh.igeofcu.unam.mx
AF: Instituto de Geofisica, UNAM, Cd. Universitaria, Mexico City, DF 04510 Mexico
AB: The source mechanism of very-long-period (VLP) signals accompanying degassing exhalations at Popocat\'{e}petl is analyzed in the $15-70$~s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two events (04/23/00, 05/23/00) representative of mild Vulcanian eruptions are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500~m below the western perimeter of the summit crater, and the modeled source is composed of a shallow-dipping crack (sill with easterly dip of $10\deg$) intersecting a steeply-dipping crack (northeast striking dike with northwest dip of $83\deg$), whose surface trace bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation --- reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of $3-5$~min. The largest moment release occurs in the sill, showing a maximum volume change of $500-1000\:{\rm m^3}$, pressure drop of $3-5$~MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is $200-300\:{\rm m^3}$, with a corresponding pressure drop of $1-2$~MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources is a single force with magnitude of $5 \times 10^8$~N, consistent with melt advection in response to the pressure transients. The source-time history of the three components of this force confirms that significant mass movement starts in the sill and triggers a mass movement response in the dike within $\sim 5$~s. Such source behavior is consistent with the opening of an escape pathway for accumulated gases from slow pressurization of the sill driven by magma crystallization. The opening of a pathway for pent-up gases in the sill and rapid evacuation of this separated gas phase induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40~MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius, (100~m), crack aperture, (5~m), bubble number density, ($10^{10} - 10^{12}\:{\rm m^{-3}}$), initial bubble radius, ($10^{-6}\:{\rm m}$), final bubble radius, ($\sim 10^{-5}\:{\rm m}$), and net decrease of gas concentration in the melt, (0.01~wt%).
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 7260 Theory and modeling
DE: 7280 Volcano seismology (8419)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting