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Morrissey, M.M., 1994

Magmatic fluids and long-period seismicity: A geological and fluid dynamical perspective

Bibliographic Reference

Morrissey, M.M., 1994, Magmatic fluids and long-period seismicity: A geological and fluid dynamical perspective: Tempe, Arizona, Arizona State University, Ph.D. dissertation, 136 p.


Results from numerical simulations of high-speed gas flow through a crack demonstrate how shock waves form and dissipate repeatedly to produce the triggering pressure transient of each of the >4,000 long-period (LP) events recorded 23 hr prior to the December 14, 1989, eruptive onset at Redoubt Volcano, Alaska. The pressure transient is thought to have triggered resonance in a fluid-filled crack by dropping and recovering the pressure by 0.4-40 bar within 0.5 sec over a 7-10 m section of the crack walls every 12-20 sec. Three fluid dynamic models are presented: (1) Continuous fluid flow with a fluctuating outflow pressure; (2) continuous fluid flow with a decreasing outflow pressure and fluctuating sound speed; and (3) intermittent injection of an over-pressured fluid. Each model puts constraints on the flow conditions under which an appropriate shock structure forms and dissipates. Only model 1 is both seismically and geologically consistent. Adverse pressure waves occur in model 3 that will result in compressional first motion. No realistic physical mechanism can produce the necessary fluctuation in fluid sound speed for model 2. In model 1, the occurrence of LP events reflects the pressurization history in shallow regions of the volcanic edifice from the interaction of magmatic fluids and pore-fluids such as groundwater and/or hydrothermal fluid. The mass of magmatic fluids transported through the LP source crack by model 1 is estimated at 3.1 x 107 tonnes over 23 hr. This value is compared to the combined mass of H2O and SO2 in the magma reservoir constrained from petrological and thermodynamic data (3.6-7.3 x 107 tonnes), and that released during the eruption constrained from geochemical and remote sensing data (103-105 tonnes). Results from the mass budget analysis of magmatic fluids in the volcanic system and crystal size population analysis, suggest the following scenario for fluid movement in the edifice: A magma mixing event occurred 10-30 yr prior to the eruption and released a sufficient quantity of volatiles to pressurize the chamber and open new pathways for the fluids to escape. The ascending fluids provided additional heat to the shallow, water-saturated region of the edifice, which enhanced the circulation rate and induced pressure fluctuations between 55 bar and 275 bar in this region. These pressure fluctuations created the necessary conditions for 'seismic' flow conditions to occur repeatedly in the main pathway within >23 hr of the eruption. The volume (0.5 km3) of fluid transported through the LP source crack likely contributed to creating the necessary conditions in the edifice for an eruption.

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