A model of the seismic wavefield in gas-charged magma: application to Soufrière Hills Volcano, Montserrat

Abstract

Abstract Abstract: A stationary two-phase magma model is used to derive the relationship between gas mass and volume fractions, gas and magma bulk densities, temperature and pressure as a function of depth. In turn, these parameters are used to obtain the vertical seismic velocity profile in the gas-charged magma. A two-dimensional finite-difference model of a magma-filled conduit embedded in an elastic medium is then employed to generate the seismic wavefield in and around the conduit. The high impedance contrast between gas-rich magma and surrounding rock results in the seismic energy being efficiently trapped in the conduit; this leads to the generation of a long-lived resonance of tens of seconds commonly observed as low-frequency earthquakes and harmonic tremor. During a single seismic event, a variety of different seismic radiation patterns along the conduit is observed, leading to the occurrence of several distinct seismic phases in the synthetic seismograms. Observations from several volcanoes show peaked amplitude spectra with integer harmonic overtones that exhibit a time-dependent gliding. These features are successfully modelled by varying the excess pressure and, consequently, the gas volume fraction and the seismic velocity, representing sudden degassing events, such as the reduction of pressure by ash-venting, a dome collapse or a Vulcanian explosion, or, in turn, the pressurization of the conduit prior to such events.