Coherent low frequency seismic signatures of unsteady eruptions predicted from numerical conduit flow models

Current seismic monitoring of volcanoes provides data that potentially could constrain eruptive processes, such as the descent of the fragmentation front in the conduit. Proper interpretation of seismograms requires quantitatively understanding the characteristic seismic radiation from such processes. Here we investigate coherent low frequency (< 1Hz) seismic signatures of unsteady eruptions by combining conduit flow simulations with seismic source representation theorems. The approach could be extended to model incoherent high frequency radiation from turbulence.

We simulate short-duration vulcanian explosions using a quasi-one-dimensional, two-phase unsteady conduit flow model. Specifically, we simulate explosions triggered by the sudden rupture of a plug or dome, which allows depressurization and eruption of overpressurized magma from the conduit. A decompression wave travels down the conduit, followed by a fragmentation front behind which magma rapidly accelerates and further depressurizes. Planned extensions to this model include accounting for changes in viscosity accompanying gas exsolution as well as gas escape, which partially control the initial overpressure distribution. The conduit flow model provides changes in pressure and shear traction on the conduit walls and changes in magma momentum, which generate seismic waves in the surrounding earth. The pressure changes define an equivalent distribution of moment tensor sources along the conduit (e.g., Chouet, 1985; Burridge et al.,1996). We also plan to account for equivalent forces due to changes in inertia. Seismic radiation is then calculated semi-analytically using closed-form expressions for far field body and surface waves.

Possible extensions of our method include coupling conduit flow to pressure changes in magma chambers/dikes/sills and introducing incoherent high frequency effects through parametrization of turbulence and mean flow properties. Study of synthetic seismograms will eventually inform studies of data from observations of volcanic seismic activity.