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

Abstract:

Increases in seismic activity coincident with volcanic eruptions have been observed, motivating investigation of seismic radiation produced by internal eruptive processes. Traditionally, seismograms are interpreted through kinematic waveform inversions for best-fitting moment tensor and single force sources. However, the interpretation of these sources is nonunique and often ambiguously related to eruptive processes. Here we take an alternative approach to rigorously explore the connection between seismic signals and eruptive processes: we first simulate unsteady eruptions, then use conduit wall tractions and magma momentum changes in seismic source representation theorems to generate synthetic seismograms. We focus on coherent low frequency (< 1Hz) body and surface wave radiation, but the approach can be extended to incoherent high frequency radiation by introducing a parameterization of turbulence and particle-wall collisions.

Using an unsteady conduit flow model, we simulate short-duration Vulcanian explosions. The quasi-one-dimensional model simulates two-phase flow, incorporating melt fragmentation according to a critical volume fraction criterion, and capturing the descent of the fragmentation front as the eruption progresses. We plan to explore alternative fragmentation criteria and associated seismic signatures. The eruption initial conditions account for the emplacement of a magma plug, with overpressure developing below the plug; then, the plug ruptures, leading to outflow accompanying depressurization and fragmentation of the magma. Future extensions of the model will incorporate variable magma viscosity and gas escape. The simulation tracks pressure changes, shear traction on conduit walls, and magma momentum changes, which all generate seismic radiation.

The conduit is represented in the seismic model as a distribution of equivalent sources, with contributions from moment tensor sources (defined by pressure changes) and forces (defined by inertia changes and shear tractions on the wall). Our semi-analytical framework uses closed-form expressions to calculate body and surface waves. Through this approach, we will be able to directly link eruptive processes to seismic signatures, exploring dominant contributions to radiation at various observing distances.