Very long period seismic signatures of unsteady eruptions predicted from
conduit flow models
Abstract
Explosive volcanic eruptions radiate seismic waves as a consequence of
pressure and shear traction changes within the conduit/chamber system.
Kinematic source inversions utilize these waves to determine equivalent
seismic force and moment tensor sources, but relation to eruptive
processes is often ambiguous and nonunique. In this work, we provide an
alternative, forward modeling approach to calculate moment tensor and
force equivalents of a model of eruptive conduit flow and chamber
depressurization. We explain the equivalence of two seismic force
descriptions, the first in terms of traction changes on the
conduit/chamber walls, and the second in terms of changes in magma
momentum, weight, and momentum transfer to the atmosphere. Eruption
onset is marked by a downward seismic force, associated with loss of
restraining shear tractions from fragmentation. This is followed by a
much larger upward seismic force from upward drag of ascending magma and
reduction of magma weight remaining in the conduit/chamber system. The
static force is upward, arising from weight reduction. We calculate
synthetic seismograms to examine the expression of eruptive processes at
different receiver distances. Filtering these synthetics to the
frequency band typically resolved by broadband seismometers produces
waveforms similar to very long period (VLP) seismic events observed in
strombolian and vulcanian eruptions. However, filtering heavily distorts
waveforms, accentuating processes in early, unsteady parts of eruptions
and eliminating information about longer time scale depressurization and
weight changes that dominate unfiltered seismograms. The workflow we
have introduced can be utilized to directly and quantitatively connect
eruption models with seismic observations.