Abstract
Fragmentation plays a critical role in eruption explosivity by
influencing the eruptive jet and plume dynamics that may initiate
hazards such as pyroclastic flows. The mechanics and progression of
fragmentation during an eruption are challenging to constrain
observationally, limiting our understanding of this important process.
In this work, we explore seismic radiation associated with unsteady
fragmentation. Seismic force and moment tensor fluctuations from
unsteady fragmentation arise from fluctuations in fragmentation depth
and wall shear stress (e.g., from viscosity variations). We use unsteady
conduit flow models to simulate perturbations to a steady-state eruption
from injections of heterogeneous magma (specifically, variable magma
viscosity due to crystal volume fraction variations). Changes in wall
shear stress and pressure determine the seismic force and moment
histories, which are used to calculate synthetic seismograms. We
consider three heterogeneity profiles: Gaussian pulse, sinusoidal, and
stochastic. Fragmentation of a high-crystallinity Gaussian pulse
produces a distinct very-long-period (VLP) seismic signature and
associated reduction in mass eruption rate, suggesting joint use of
seismic, infrasound, and plume monitoring data to identify this process.
Simulations of sinusoidal injections quantify the relation between the
frequency or length scale of heterogeneities passing through
fragmentation and spectral peaks in seismograms, with velocity
seismogram amplitudes increasing with frequency. Stochastic composition
variations produce stochastic seismic signals similar to observed
eruption tremor, though computational limitations restrict our study to
frequencies less than 0.25 Hz. We suggest that stochastic fragmentation
fluctuations could be a plausible eruption tremor source.