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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 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. 

Abstract All instrumented basaltic caldera collapses have generated M_w > 5 very long period earthquakes. However, previous studies of source dynamics have been limited to lumped models treating the caldera block as rigid, leaving open questions related to how ruptures initiate and propagate around the ring fault, and the seismic expressions of those dynamics. We present the first 3D numerical model capturing the nucleation and propagation of ring fault rupture, the mechanical coupling to the underlying viscoelastic magma, and the associated seismic wavefield. We demonstrate that seismic radiation, neglected in previous models, acts as a damping mechanism reducing coseismic slip by up to half, with effects most pronounced for large magma chamber volume/ring fault radius or highly compliant crust/compressible magma. Viscosity of basaltic magma has negligible effect on collapse dynamics. In contrast, viscosity of silicic magma significantly reduces ring fault slip. We use the model to simulate the 2018 Kı̄lauea caldera collapse. Three stages of collapse, characterized by ring fault rupture initiation and propagation, deceleration of the downward‐moving caldera block and magma column, and post‐collapse resonant oscillations, in addition to chamber pressurization, are identified in simulated and observed (unfiltered) near‐field seismograms. A detailed comparison of simulated and observed displacement waveforms corresponding to collapse earthquakes with hypocenters at various azimuths of the ring fault reveals a complex nucleation phase for earthquakes initiated on the northwest. Our numerical simulation framework will enhance future efforts to reconcile seismic and geodetic observations of caldera collapse with conceptual models of ring fault and magma chamber dynamics. 

The intensity of explosive volcanic eruptions is correlated with the amplitude of eruption tremor, a ubiquitously observed seismic signal during eruptions. Here we expand upon a recently introduced theoretical model that attributes eruption tremor to particle impacts and dynamic pressure changes in the turbulent flow above fragmentation (Gestrich et al., 2020). We replace their point source model with Rayleigh wave Green's functions with full Green's functions and account for depth variation of input fields using conduit flow models. The latter self-consistently capture covariation of input fields like particle velocity, particle volume fraction, and density. Body wave contributions become significant above 2-3 Hz, bringing the power spectral density (PSD) closer to observations. Conditions at the vent are not representative of flow throughout the tremor source region and using these values overestimates tremor amplitude. Particle size and its depth distribution alter the PSD and where dominant source contributions arise within the conduit. Solutions with decreasing mass eruption rate, representing a waning eruption, reveal a shift in the dominant tremor contribution from turbulence to particle impacts. Our work demonstrates the ability to integrate conduit flow modeling with volcano seismology studies of eruption tremor, providing an opportunity to link observations to eruptive processes.