Earthquake Cycle Mechanics During Caldera Collapse: Simulating the 2018 Kı̄lauea Eruption

In multiple observed caldera-forming eruptions, the rock overlying a draining magma reservoir dropped downward along ring faults in sequences of discrete collapse earthquakes. These sequences are analogous to tectonic earthquake cycles and provide opportunities to examine fault mechanics and collapse eruption dynamics over multiple events. Collapse earthquake cycles have been studied with zero-dimensional slider-block models, but these do not account for the complicated interplay between fluid and elastic dynamics or for factors such as the heterogeneous fault properties and non-vertical ring fault geometries often inferred at volcanoes. We present two-dimensional axisymmetric mafic piston-like collapse earthquake cycle models that include rate-and-state friction, fully-dynamic elasticity, and compressible viscous fluid magma flow. We demonstrate that collapse earthquake intervals and magnitudes are highly sensitive to inertial effects, evolving stress fields, fault geometry, and depth-varying fault friction. Given the consistent earthquake cycles observed in most eruptions, this suggests that ring faults can quickly stabilize and often become nearly vertical at depth. We use the well-monitored 2018 collapse sequence at Kilauea as a case study. Our model can produce many features of Kilauea seismic and geodetic observations, except for a significant amount of interseismic slip, which cannot be readily explained with simple rate-and-state friction parameterizations. Plain Language Summary Many large volcanic eruptions involve caldera collapse, where the rock above a magma reservoir abruptly subsides as the reservoir drains. Collapse can cause large earthquakes and repressurize the magma reservoir, creating a repeating cycle that helps sustain eruptions over days-months. We develop computational models to understand the rock and magma interactions involved in collapse earthquake cycles, using data from the 2018 eruption of Kilauea. The models can explain many geophysical measurements and show that collapse eruptions are very sensitive to fault geometry and spatially variable rock properties. However, standard friction models do not readily explain large amounts of fault slip measured between collapse earthquakes, suggesting that other complexities should be considered.