Modeling the along-fault variability of slow earthquakes in subduction zones based on a combined Maxwell Viscoelastic and damaging rheology

Slow earthquakes are transient events detected with geodetic (slow slip) and/or seismic (low-frequency earthquakes and tremors) observations that release energy over a period of hours to months, i.e., much longer than regular earthquakes. These events are observed in several subduction at a specific range of depths and with their properties, such as strength, duration, recurrence interval, and predominance of either slip or seismic components varying along the subduction interface. In this study, we investigate slow earthquakes based on numerical modeling by applying a combined Maxwell viscoelastic and damaging rheology. Unlike conventional rate and state modeling practices utilizing infinitely thin fault assumptions with fault friction laws, our approach incorporates a finite fault thickness through a damage mechanism and suitable failure criteria. Our model directly predicts a geodetically observable slow displacement, while rock damaging rate is considered as a proxy to seismic tremors.

We run a set of numerical simulations to systematically explore the effects of model parameters such as viscosity, yield stress, and healing time on slow slip and tremors. We then incorporate along-fault variations in temperature, pore pressure, and permeability rate to infer changes in viscosity, yield stress, and healing time. The model successfully reproduces the observed synchronous episodes of slow slip and tremors in the initial stage. We show that decreasing strength and healing time results in reducing the recurrence intervals between these episodes and the amplitude of slow slip. Conversely, decreasing viscosity leads to a larger recurrence time and increased amplitude. After introducing the along-fault variability of main mechanical parameters, the model successfully predicts tremor concentration in the downdip zone which can be explained by the influence of high pore pressure in the yield stress profile as a function of depth. The capacity of the model to reproduce some patterns seen in observations underscores its capacity to capture the physics of slow earthquakes, emphasizing its potential to improve our understanding of seismic phenomena in subduction zones.