Slow earthquakes manifest as unstable fault slips, but their slip rates, rupture velocities, and radiation efficiencies are significantly lower than fast (regular) earthquakes. The classic model derived from fast earthquake observations cannot explain the first-order characteristics of slow earthquakes. 

Many recent studies suggest that these slower characteristics may be associated with structural heterogeneity within the fault zone, rather than specific friction or rheological properties. In this study, I theoretically investigate why a heterogeneous setup may lead to slow rather than fast earthquakes. I identify three features of a general frictional-viscous heterogeneous model that help generate slow earthquake characteristics.

First, the frictional components in the fault zone, even if only being a small portion, could pin the fault zone due to their spatially heterogeneous distribution. Consequently, the fault can still exhibit stick-slip behaviors as a whole. 

Second, in-between slip events, the frictional components that pin the fault bear most of the shear loading. The local shear stresses on the frictional components would be greater than the average shear stress due to stress concentration. As a result, the fault might fail more easily as a whole, leading to a shorter inter-event interval and a smaller average stress drop during events. 

Third, the seismic radiation from the frictional components may be too small and incoherent to lead the rupture propagation, even though seismometers may detect them. As a result, the large-scale rupture propagation is primarily facilitated by slip cascading of the bulk fault zone, as opposed to dynamic stress perturbation, leading to slow rupture speed and occasional "stress diffusion" behaviors. 

With analytical and numerical models, I provide some quantitative relations between the heterogeneous setup and its resulting slip behaviors. The results imply that a frictional-viscous heterogeneous model can potentially reconcile multiple geophysical observations to first order.

Learn more in my PhD dissertation and my JPGU 2022 presentation. 

Last updated Aug-28-2023

This project started in 2017. There is a debate over whether very low frequency earthquakes (VLFEs), which are considered a type of slow earthquake signal in the 0.02 - 0.05 Hz range, always coincide with and are colocated with tectonic tremors, another type of slow earthquake signal in the 2-8 Hz range. While many literature suggests this is the case, several reports have shown that VLFEs can occur independently of tremors. At that time, there were only a few models for VLFEs, and all predicted VLFEs with tremor signals. 

We were curious whether a model could generate VLFEs without tremors. We delved into a frictional-viscous model from Nakata et al. (2011), which was the only mechanical VLFE model available at the time. Using our own analytical and numerical tools, we analyzed their model and found that it is the frictional-viscous rheology in the model that causes slip to be slow, rather than the spatial mixtures of weakening and strengthening patches, as argued in the original paper. In particular, we discovered that a spatially homogeneous fault with frictional-viscous rheology could generate VLFEs without tremor signals.

Learn more in our publication Wu et al., 2019

Last updated Sept-30-2023