Generation mechanism of slow earthquakes: Numerical analysis based on a dynamic model with brittle-ductile mixed fault heterogeneity

Various characteristics have been discovered for small, slow earthquakes occurring along subduction zones, which are deep nonvolcanic tremor, low-frequency earthquakes (LFEs), and very low frequency earthquakes (VLFs). In this study, we model these slow earthquakes using a dynamic model consisting of a cluster of frictionally unstable patches on a stable background. The controlling parameters in our model are related to the patch distribution and the viscosity of both the patches and the background. By decreasing patch density or increasing viscosity, we observed the transition in rupture propagation mechanism, that is, from fast elastodynamic interactions characterized by an elastic wave propagation to slow diffusion limited by viscous relaxation times of traction on fault patches and/or background. Some sets of these geometrical and frictional parameters collectively explain the moment rate functions, source spectra, and scaled energy of observed slow earthquakes. In addition, we successfully explain both parabolic and constant velocity migrations in the case of the diffusion-limited rupture. Therefore, the observed various characteristics of tremor, LFEs, VLFs, and, potentially, slow slip events, may be essentially explained by our simple model with a few parameters describing source structures and frictional properties of brittle-ductile transition zones along plate boundaries.