[1] Slow earthquakes called episodic tremor and slip (ETS) events propagate over 100 km at low average velocities, $10 km per day, along several plate interfaces accompanying seismic and aseismic slip. These low propagation velocities differentiate slow earthquakes from ordinary earthquakes, and thus understanding their propagation processes is fundamental to understanding the poorly constrained physics governing the diversity and universality of earthquakes. We show that rheological heterogeneity on faults primarily governs ETS propagation on the basis of comprehensive modeling and observations that correlate migration patterns with the energetics of tremor on a plate-bounding fault in southwest Japan. The fault has persistent small-scale segmentation, in which ETS events started propagating energetically in relatively brittle sections and decelerated generally with a parabolic pattern in relatively ductile sections. Simulated spontaneous ruptures that are based on these parabolic tremor migration patterns constrain the cause of ductility to Newtonian plastic flow or perhaps dilatant strengthening, but reject large-scale fluid flows. We discuss possible elementary processes underlying the Newtonian rheology. This model is also consistent with the observed seismicity migration pattern before the 2011 M w 9.0 Tohoku-oki earthquake, suggesting delayed triggering by the M w 7.3 foreshock.Citation: Ando, R., N. Takeda, and T. Yamashita (2012), Propagation dynamics of seismic and aseismic slip governed by fault heterogeneity and Newtonian rheology,

[1] Deep low-frequency earthquakes (LFEs) and nonvolcanic tremor have distinctive characteristics unlike those of regular earthquakes, including strong anisotropy in their migration velocity and source spectra displaying 1/f decay. We show that a physical model can explain these features in a simple framework with slip pulses originating on fault heterogeneity and triggered by slow-slip events. LFE/tremor source areas in the model consist of unstable patches sparsely and heterogeneously distributed following a Gaussian distribution. The difference in their migration speeds along dip and along strike was reproduced, without anisotropic rheological properties, by introducing alignments of their sources similar to observed streaks of LFEs/tremor. The key to reproducing inverse linear spectral decay is that the slip pulse has a constant mean moment rate. This model provides new insights into the physical source process of LFEs and tremor and should find practical use in assessing properties of deep plate interfaces. Citation: Ando, R., R. Nakata, and T. Hori (2010), A slip pulse model with fault heterogeneity for low-frequency earthquakes and tremor along plate interfaces, Geophys.

[1] 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.Citation: Nakata, R., R. Ando, T. Hori, and S. Ide (2011), Generation mechanism of slow earthquakes: Numerical analysis based on a dynamic model with brittle-ductile mixed fault heterogeneity,