Propagation dynamics of seismic and aseismic slip governed by fault heterogeneity and Newtonian rheology

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.