Yingdi LUO, Jean Paul Ampuero 

Slow-slip events (SSE) and non-volcanic tremors have revealed a broad spectrum of earthquake behavior, involving entangled seismic and aseismic slip, and offer a unique window into fault mechanics at the bottom of seismogenic zones. A hierarchy of migration patterns of tremors has been observed in the Cascadia subduction zone, including large-scale along-strike tremor propagation and Rapid Tremor Reversals (RTR) migrating in opposite directions with much higher propagation speeds. Here we show that these tremor migration patterns can be reproduced by two end-member models of a fault with heterogeneous mechanical properties, composed of competent asperities embedded in a more frictionally stable, incompetent matrix. In the SSE-driven-tremor model, SSEs are spontaneously generated by the matrix, even in absence of seismic asperities, and drive tremor. In the tremor-driven-SSE model the matrix is stable, it slips steadily in absence of asperities, and SSEs result from the collective behavior of tremor asperities interacting via transient creep in the form of local afterslip fronts. We study these two end-member models through 2D quasi-dynamic multi-cycle simulations of faults governed by rate-and-state friction with heterogeneous frictional properties and effective normal stress, using the earthquake simulation software QDYN . In both models, tremor migration patterns emerge from interactions between asperities mediated by creep transients. The models successfully reproduce forward tremor propagation and RTRs, as well as various other observed tremor migration patterns, without the need to finely tune model parameters. Our modeling results suggest that, in contrast to a common view, SSE could be a result of tremor activity. Also, the hierarchical pattern of tremor migrations provides general constraints on fault zone rheology, and the location of RTRs and other tremor patterns might shed light on the finer scale spatial variability of fault properties. We also find that, despite important interactions between asperities, tremor activity rates are proportional to the underlying aseismic slip rate, supporting an approach to estimate SSE properties with high spatial-temporal resolutions via tremor activity.