Migration Patterns and Scaling Laws of Slow Slip and Tremor Resulting From the Collective Behavior of Fault Asperities Mediated by Transient Creep (Invited)

The coupled phenomena of episodic slow slip and tectonic tremor offer an exceptional opportunity to investigate the rheology and heterogeneity of active faults at depth. Tremor activity might provide a natural creepmeter to monitor aseismic slip with high resolution, including possible precursory slip associated with the nucleation of large earthquakes. Recently, a hierarchy of tremor migration patterns has been observed in Cascadia. On the longest time scales, tremor migrates along strike at a speed of order 10 km/day, coincident with the propagating front of slow slip events. At shorter time scales, tremor swarms coined “rapid tremor reversals" propagate backwards along-strike at speeds of order 100 km/day. At even shorter time scales, tremor streaks propagate along-dip at speeds of order 1000 km/day. Interestingly, some of the natural tremor migration patterns are also observed in laboratory experiments of slow sliding in gels and of slow rupture along weak interfaces in plexiglass. Whereas the largest scale migration pattern is naturally explained by triggering of tremor by a propagating slow slip pulse, the origin of the two other patterns remains unknown. I propose a unifying framework to understand these three patterns and their relation to the spatial distribution of slip rate within the underlying slow slip pulse. Numerical models of tremor generated by brittle asperities present in the deep, mainly ductile portions of a fault reveal that migrating tremor swarms arise from a cascade of triggering between asperities mediated by propagating creep perturbations analogous to afterslip. The speed of these creep waves controls the tremor migration speed. Theoretical arguments and numerical simulations show that the propagation speed of creep transients correlates strongly with the background slip rate, implying that larger slip rates at the leading front of a large scale slow slip pulse produce faster tremor migration, slower slip within the pulse implies slower tremor migration. The model also predicts that the source areas of tremor swarms have large aspect ratios. This source shape implies pulse-like ruptures that are consistent with the proportionality between moment and duration observed for slow earthquakes. The universality of that scaling law is linked to the relations between the characteristic length scales of the slow slip pulse. The model also yields further predictions that could be tested by observations affordable in the near future.