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Recently discovered slow-slip events (SSE) and non-volcanic tremors have greatly enriched the spectrum of earthquake behavior. These phenomena offer a unique window into the mechanics of the bottom of the seismogenic zone of active faults. In an emergent view, this transition region has heterogeneous frictional properties, and is composed of frictionally unstable, velocity-weakening patches (" brittle asperities") embedded in a frictionally stable fault region. Tremor swarms are viewed as the collective response of brittle asperities interacting through transient aseismic slip in their surroundings. A hierarchy of migration patterns of tremors has now been observed in the Cascadia subduction zone, including large-scale along-strike tremor propagation at~ 10km/day and rare swarms that propagate 10 times faster in the opposite direction (" rapid tremor reversals" or RTR). A cascade of brittle asperity failures …