Rate‐and‐State Model Casts New Insight into Episodic Tremor and Slow‐slip Variability in Cascadia

Advances in geodetic and seismic observations have led to the discovery of Episodic Tremor and Slow‐slip (ETS). ETS in Cascadia subduction zone occurs semiregularly and shows intriguing spatio‐temporal variability reportedly associated with frictional properties and stress conditions. Yet the origin of complex ETS behaviors remains largely unknown. Here we develop a laboratory‐based rate‐and‐state asperity‐in‐matrix subduction fault model, supported by geological observations of exhumed fault with heterogeneous frictional properties and pore pressure variation, to reproduce all ETS variability in good agreement with observations. Our results show that differential pore pressure plays a crucial role in affecting fault behaviors. Regions of asperities with decreased pore pressure tend to have increased tremor. Our study suggests that ETS variability can be used to probe otherwise enigmatic fault zone properties. Plain Language Summary The discovery of slow earthquakes has greatly broadened our view of faulting processes and earthquake dynamics. The Episodic Tremor and Slow‐slip (ETS) is one kind of slow earthquakes featuring slow‐slip (fault moves very slowly yet still higher than plate motion, emitting no seismic signals) and accompanying tremors (weak, nonimpulsive, and continuous “humming” of fault). Intriguing ETS behaviors have been observed in Cascadia subduction zone such as broad‐scale segmentation and local transient features including rapid tremor reversals, ETS “gap,” and “halt.” But physical explanation of these complex ETS behaviors is elusive. In this study we propose a rate‐and‐state fault model consisting of a mixture of competent tremor asperities embedded in incompetent matrix with heterogenous pore pressure. For the first time we show that the broad spectrum of observed ETS complexity can be reproduced in a unified mechanical model. We find that the variation in pore pressure (thus effective normal stress) can play a crucial role in affecting various‐scale fault behaviors. Our study provides new insights into the physics of slow earthquakes, suggesting that the observation of ETS variability can be a useful tool, when combined with numerical model, to probe otherwise enigmatic fault zone properties and stress conditions on the subduction megathrust fault.