Episodic Slow Slip Events and Tremors in Subduction Zones and Relation to Megathrust Earthquakes

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Recent observations of episodic tremor and slip (ETS) events in multiple subduction zones constitute a new deformation mode at active plate boundaries. ETS consists of low-frequency non-impulsive seismic radiation (``tremor"), coupled with geodetically observed slow slip events (SSE) with durations of order weeks. The occurrence of ETS events poses significant questions as to their origin, and also relative to existing concepts of interseismic loading of the locked seismogenic regions. For example, what are the physical causes of ETS? What controls the observed features such as the recurrence period and migration pattern? What are the seismic consequences of imposing steps in the otherwise steady stress accumulation on the updip seismogenic zone?Previous numerical studies in the framework of rate and state-dependent friction can produce slow slip events with features, such as the slip velocity, recurrence period and slip propagation speed, that are qualitatively similar to those inferred for natural events. However, slip is assumed to take place on a planar fault embedded in a half space with uniform elastic moduli, under time-independent fluid pressure. Such model simplifications have prevented us from using the increasingly rich SSE geodetic data to constrain the downdip fault zone rheology and to investigate its effects on the strength of the seismogenic zone. The goal of this study is to construct a more realistic subduction fault model that encapsulates the essential physics of SSEs and tremors as well as constraints from laboratory and field observations, and to study their relation to megathrust earthquakes. The PI will develop a more complete physical description of fluid transport, pore dilation and pressurization, specifically, to incorporate the dilatancy-strengthening and shear heating induced fluid pressurization into the rate and state modeling. This will allow the researchers to investigate how the occurrence of SSEs could affect the nucleation of megathrust earthquakes and the spatial extent of earthquake ruptures. Second, to constrain the assumed rheology of the fault zone, they will apply frictional properties from lab measurements of different types of rocks, and investigate the fitting of 3D model results to geodetic data on SSEs in the Cascadia margin. They will use finite element method (FEM) in the construction of a non-planar fault model that more realistically represents the Cascadia subduction interface. Finally, they will study the relation between slow slip events and tremors (including low-frequency earthquakes) by identifying the favorable conditions for tremor excitation due to tidal or teleseismic stress triggering.The discovery of ETS events in subduction zones poses significant puzzles and changes the way we think about the interseismic loading and overall mechanical budget on active plate boundaries. Improved understanding of their physical basis and their implications for the updip and downdip limits of megathrust ruptures may increase our knowledge for earthquake forecast and seismic hazard assessment. The development of a 3D Cascadia subduction fault model with rate and state friction will help interpret EarthScope geodetic data sets. The FEM approach will allow general applicability of this model to simulate SSEs and earthquakes rupture processes in other subduction zones and continental faults. The area as it will be addressed is rich, involving inputs from several disciplines, including laboratory rock physics, geodesy, petrology, seismology, and mechanical and hydraulic modeling concepts. This proposed project also contributes to the education of a WHOI/MIT Joint Program graduate student.