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The updip limit of the seismogenic zone of megathrusts is poorly understood. The relative absence of observed microseismicity in such regions, together with laboratory studies of friction, suggests that the shallow fault is mostly velocity strengthening, and likely to creep. Inversions of geodetic data commonly show low to zero coupling at the trench, reinforcing this view. We show that the locked, downdip portion of the megathrust creates an updip stress shadow that prevents the shallow portion of the fault from creeping at a significant rate, regardless of its frictional behavior. Our models demonstrate that even if the shallowest 40% of the fault is frictionally unlocked, the expected creep at the fault tip is at most 30% of the plate rate, often within the uncertainties of surface geodetic measurements, and below current resolution of seafloor measurements. We conclude that many geodetic models significantly underestimate the degree of shallow coupling on megathrusts, and thus seismic and tsunami hazard.

Plain Language Summary When one tectonic plate dives beneath another, the fault between them is called a megathrust. The shallow part of these faults is not well understood. Generally, it is thought that if this area is pushed, it will freely slip (and will not store energy that would be released as earthquakes). Researchers make models of megathrusts using GPS measurements to determine which parts of it are slipping and which are not (which means they are storing energy that will be released as earthquakes). These models are not well constrained far from the GPS measurements, and in many areas it is difficult to make measurements near the shallow megathrust because they are under the ocean. We use a simple model that considers the forces acting on the fault to show that if the deeper megathrust is not slipping, then it will act as a buffer to prevent the shallow part from moving. We compare our model to megathrusts with many measurements in Japan and Nepal and show that the GPS data cannot tell us if the shallow fault is stuck together by friction or not. This is important because the behavior of the shallow fault affects potential earthquake size and tsunami risk.