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Abstract

Subduction zone forearcs deform transiently and permanently due to the frictional coupling with the converging lower plate. Transient stresses are mostly the elastic response to the seismic cycle. Permanent deformation is evidenced by forearc topography, upper plate faulting, and earthquakes; its relation to the megathrust seismic cycle is debated. Here we study upper plate seismicity in the northern Chile subduction zone as a proxy for forearc brittle deformation. We find that seismicity is distributed unevenly and a dramatic increase correlates with the onset of a change in subduction obliqueness. Earthquakes in the South American crust show a remarkably homogenous trench-parallel, compressional stress field. Earthquake fault mechanisms are dominated by trench-perpendicular thrusts. Further inland, where the lower plate becomes uncoupled, the stress field is more varied with a compression direction approx. convergence parallel. The stress regime above the plate-coupling-zone, almost perpendicular to the plate convergence direction, may be explained by a change in subduction obliqueness due to the concave shape of the plate margin, which we demonstrate by investigating inter-plate earthquake slip vectors. From these, we derive a strain rate and compare it to one derived from upper plate earthquakes and geological time-scale shortening. Based on the distribution of the type of faulting we investigate the trench-perpendicular stress field with a force balance model taking into account gravitational stresses and the traction along the megathrust. The observed deep strike-slip earthquakes, expression of trench-perpendicular tension, require the deepest extent of the megathrust to be very weak.