Trench‐Parallel Shortening as a Key Driver of Forearc Deformation Along the Seaward‐Concave Northern Chilean Margin

Abstract Oblique plate convergence globally partitions strain into trench‐normal and trench‐parallel components, inducing along‐strike and downdip shortening that drives distributed deformation and segmented seismicity on the megathrust and in the upper plate. The Arica Bend of the Peru–Chile margin, with seaward‐concave curvature, shows increasing subduction obliquity from south to north, defined as the angle between the plate convergence vector and the trench‐normal direction. In this study, we analyze high‐resolution multichannel seismic reflection data across the northern Chile margin to examine the regional distribution of forearc deformation in the upper plate and its relationship to the largest foreshock of the 2014 Mw 8.1 Iquique earthquake. Our results reveal high‐amplitude seismic reflections within the marine forearc, indicative of deformation in both trench‐perpendicular and trench‐parallel directions. These features are interpreted as oblique‐slip thrust faults and related fault‐propagation folds, driven by combined trench‐normal and trench‐parallel deformation. A prominent forearc bulge, oriented primarily parallel to the trench axis, results from oblique subduction and aligns with a low‐velocity zone that concentrates foreshock seismicity, with the Mw 6.7 foreshock nucleating on a mapped oblique thrust fault. These findings demonstrate that trench‐parallel shortening extends offshore the coastal zone, shaping the marine forearc architecture and modulating seismic behavior by localizing stress in mechanically weakened domains. This framework links pre‐existing upper‐plate heterogeneity to foreshock nucleation and rupture, with implications for oblique subduction zones worldwide.