Does permanent extensional deformation in lower forearc slopes indicate shallow plate-boundary rupture?

11 pages, 7 figuresSeismic rupture of the shallow plate-boundary can result in large tsunamis with tragic socio-economic consequences, as exemplified by the 2011 Tohoku-Oki earthquake. To better understand the processes involved in shallow earthquake rupture in seismic gaps (where megathrust earthquakes are expected), and investigate the tsunami hazard, it is important to assess whether the region experienced shallow earthquake rupture in the past. However, there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture, with the exception of mechanical studies on subducted fault-rocks. Here we combine new swath bathymetric data, unpublished seismic reflection images, and inter-seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the Northern Chile seismic gap allows inferences on past earthquake behavior. While the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the North Chilean margin, we document permanent extensional deformation of the lower slope localized to the region 20.8°S–22°S. Critical taper analyses, the comparison of permanent deformation to inter-seismic seismicity and plate-coupling models, as well as recent observations from other subduction-zones, including the area that ruptured during the 2011 Tohoku-Oki earthquake, suggest that the normal faults at the lower slope may have resulted from shallow, possibly near-trench breaking earthquake ruptures in the past. In the adjacent margin segments, the 1995 Antofagasta, 2007 Tocopilla, and 2014 Iquique earthquakes were limited to the middle and upper-slope and the terrestrial forearc, and so are upper-plate normal faults. Our findings suggest a seismo-tectonic segmentation of the North Chilean margin that seems to be stable over multiple earthquake cycles. If our interpretations are correct, they indicate a high tsunami hazard posed by the yet un-ruptured southern segment of the seismic gapJacob Geersen was funded by a grant (CP1404) of the Cluster of Excellence 80 ‘The Future Ocean’, funded within the framework of the Excellence Initiative by Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. César R. Ranero was supported by the Grup de Recerca 2014SGR940 de la Generalitat de Catalunya. R/V SONNE Cruises SO-104 (Grant No. 03G104A) and SO-244 (Grant No. 03G0244A) were supported by the German Bundesministerium für Bildung und Forschung (BMBF)Peer Reviewe