Marine forearc tectonics in the unbroken segment of the Northern Chile seismic gap

Abstract

While releasing stress of the well-defined Northern Chile seismic gap, the 2014 Mw. 8.1 Iquique Earthquake only ruptured part of this gap, leaving large and possibly highly coupled areas untouched. These non-ruptured areas now may pose an elevated seismic hazard due to the transfer of stresses resulting from the 2014 rupture. Here we use multibeam bathymetric data, covering ~90% of the North Chilean marine forearc, in combination with seismic reflection images to derive a tectonic map of the marine forearc in the seismic gap. In the entire study area we find evidence for widespread normal faulting. On the upper continental slope, some of the normal faults dip towards the continent, defining N-S trending ridges that can be traced over tens of kilometers. Seaward dipping normal faults however displays segmentation in it distribution along the margin. In the unbroken southern segment of the seismic gap, seaward dipping normal faults cutting the forearc locally extend close to the slope toe and the deformation front at the deep-sea trench, under ~8 km of water. Similar normal faults cutting the lower slope are neither observed further north (2014 Iquique earthquake area) nor further south (2007 Tocopilla earthquake area). The spatial variation in lowermost-slope normal faulting does not correlate with obvious changes in the structural and tectonic setting of the subduction zone (e.g. plate convergence rate and direction, trench sediment thickness, subducting plate roughness). Thus, the changes in permanent deformation recorded by the spatial distribution of faults may hold crucial information about the long-term seismogenic behavior of the Northern Chile seismic gap over multiple earthquake cycles. Although the structural interpretations cannot directly be translated into seismic hazard, the tectonic map may hold key information to better understand deformation in the marine forearc related to the seismic cycle, historic seismicity, and spatial changes in plate-coupling