Seismic images of the North Chilean subduction zone prior to the 2014 Iquique earthquake

On April 1, 2014, a Mw 8.1 megathrust earthquake occurred within a previously locked section of the North Chilean subduction zone. In order to understand the pre-rupture conditions, we have reprocessed an offshore seismic reflection line from the CINCA'95 experiment using advanced seismic depth imaging methods with the aim of obtaining a high resolution image of the structures within the locked zone. This seismic profile crosses the rupture zone and provides an image of the locked plate boundary prior to the megathrust rupture. The reprocessed CINCA'95 profile line 22 crosses the North Chilean subduction zone at 19°40'S, extending 180km over the oceanic Nazca plate, the trench, and the continental margin. Following the data pre-processing, velocity model building, and focusing prestack depth migration, we have imaged the subduction interface down to a depth of approximately 16 km. The image clearly shows several large splay faults in the frontal prism and crystalline forearc basement. Moreover, we have imaged complex structures in the crystalline forearc basement as well as in the overlaying sediments. The oceanic plate shows a significant horst-and-graben structure caused by the downward bending of the plate. These bending-related faults appear to cross-cut the entire oceanic crust down to the Moho.The resulting high resolution seismic depth image reveals the plate interface as a boundary with varying reflectivity. We interpret these variations as lenses with enhanced fluid pressure, possibly caused by subducted sediment-filled grabens and half-grabens. This interpretation is in contrast to previous explanations of this varying reflectivity to be caused by a subducted sea mount which controls the interplate coupling and seismic rupture of the Iquique earthquake.