Description: The main objective of this study was to investigate the influence of the subducting Juan Fernandez aseismic ridge on the central Chilean margin structure. Seismic wide-angle data were collected during the cruise of RV SONNE 103 (S0103) in 1995. At the latitude of Valparaiso (≈ 32° S), a fundamental change in the configuration of the Benioff zone, volcanic activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. The seismic surveying of the continental margin and the oceanic plate offshore Valparaiso reveal the crustal structure and possible causes for the change in slab configuration. Coincident near-vertical (S0104) and wideangle measurements were made along two profiles. Profile 1, located at the south of the study area, away from the influence of the subducting ridge, crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed. Profile 2, located in the north, runs from the Juan Femandez ridge to the Chilean coast The crustal velocity models ohtained for the two profiles reveal that the continental ernst extends to the middle-lower slope boundary, which is also reflected in morphology. In addition, they show that the crustal structure of the oceanic plate is rather similar, but the plate seems to be slightly more inclined along the nothern profile (13° versus 10° in the south). The two profiles are only 70 km apart but their structures differ significantly. No correlation amongst the two profiles existed which could be attributed to ridge collision. The data support that the 1985 earthquake ruptured the plate boundary in this specific area that includes the segment boundary and mainly where continental ernst forms the upper plate. A supplementary 3-D wide-angle survey with an experimental geometry has been carried out on top of the Valparaiso Basin. The Valparaiso forearc basin lies opposite the ridge at a midslope depth, the most northern basin along the margin. 3-D data were acquired with ocean bottom hydrophones using a star-shaped network of 10 airgun profiles with 10 intersection points. The data constrain the velocity structure over a 85 x 85 km region to a maximum depth of about 20 km, including the Valparaiso Basin, the frontal collision zone, the continental backstop, and the top of the subducting oceanic plate. The results of a first arrival inversion using a 3-D tomographic method is presented. Swathmap (SO101) and coincident deep and high-resolution seismic reflection data were taken into account to develop a 2-D starting model for each profile. The reflecting interfaces within the 10 2-D profiles represent the base of the slope sediments, accretionary wedge, the Valparaiso forearc basin, and the subducting oceanic crust. Simultaneous 2-D inline modeling of refracted and reflected arrivals was used to construct the 3-D starting model. 3-D traveltimes and raypaths were calculated using a finite difference solution of the eikonal equation that is accurate for large velocity contrasts. The 3-D dataset consists of about 9,500 first arrivals recorded at seven ocean bottom hydrophones (3-D experiment) and approximately 8,000 first arrivals recorded at thirteen ocean bottom hydrophones located at profile 2. The 3-D tomographic method involves a simultaneous refraction inversion incorporating smoothness and smallest perturbation constraints so that a minimum structure model with minimum perturbation from the starting model can be obtained. The selection of the starting model and the type of regularization is important given the ray coverage and strong lateral variations of the study area. Model constraint is explored by clipping the velocity values in regions of relatively large total perturbation with respect to the starting model at each iteration. This approach yields realistic velocities in regions that would otherwise contain unrealistic velocities using conventional forms of regularization alone, yet minimizes the amount of bias introduced into the solution. The 3-D model shows several large-scale features including the Valparaiso Basin, the forearc wedge, the hackstop, the Punta Salinas Ridge; and possibly the top of the subducted Papudo seamount. From this study it can be concluded that a different 3-D shot receiver geometry will significantly improve the possibility of exploitation of rays travelling through the subsurface.