High-Resolution Subducting Slab Structure Beneath Northern Chile Using the Double-Difference Tomography Method

The body-wave velocity structure beneath northern Chile was obtained using the double-difference tomography method with the relative arrival times derived from absolute catalogue data. We used 3992 events distributed within the coverage area of the RESISTE local permanent network (18.1° -20.2° S and 68.8° -70.5° W) between 1997 and 2000. During this period about 75.000 phase arrivals ( ∼50% each P and S waves) were observed at 14 stations. From these absolute picks, about 415.000 catalogue differential times ∼50% each P and S waves) were constructed from event pairs with interevent distance <10 km observed at common stations. Both the mantle wedge and subducting slab are well sampled by the absolute and differential ray paths. In this study we used 1728 nodes distributed between the surface and 140 km depth in 16 layers, with an average separation of 10 km. The inversion strategy started from with 1D velocity model, and the velocities obtained in the 1D inversion were used as the initial model for the 3D inversion. The inversion was done for P and S wave velocities, with the initial 3D model starting with a Vp/Vs ratio of 1.73, for 12 iterations. The hypocenters and Vp and Vp/Vs models exhibit more detailed features of the subducting slab and the continental lithosphere than those observed before with the same dataset, but with different relocation and tomography inversion techniques. The intermediate depth double seismic zone dipping at ∼30° E, reported previously, appears with a clear separation of 20 km, and the thickness of each plane is less than 5 km. Vp between the two layers of the double seismic zone is higher than that obtained for each layer of seismicity, similar to that observed in northern Honshu, Japan. The double seismic zone is better observed close to the Arica Bend, and its expression is diminished southward. The shallower structures obtained are in good agreement with active fault systems present in the forearc of the region.