Accurate hypocentre determination in the seismogenic zone of the subducting Nazca Plate in northern Chile using a combined on-/offshore network

The coupled plate interface of subduction zones-commonly called the seismog enic zone-has been recognized as the origin of fatal earthquakes. A subset of the aftershock series of the great Antofagasta thrust-type event (1995 J uly 30; M-w = 8.0) has been used to study the extent of the seismogenic zon e in northern Chile. To achieve reliable and precise hypocentre locations w e applied the concept of the minimum 1-D model, which incorporates iterativ e simultaneous inversion of velocity and hypocentre parameters. The minimum 1-D model is complemented by station corrections which are influenced by n ear-surface velocity heterogeneity and by the individual station elevations . By relocating mine blasts, which were not included in the inversion, we o btain absolute location errors of 1 km in epicentre and 2 km in focal depth . A study of the resolution parameters ALE and DSPR documents the importanc e of offshore stations on location accuracy for offshore events. Based on p recisely determined hypocentres we calculate a depth of 46 km for the lower limit of the seismogenic zone, which is in good agreement with previous st udies for this area. For the upper limit we found a depth of 20 km. Our res ults of an aseismic zone between the upper limit of the seismogenic zone an d the surface correlates with a detachment zone proposed by other studies; the results are also in agreement with thermal studies for the Antofagasta forearc region.