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The spatial distribution of the intermediate‐depth earthquakes in the central Vanuatu (New Hebrides) islands includes a remarkable gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km. No well‐located earthquakes determined from teleseismic data during a 16‐year period nor any microearthquakes located by a local seismograph network during a 5‐year period have occurred in the gap. A zone of attenuation of high‐frequency shear waves overlaps the seismicity gap. No contortion of the Benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent. However, the gap is close to the extrapolated location of the subducted part of the D'Entrecasteaux Fracture Zone (DFZ). The DFZ is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island arc. A positive thermal anomaly associated with the DFZ may cause the gap of intermediate‐depth seismicity and produce anomalous absorption of high‐frequency shear waves. Alternatively, a zone of strong scattering associated with the subducted part of the DFZ may also account for the anomalous attenuation.

International audienceThis paper illustrates the efficiency of vertical seismic profiling (VSP) for the investigation of dipping and hydraulically conductive structures affecting a granitic basement covered by sediments. A three-component (3C) VSP data set has been acquired in the GPK1 and EPS1 wells of the Soultz-sous-Foreˆts enhanced geothermal system (EGS) located within the Upper Rhine Graben (URG). Our study focuses on the isotropic processing of profiles acquired with vertical vibrator P and their subsequent interpretation. Mainly P-S converted reflections are identified from the analysis of the 3C records. These P-S conversions occur on steep permeable faults that are positioned in space by traveltime modelling. These faults cut the granite basement in several hectometric-scale blocks, and represent the main fluid paths between the boreholes. These faults are thought to be inherited from late Variscan and Alpine deformation periods, reactivated by the current stress field. When properly processed and interpreted, VSP allow the scale gap between surface and borehole data to be bridged

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