Seismic Structure of the Oceanic Crust Around Petit-Spot Volcanoes in the Outer-Rise Region of the Japan Trench

The structure of an incoming oceanic plate in a subduction zone systematically changes toward the trench because of the flexure of the plate. Changes in seismic velocity from the outer rise toward the trench are widely considered to be caused by the development of bending-related faults. However, few studies have focused on the effects of igneous activities preceding the subduction, which lead to structural changes of the incoming plate. Here we present active-source seismic data acquired around petit-spot volcanoes situated in the outer-rise region of the Japan Trench. The resultant velocity model showed low seismic velocities (V-p < 4.0 km/s) at the top of the oceanic crust under the petit-spot volcanoes. Moreover, the velocity reduction was significantly larger than that caused only by bending-related faults. Our results suggest that petit-spot volcanism is a contributing factor affecting structural variation and modification of the incoming oceanic plate in this subduction zone. Plain Language Summary In subduction zones, the oceanic plate is bent in trench-outer-rise region before its subduction. By using marine seismic methods with airgun seismic sources, many studies have shown that the seismic velocity of the incoming oceanic plate reduces in the trench-outer-rise region due to bending-related faults. Here we present the results of marine seismic experiments in trench-outer-rise region of the Japan Trench, which showed a large velocity reduction (V-p < 4.0 km/s) under the petit-spot volcanoes that cannot be explained only by the effects of bending-related faults. This implies that petit-spot volcanism is one of the contributing factors leading to structural variation and modification of the incoming oceanic plate before its subduction.