Upper Mantle Structure Beneath the Japan Trench and Tohoku Arc Viewed from Mantle Bouguer Anomalies

Abstract

Mantle Bouguer gravity anomaly gives density anomaly in the upper mantle. We compiled the mantle Bouguer anomaly in the Japan Trench and Arc using data of crustal structure given from explosion seismology. A profile of the gravity anomalies across the subduction zone shows a regional positive anomaly over the island arc. The profile also shows local low-density anomalies beneath the central part of the island arc and near the trench axis. A relatively positive anomaly of about 70 mgals still exists over the island arc after removing the effect of the subducting slab. We suspect missing of the low-density layers on the surface of the slab can explain the result. The low-density zone beneath the central part of the island arc relates to the zone of volcanic activity expected from petrological studies. Another low density zone relates to accretion of sediment near the trench axis.