Abstract
The crustal structure across the northern part of the Izu-Ogasawara arc and trench system was investigated by seismic refraction and reflection methods. A schematic crustal section across the island arc and trench along latitude 32°N is presented. The 5.4-km/sec layer in appreciable thickness was detected in the narrow area between the trench and the Shichito-Iwojima ridge as same as in the Kuril arc and the southern part of this arc in the junction to the Mariana arc. If the 5.4-km/sec layer was concerned with the outer arc formation in the southern double arc, south of 29°N, then the presence of the layer may indicate the continuation of a recent non-volcanic outer arc not reflected on the topography to the north. There is inferred a fault which reaches down to the mantle under the ridge side slope of the trench. The fault is suggested by a study on the force mechanism in the island arc and trench systems and supported by a recent study on the spacial distribution and focal mechanisms of shallow to deep earthquakes.
