Two-dimensional depth structure of the crust beneath the Tohoku District,the northeastern Japan arc Part I. Method and Conrad discontinuity.

Abstract

A new technique for determining the three-dimensional seismic velocity structure of a crust is proposed. The principle of the technique is to relate travel time anomalies only to those thickness anomalies of the granitic and the basaltic layers, whose seismic velocities have been shown to be almost constant in the lateral direction at least beneath the land area of Japan. We determine the lateral variations of the depth of the Conrad discontinuity beneath the Tohoku District, the northeastern part of Honshu, Japan by applying the technique to arrival time data from local earthquakes. The results show that the Conrad is deep beneath the central part of the Tohoku District and becomes shallower toward both the eastern and western coast lines. They also show that the Conrad becomes deeper to the north and is located at a depth ranging from 14 to 18km. The resulting Conrad depth at the western part of the profile along the Kesennuma-Oga is consistent with that derived from explosion seismic experiment. However, our result in the eastern part of the profile suggests that the Conrad is located at a depth of about 14km, which is deeper by 6km than that derived from explosion seismic experiment.