On the Two-Layered Structure of Deep Seismic Plane in Northeastern Japan Arc

A two-layered structure of deep seismic plane is ascertained in the central part of the Tohoku District (39°N-40°N), the northeastern part of Honshu, Japan.This two-layered structure is found for the microearthquakes, the hypocenters of which are determined by the seismological network of Tôhoku University. A similar trend is not seen at all as for the small, moderate and large earthquakes, the hypocenters of which are determined in the same region by JMA. Our redetermination of the hypocenters of these earthquakes by using the same data of JMA, however, makes it clear that the deep seismic plane is distinctly separated into two layers.The focal mechanism solutions for these two groups of earthquakes are derived from the superposition of the initial motions of P waves, and the opposite sense of focal mechanisms is found with each other for these two groups of earthquakes. The earthquakes that occurred in the upper layer, not only microearthquakes but also larger earthquakes, are characterized by down dip compression, that is, the pressure axis is nearly parallel to the dip of the deep seismic plane. On the other hand, microearthquakes as well as larger earthquakes, which occurred in the lower layer, are characterized by down dip extension, that is, the tension axis is nearly parallel to the dip of the deep seismic plane.In the neighbouring regions, similar features are seen slightly, but not so distinctly compared with those in 39°N-40°N region.