Intermediate and deep-focus earthquakes under the Japanese arcs delineate seismic zones in which there are distinct and abrupt discontinuities that strike transverse to the arcs. The major types of discontinuities are offsets of the seismic zone (up to 50 km) and changes in strike (up to 30°). The projection of these deep discontinuities back to the trench gives the arcs a segmented pattern which appears to be closely related to the surface geology. The geologic features that give the surface of the arcs a blocklike character and that are closely correlated to the deep discontinuities are offsets or strike changes of the axis of the trench, clusters or lines of volcanos transverse to the arc, prominent recent structures transverse to the arc, and lateral margins of the focal regions of great earthquakes.

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