DOUBLE-PLANED DEEP SEISMIC ZONE AND ANOMALOUS STRUCTURE IN THE UPPER MANTLE BENEATH NORTHEASTERN HONSHU (JAPAN)

Seismicity located by using the most recent data obtained from the high-gain seismograph network of Tohoku University shows that the deep seismic zone beneath northeastern Honshu, Japan, is composed of two thin planes which are parallel to each other and are 30-40 km apart. Focal mechanisms derived from the earthquakes in the upper plane are reverse-faulting, or, some of them, down-dip compression. As a contrast, those in the lower plane are down-dip extension. The location of the upper boundary of the descending lithospheric slab, inferred from the arrival-time difference between ScS and ScSp waves and from the travel-time anomaly of intermediate-depth earthquakes observed at the small-scale seismic array, coincides exactly with the upper plane of the double-planed deep seismic zone. Anelasticity (1/Q) structure of the upper mantle consists of three distinct zones: a high-Q (Qs- 1500) inclined lithospheric slab, an intennediate-Q (Qs-350) land-side mantle between the Pacific coast and the volcanic front, and a low-Q (Qs - 100) land-side mantle between the volcanic front and the coast of the Japan Sea. The evidence obtained here provides valuable information as to the definition of the type of mechanism producing the plate motion beneath island arcs. © 1979.