A detailed cross-section of seismicity combined with types of earthquake mechanisms was constructed for the subduction zone off Miyagi Prefecture, northern Honshu, Japan, where a large Ms= 7.5 earthquake occurred on June 12, 1978. Nodal-plane solutions were determined for fourteen earthquakes with body wave magnitudes ≥5.4 using data recorded at WWSSN and Japanese stations. For smaller-magnitude events, the types of focal mechanism were analysed using P-wave first-motion data recorded at regional stations. The results obtained are: 1. (1) For the area 150-200 km landward from the trench axis, the cross-section suggests a triple-planed seismic zone; that is, the zone of thrusting overlaps the so-called double-planed seismic zone. The upper and lower planes of the double-planed seismic zone are characterized by down-dip compression and down-dip tension, respectively. 2. (2) Thrusting terminates at a depth of 60 km, where the aseismic front in the area is located, and it merges into the zone of down-dip compression in the deeper part, in a down-dip direction. In contrast, the depth of the zone of down-dip compression appears to shift by 10 km at the aseismic front. 3. (3) The extension of the zone of down-dip compression seaward beyond the aseismic front and the shift of the depth of the zone may be caused by loading within the descending slab through a mechanical coupling between the plates at the focal area of the 1978 earthquake prior to its occurrence. © 1981.
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