• Japan Trench;
  • Kuril Trench;
  • normal faulting.

Elongated fault escarpments on the outer slopes of the western Kuril and Japan trenches have been investigated through detailed swath bathymetric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70°E linear alignment of the escarpments, parallel to the magnetic anomaly lineations, was revealed on the outer slope of the western Kuril Trench. In the Japan Trench north of 39°00′N, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of faulting exists south of 39°00′N. Each topographic profile was decomposed by computer analysis into two curves representing (1) the smoothed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (<10 km) roughness of plateaus and valleys edged by outward- and inward-facing fault escarpments. Throughout the surveyed areas, escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall, but with no distinct increase below that depth. No significant difference is recognized in fault throws towards and away from the trench. It can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis, which is caused by downward bending of the subducting lithosphere. The relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the East Pacific Rise crest. The maximum length and height of escarpments are small in the Kuril Trench compared with those in the Japan Trench, implying a difference in mechanical strength depending on the fault orientation. The crust is weakest along the inherited spreading fabric, second weakest probably along the non-transform offset direction and strongest in directions very oblique to these orientations. Seamounts appear to be more rigid than normal ocean crust, with no particular weak orientations, resulting in fewer but larger faults along the axis of plate bending, as most clearly represented in the subducting Daiichi–Kashima Seamount.