Outer slope faulting associated with the western Kuril and Japan trenches

Abstract

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.

A precise bathymetric map of the world’s deepest seafloor, Challenger Deep in the Mariana Trench

“…The control of the inherited seafloor spreading fabric operates when it is oblique to the trench axis by less than 25-30°. In all other cases, the elongated topographic structures are parallel or subparallel to the trench axis (Masson 1991;Kobayashi et al 1998;Nakanishi 2011b). The discrepancy between the elongated structures striking N70°E and the trench axis implies that the structures result from reactivation of the seafloor spreading fabric.…”

“…The structures parallel to the trench axis are located only near the trench axis. The strike of a trench and the inherited seafloor spreading fabric in the subducting plate are commonly considered to be factors controlling the strike of elongated topographic structures on the oceanward slope (e.g., Masson 1991;Kobayashi et al 1998;Nakanishi 2011b). The control of the inherited seafloor spreading fabric operates when it is oblique to the trench axis by less than 25-30°.…”

Constraint on the recurrence of great outer-rise earthquakes from seafloor bathymetry

“…I proceed with a method that does not require knowing when particular faults slipped. Kobayashi et al (1998) obtained detailed bathymetric maps of seafloor approaching the Kurile and Japan trenches. Faults dip both toward and away from the trench.…”

“…The neutral plane is likely at a comparable but somewhat deeper depth. The radius of curvature from the maps of Kobayashi et al (1998) is ∼1000 km. So the bending strain is also ∼3%.…”

“…The largest instrumentally recorded of these events are the 1933 Sanriku Japan earthquake (M w = 8.4) and 1977 Sumba Indonesia earthquake (M w = 8.3) (Lay et al, 2009). Seafloor bathymetry (Kobayashi et al, 1998) and plate kinematics indicate a great M w ≥ 8 outer rise earthquake on the zone of the great Tohoku M w 9 event can occur at most once in 7 megathrust cycles. …”