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Residual topography and gravity anomalies reveal a tectonic boundary in northeast Japan, which is proposed to represent the offshore continuation of the Median Tectonic Line; the contrast in frictional properties across this structure may control earthquake behaviour there, as recently demonstrated by the giant 2011 Tohoku-oki earthquake.Geological context of the Tohoku-oki earthquake: Dan Bassett et al. use residual topography and gravity anomalies to characterize geological structures in the region of the Tohoku-oki giant megathrust earthquake of March 2011. Their data point to a possible tectonic boundary in northeastern Japan, which may represent the offshore continuation of the Median Tectonic Line. The authors propose that the structure and frictional properties of the overthrusting (upper) plate offshore of northeastern Japan control megathrust coupling and seismogenic behaviour in this earthquake-prone region.The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw ≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture1, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface2. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest–northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150–200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south)3. The megathrust north of the Median Tectonic Line is interseismically locked2, has a history of large earthquakes (18 with Mw > 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake1. In contrast, the megathrust south of this boundary has higher rates of interseismic creep2, has not generated an earthquake with MJ > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 20111. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan. [ABSTRACT FROM AUTHOR]

The structure of a mountain belt reflects the manner in which plate convergence is accommodated in Earth's lithosphere. However, the extent to which orogenic structure is preconditioned by the thermo-mechanical conditions of the converging plates is debated. Here we re-process and analyse existing data on the amount and style of contractional deformation in 30 orogens worldwide and compare this with the lithospheric strength and age of the colliding plates. We find a correlation between orogenic deformation, and specifically the depth at which the crust decouples from the underlying plate, and the age of the lithospheric plate at the time of collision. Orogens formed from Phanerozoic lithosphere, which has high geothermal gradients and weak mantle, are characterized by several under-thrust faults that form in the mid-to-lower crust and moderate amounts of deformation, at less than 35% crustal strain. In contrast, orogens formed on older lithospheric plates, which have greater strength and higher-viscosity mantle, are characterized by a large detachment fault and large amounts of deformation, at about 70% crustal strain. We conclude that inherited lithospheric strength influences the style and amount of plate-tectonic contraction during mountain building, and thus the stability of continental subduction. Our results emphasize the influence of the deep Earth on the structural style of collisional orogens. [ABSTRACT FROM AUTHOR]