ALBERT

Beschreibung: The kinematics of deformation in the overriding plate of convergent margins may vary across timescales ranging from a single seismic cycle to many millions of years. In Northeast Japan, a network of active faults has accommodated contraction across the arc since the Pliocene, but several faults located along the inner forearc experienced extensional aftershocks following the 2011 Tohoku-oki earthquake, opposite that predicted from the geologic record. This observation suggests that forearc faults may be favorable for stress triggering and slip inversion, but the geometry and deformation history of these fault systems are poorly constrained. Here we document the Neogene kinematics and subsurface geometry of three prominent forearc faults in Tohoku, Japan. Geologic mapping and dating of growth strata provide evidence for a 5.6-2.2 Ma initiation of Plio-Quaternary contraction along the Oritsume, Noheji and Futaba faults, and an earlier phase of Miocene extension from 25 to 15 Ma along the Oritsume and Futaba faults associated with the opening of the Sea of Japan. Kinematic modeling indicates these faults have listric geometries, with ramps that dip ~40-65° W and sole into subhorizontal detachments at 6-10 km depth. These fault systems can experience both normal and thrust sense slip if they are mechanically weak relative to the surrounding crust. We suggest that the inversion history of Northeast Japan primed the forearc with a network of weak faults mechanically and geometrically favorable for slip inversion over geologic timescales, and in response to secular variations in stress state associated with the megathrust seismic cycle.