Slip inversion along inner fore-arc faults, eastern Tohoku, Japan

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 fore arc experienced extensional aftershocks following the 2011 Tohoku-oki earthquake, opposite that predicted from the geologic record. This observation suggests that fore-arc 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 fore-arc faults in Tohoku, Japan. Geologic mapping and dating of growth strata provide evidence for a 5.6-2.2Ma initiation of Plio-Quaternary contraction along the Oritsume, Noheji, and Futaba Faults and an earlier phase of Miocene extension from 25 to 15Ma along the Oritsume and Futaba Faults associated with the opening of the Sea of Japan. Kinematic modeling indicates that these faults have listric geometries, with ramps that dip similar to 40-65 degrees W and sole into subhorizontal detachments at 6-10km 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 fore arc 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.