Slip Inversion Along Inner Fore‐Arc Faults, Eastern Tohoku, Japan

Abstract

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 5.6–2.2 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 that 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 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.

Tectonic Influences on Trench Slope Basin Development via Structural Restoration Along the Outer Nankai Accretionary Prism, Southwest Japan

“…These parameters include displacement, trishear angle, trishear angle offset, trishear apex, and propagation to slip (p/s) ratio (Allmendinger, 1998;Allmendinger et al, 2004) and are further defined in the supporting information file ( Figure S1). This approach has also been successfully used in other trishear restoration models in convergent margin settings (Boston et al, 2016;Ghisetti et al, 2016;Regalla et al, 2017). 2011; our horizon M is just below their Horizon N; MTD indicates a mass transport deposit.…”

K‐Ar Dating of Fossil Seismogenic Thrusts in the Shimanto Accretionary Complex, Southwest Japan

“…At the Middle America Trench in Costa Rica, an inner forearc mountain range and active thrust belt is related to subduction of the Cocos Ridge (Fisher et al, ; Morell et al, ; Sitchler et al, ). Active out‐of‐sequence faults are also found in the inner wedge along the Cascadia margin (Morell et al, ; Sherrod et al, ) and along the Japan margin (Regalla et al, ). In the Kodiak accretionary complex, Paleocene igneous intrusives have been attributed to subduction of the Kula‐Farallon (Moore et al, ) or Kula‐Resurrection (Haeussler et al, ) spreading center.…”

Stress Evolution During the Megathrust Earthquake Cycle and Its Role in Triggering Extensional Deformation in Subduction Zones

“…The relationship between megathrust earthquakes and extensional forearc structures in the absence of recent activity is more speculative. Several other normal faults in eastern Japan may accumulate their offsets during megathrust events over multiple earthquake cycles (Regalla et al, 2017). Similarly, mapped normal faults in southwestern Mexico (Gaidzik et al, 2016) and southern Iran (Normand et al, 2019) occur in areas otherwise characterized by shortening.…”