Fault reactivation and active tectonics on the fore-arc side of the back-arc rift system, NE Japan

Positive basin inversion is documented using seismic reflection profiles and seismic data on the fore-arc side of the back-arc rift system of northern Honshu, Japan. This finding within a seismically active region is used to understand source fault geometry based on fault reactivation models. Seismic reflection profiles and Bouguer gravity anomalies along with available surface and subsurface evidence define a >300-km-long normal fault system, formed in Miocene time, that bounds the fore-arc side of the Miocene back-arc basin.Based on the geometry of the seismogenic source fault which was estimated from aftershock distribution and seismic reflection profiles, reverse fault reactivation of former Miocene normal faults occurred and is presently occurring across the entire seismogenic zone. This contractional event began in late Neogene time. The total amount of shortening as inferred from sequential cross section restoration is approx. 20% of the total amount of Miocene extension in the fore-arc side of the northern Honshu. Thus, the overall extensional architecture of the region inherited from Miocene rifting was only slightly modified by the superimposed contractional deformation. Reverse fault reactivation seems to occur only when the dip angle of precursor normal faults is 60° or less. The geometry and evolution of active thrust faults are strongly controlled by fault reactivation processes and are found in conjunction with the development of new thrust faults that propagate across the normal fault footwalls with shortcut trajectories.