Historical activity of an onshore subduction thrust and related geomorphic changes, northeastern Nankai subduction zone

We show new geomorphic and geologic evidence for historical activity of the onshore deformation front of the Nankai subduction zone, constrained by Holocene tectonic geomorphology, high-resolution borehole stratigraphy, and seismic reflection profile.  Borehole transect across a newly recognized late Holocene fold scarp contains middle to late Holocene fluvial sedimentary units. Structures of these units correlated based on sedimentary facies, diatom assemblages and radiocarbon dating illuminates that around 12-15th century to middle Holocene units are repeatedly folded and cut by a shallowly dipping thrust fault, suggesting multiple seismic events.  In addition, a new high-resolution seismic reflection profile suggests that these structures are associated with west-dipping imbricate thrust faults comprising a deformation front of the onshore subduction zone. This historical fault activity is also consistent with land use changes linked with pre- and post-earthquake flood events revealed by geographic analysis on early-modern maps drawn in early to middle 19th century and contemporaneous documents.  These multidisciplinary observations suggest that the onshore deformation front activated during the AD 1854 Ansei Tokai megathrust earthquake (M8.4),  the most recent historical event,  comprising the northern end of the ruptured area.