A deep strong reflector in the Nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release

Abstract

We present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the Nankai accretionary wedge off Shikoku Island. On the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both Oligocene‐Miocene accretionary prism and the sedimentary cover (unit A), a unit of underthrust Miocene‐Pliocene hemipelagic sediments (unit B), and a unit of subducting pre‐Miocene oceanic crustal rocks (unit C). According to relative strength of the topmost reflector of unit B the Nankai accretionary wedge can be divided into three different structural zones along the cross‐strike direction: apparent décollement reflector zone, low‐amplitude reflector (LAR) zone, and deep strong reflector (DSR) zone. Despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error, the décollement step‐down and the possible duplex structures in the LAR and DSR zones support that the landward thickness variation of unit B is caused by the structural thickening such as the underplating process. Structural features and reverse polarity suggest that the DSR as an upper interface of the underplated layer (unit B) corresponds to a shear plane or a fault zone with high pore fluid pressures. We found that most of the DSR was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period. The DSR as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the Nankai seismogenic zone. Considering all of the DSR characteristics, we propose a hypothesis for the DSR's development and role. The DSR may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the Nankai seismogenic zone. A good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the DSR as a shear plane.