An Andean model of interplate coupling and strain partitioning applied to the flat subduction zone of SW Japan (Nankai Trough)

Abstract

Transcurrent motion is commonly observed in cases of oblique subduction, yet the causes remain controversial. A model of increased interplate coupling linked to flat subduction is proposed as a mechanism for producing strain partitioning at a great distance (several hundred km) from the trench. The model, originally developed for the North Andean margin, is applied to the geodynamics of SW Japan. Here, along the Nankai Trough and below adjacent SW Honshu, flat subduction occurs due to the relatively young age (<20Ma) of the oceanic lithosphere and the buoyancy of the flanking Palau–Kyushu Ridge and the Izu Bonin arc. Although dextral strike-slip motion occurs along the median tectonic line (MTL) in Shikoku as shown by GPS studies, it has remained seismically inactive for the last 1000years. Evidence is presented supporting a second transcurrent fault system to the north of the MTL, the North Chugoku Shear Zone, associated with four M⩾7 and nine additional M⩾6 earthquakes in the past 300years. Dextral strike-slip focal mechanisms for 15 modern and historical events indicate a significant degree of subduction-related strain partitioning manifested by right-lateral strike-slip faulting along the adakitic volcanic arc, some 400km from the Nankai Trough.