The Nankai Trough marks the convergent boundary between the oceanic Philippine plate and the continental Eurasian plate. The trough extends from central Honshu Island, where a triple junction separates it from the Japan Trench, southwest along the platform from which the southwestern Japanese islands emerge. The direction of plate convergence is nearly normal to the regional trend of the continental slope and proceeds at about 2 mm/yr.

The northern Philippine plate is covered with a 1.5- to 2-km thick sediment sequence near the area of the trough. The ocean crust flexes downward as it enters the trough, but apparently flexure is not enough to produce the normal faulting commonly found on the seaward slope of deep ocean trenches. The Philippine plate sediment sequence was sampled at DSDP site 297 (Karig et al., 1975). The sediment consists mainly of hemipelagic muds, thin layers of sand and silt, and some local sand turbidites. This sediment sequence begins to fail compressionally in front of the trough, and at the topographic trough it separates into an offscraped sequence above a major decollement and a subducted sequence below it. Approximately 1300 m of soft sediment are thus detached from the subducting plate to form an accretionary complex at the front of the Eurasian plate, whereas the lower 600 m remain attached to the oceanic plate and are subducted below the front of the margin.

The structure and tectonic history of the Eurasian plate is poorly known in the area from the front of the margin