The tectonics of the southwestern Kuril arc are a result of the oblique subduction of the Pacific plate at the Kuril Trench. In association with forearc sliver migration caused by the oblique subduction, collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin. As a result of the collision, a deep crustal section of island arc is observed at the leading margin of the forearc sliver. Tectonics of the Kuril arc related to oblique subduction are different from those of the western Sunda arc, where backarc spreading occurs at the leading margin. This difference is due to margin morphology of the oblique subduction zone.