Upper Plate Geology Controls the Rupture Area Segmentation of Subduction Zone Earthquake - A Case Study of the Nankai Trough

Abstract:

What controls the earthquake rupture area of megathrust is one of the most fundamental questions in subduction zones. In the Nankai Trough, Japan, three major controlling factors have been proposed; surface topographic highs of the subducting plate such as seamounts, locally strong rigidity of upper plate, and friction property of the plate boundary megathrust with abnormal pore fluid pressure. For example, the topographic highs may control the location of asperity due to stronger coupling. The topographic highs also work as a barrier for rock deformation. From the geological point of view, plate boundary megathrust in the seismogenic zone must be composed of fault rock in the brittle regime because of its temperature range from ~150℃ to ~350℃. The friction behaviors of these fault rocks and the effects of abnormal pore fluid pressure are recently one of the major concerns. The third factor is the mechanical property of the upper plate. The property changes through geological time scale due to the growth of the accretionary prism or tectonic erosion. In the case of the Nankai Trough, a pultonic body is situated beneath the place of epicenter and is suggested to have been functioned as an asperity of 1944 and 1946 earthquakes. We examined the on-land geology of the Shikoku island and the Kii Peninsula, SW Japan, and their basement structure and composition beneath the forearc Kumano Basin. We conclude that middle to late Miocene episodic magmatic intrusion and extrusion now controls the rupture area segmentation of the great earthquakes in the Nankai Trough.