**1. Introduction**

Southwest Japan is a continental sliver rifted during the Miocene opening of the Japan Sea [1]. Its post-rifting tectonic architecture is closely related to the mode of convergence of oceanic plates around the eastern Eurasian margin. It seems that neotectonic deformation in southwest Japan becomes increasingly intense from west to east, which has long been understood as a contraction regime caused by the westerly subducting Pacific Plate (e.g., [2]). Collision between the Eurasian and North American Plates around the eastern margin of the Japan Sea and the Itoigawa-Shizuoka Tectonic Line (Figure 1) is also responsible for the deformation trend [3]. Another remarkable structural trend, a late Neogene strong inversion along the backarc coast of the island arc, is sometimes related to the subduction of the Philippine Sea Plate [4,5]. The Philippine Sea Plate changed its convergent direction in the Quaternary [6], and enhanced right-lateral wrenching within southwest Japan [7].

The authors concentrate upon the Quaternary morphological features in the eastern part of the island arc, and describe conspicuous basin-forming processes in the study area. In a general context, we aim at comprehension of the regional tectonic zones controlling the vigorous formation of intra-arc basins, and at identifying the neotectonic domains divided by them. Geophysical information such as gravity anomalies [8] and reflection seismic data are utilized to visualize the deep interiors of the intra-arc basins and evaluate structural trends in the damaged upper crust surrounded by the tectonic zones. Closer observation of geologic structures leads us on the path to understand the formation processes of subordinate structures in the study area.

© 2013 Itoh et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Itoh et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
