Geological Background

A tectonic graben within the central Kyushu Island, named the Hohi Volcanic Zone (HVZ), is buried by Plio-Pleistocene volcaniclastic materials. It is accompanied by affluent indicators of tensile stress and is often related to the N-S breakup of the continental crust of Kyushu. Such a hypothesis is, however, discordant with recent horizontal movements detected by global navigation satellite system (GNSS)-based analysis. The latest remote sensing data rather accord with the deformation field being provoked by westward indentation of the forearc sliver of southwest Japan according to intermittent dextral slips along the Median Tectonic Line (MTL). Although the MTL has a long-standing and complicated history of movements since the Cretaceous, it has been activated as a reverse, and then, right-lateral fault system under control of the convergence modes of the Philippine Sea Plate. The active MTL trace coincides with southern margin of the HVZ, and the right-stepping configuration of the dextral fault system resulted in the growth of a pull-apart basin around Beppu Bay