Abstract Devastating earthquakes and tsunamis capable of causing catastrophic damage to human societies and economies have frequently occurred on and around the Japanese islands. Because Japan is a long and narrow island arc in the Pacific Ocean located at the junction of four plates (the Eurasian and Philippine Sea Plates in the SW and the North America and Pacific Plates in the NE), it has probably suffered the highest number of earthquake and tsunami events anywhere in the world. Hence, geological and geoinformation investigations are supremely important, not only for understanding the geological development of Japan but also for forecasting the risks associated with geohazards and securing the safety of human lives and infrastructures. For these reasons, the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ-AIST) has been conducting marine geological surveys since 1974 and, as of the end of 2019, produced a Marine Geology Map Series consisting of 90 geological maps. In addition, a coastal zone research project that aims to connect marine, coastal and land area geoinformation seamlessly was launched in 2008 to survey areas that have not yet been investigated, and six maps of geologically distinct areas have already been published based on that collected data.

ABSTRACT We describe and interpret a system of well-preserved normal and reverse faults in the Kayo Formation of the Miocene Shimanto belt, an exhumed accretionary complex exposed on Okinawa Island. The normal and reverse fault systems both strike NE-SW, suggesting systematic horizontal stress variations between compression and extension. Temperature and pressure conditions for the normal and reverse fault systems were estimated from the densities of water in fluid inclusions in the veins along the faults, and previously reported maximum paleotemperature based on values of vitrinite reflectance and illite crystallinity. The fluid inclusion analyses yielded similar estimates for water density in both normal and reverse fault systems. The minimum geothermal gradient was constrained to a narrow range of 40–50 °C/km. These results suggest that the normal and reverse fault systems developed at a similar depth within the seismogenic zone. This can be interpreted as a change between horizontal compression and horizontal extension occurring at a maximum depth of 3.8–7.5 km below the seafloor, assuming lithostatic fluid pressure. This 90° rotation of the principal stress could be controlled by the seismic cycle, as exemplified by the rotation of stresses that occurred after the Tohoku-Oki earthquake.