Holocene activity of the Asamigawa fault detected from sediment cores and a ground-penetrating radar cross-section in the Beppu area, southwestern Japan

Abstract

The Beppu area, on the northeast coast of Kyushu Island, southwest Japan, is characterized by hydrothermal activity, tectonic deformation, and recent volcanism related to subduction of the Philippine Sea plate under the Japan arc. Since ca. 5 Ma, subsidence has occurred in this region. We analyzed the sedimentary facies, diatom assemblages, and radiocarbon ages of four drill cores to estimate Holocene activity on the Asamigawa fault in Beppu City. A ground-penetrating radar profile was recorded across one of these drilling sites to improve our understanding of subsurface structures. Analyses of these data showed the following sedimentary facies: the Middle Pleistocene Otobaru lava, a lower fluvial sediment, marsh to shallow marine sediment, and an upper fluvial sediment, in ascending order, and an overlying layer of artificial soil. The vertical offsets at the fault of an uplifted marine terrace and the Kikai-Akahoya volcanic ash indicate an average vertical slip rate on the Asamigawa fault of ca. 5 mm/yr since 7300 cal BP. Floodplain sediments observed 3–4 m below present sea level indicate that the latest fault slip probably occurred after 600 cal BP, and may have been related to the 1596 CE Bungo-Keicho Earthquake. Evidence of rapid accumulation of floodplain sediment and faulting-related tilting of strata near the fault indicate that the latest faulting event was at ca. 1900 cal BP, so the minimum interval between these two events was about 1300 years.