On-land active thrust faults of the Nankai–Suruga subduction zone: The Fujikawa-kako Fault Zone, central Japan

Abstract

This paper describes the tectonic topography that characterizes recent thrusting along, the on-land active fault zone of the Nankai–Suruga subduction zone, called the Fujikawa-kako Fault Zone, located near the triple junction of the Eurasian (EUR), Philippine Sea (PHS), and North American (NA) plates, in the western side of Mt. Fuji, central Japan. The analysis was based on interpretations of aerial photographs and 3D perspective images made with Digital Elevation Model (DEM) data, field investigations, and trench excavations. Our study shows the following new observations: 1) distinct east-facing fault scarps are developed on the west-facing slopes, alluvial fans, and terraces of western Mt. Fuji; ii) the total length of the fault zone is ~ 40 km; iii) the Older stage (ca. 8000–14, 000 yr) Fuji lavas have been displaced by as much as 70 m; and iv) the 864–865 AD Jogan lava flow is displaced by 2–4 m vertically along the scarp at the northeastern end of the fault zone. Based on the offsets of lavas and mudflow deposits, as well as historical records, it is found that i) the vertical slip rate for the fault zone is up to 5–8 mm/yr, ii) the recurrence interval of morphogenic earthquakes is estimated to be 150–500 yr, and iii) the most recent seismic faulting event along the Fujikawa-kako Fault Zone is inferred to be related to the 1854 AD (M 8.0–8.5) Ansei-Tokai earthquake. When compared with the active intraplate faults of Honshu, Japan, the relatively high slip rates and short recurrence intervals for morphogenic earthquakes in the Fujikawa-kako Fault Zone indicate that the activity of this fault zone is closely related to subduction-zone earthquakes and plate convergence near the triple plate junction of the EUR, PHS, and NA plates.