Faulting history of the Median Tectonic Line active fault system at Tokushima plain in the easternmost part of Shikoku, southwest Japan

Abstract

The Median Tectonic Line (MTL) active fault system is one of the most active intraplate faults in Japan. The fault system, more than 300 km long, is a right-lateral strike-slip fault with average slip rate of 5-10 mm/y in east Shikoku and 1-3 mm/y in western Kii Peninsula.We have investigated the faulting history of the Naruto-minami and Itano faults of the MTL active fault system at Tokushima plain in the easternmost part of Shikoku. The Naruto-minami and Itano faults juxtapose the north-side Cretaceous Izumi Group and the south-side Sambagawa Metamorphic Rocks. The south-side-down vertical displacement of the basement rocks attains to more than 1, 000 m. These faults also displace the Late Pleistocene and Holocene deposits and warp the ground surface. The average vertical slip rate of the Naruto-minami fault is estimated at approximately 1 mm/y.We recognized four surface faulting events of the Naruto-minami fault at Danzeki Ooshiro site, Naruto City, from angular unconformity. The Itano fault at Kawabata B site in Itano Town cuts all the sediments except for cultivated soils. Three surface faulting events were recognized from upward fault terminations. Integrating our survey results with previous reports, we conclude that the MTL active fault system from the Naruto-minami fault to at least the Mino fault forms a single rupture segment. The most recent rupture on this segment occurred in the 16th century A.D. or later. The timing of the penultimate event is around 2, 000 years ago. The rupture segment in east Shikoku may extend to the Kii Strait on the south of Awaji Island, but more precise investigations are necessary to determine the boundary between the rupture segments in east Shikoku and western Kii Peninsula.