Geologic evidence for prehistoric tsunamis and coseismic uplift during the ad 1854 Ansei-Tokai earthquake in Holocene sediments on the Shimizu Plain,central Japan

Abstract

This study examines geological evidence for earthquakes and tsunamis based on data obtained from 14 sediments cores and one trench in the coastal area of the Shimizu Plain, Japan. The Shimizu Plain is located onshore of the Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. On the basis of lithologies, fossil contents, and radiocarbon dating, we identified geological and paleontological evidence for abrupt changes in depositional environments related to coseismic uplift associated with the ad 1854 Ansei-Tokai earthquake. We estimated a maximum coseismic uplift of 1.2 m and post-earthquake gradual subsidence of c. 0.6 m. We also identified four prehistoric tsunami deposits (T-I, T-II, T-III, and T-IV) in shallow-water (from low tide to 40 m depth) muddy bay deposits. The tsunami deposits are 20–70 cm thick, have sharp erosional bases, typically a graded structure, and contain rip-up clasts. ¹⁴C ages date the deposits to 6180–6010 to 5700–5580 cal. yr BP for T-I, 5700–5580 to 5520–5320 cal. yr BP for T-II, 4335–4125 to 4250–4067 cal. yr BP for T-III, and 3670–3540 to 3500–3360 cal. yr BP for T-IV. The interval between the tsunamis is in the order of 500–1000 years, which is significantly longer than the intervals between the four historical tsunamis that have inundated the Shimizu Plain since ad 1096. We also identified three event deposits that may have formed during the ad 1498 Meio or ad 1707 Hoei earthquakes.