Transportation and Sedimentation of Beach and Sea-floor Gravels Caused by Tsunami

An Example at Attari Coast, Iwate Prefecture

Abstract

Gravel beds presumably formed by the Meiji-Sanriku-Tsunami (1896) Occur immediately below a large tsunami stone at Attari Coast, Ohfunato-City, northeast Japan. The stone weights approximately 10 tons and was transported by the tsunami from a nearby sea-floor. Gravel (mostly coarse-grained slate) beneath the tsunami stone consists of either rounded beach gravel or angular breccias that pre-existed on the beach or on the sea floor (probably deeper than the upper shoreface). Granulometric analyses of the breccias show a consistent decreae of size and number toward inland, whereas there is an increasing size and number of rounded beach gravel toward the foot-hill. The facts suggest that the tsunami (flood) was transported beach gravel inland during the deposition of angular clasts from the sea-floor.