2017 Volume 76 Issue 1 Pages 3-16
The 2011 Tohoku-oki tsunami inundated the Idagawa estuarine lowland in Odaka District, Minamisoma City, to a distance of up to 3.2km inland from the shoreline. In this study, the formation processes of the tsunami deposits is reconstructed based on the pathways of the tsunami, facies of the tsunami deposits. The tsunami deposits are divided into three main units (Units 1-3, in ascending order), and Unit 1 is subdivided into Subunits 1A-1C. Subunit 1A consists of fine sand and was deposited by the encroached flow through the drainage channel in the earliest stage of the tsunami. Subunit 1B comprises an assortment of gravels and was formed from the overflow of a sea dike and river bank. Subunit 1C consists of medium to fine sand and was formed from the flooded flow. Most of these subunits formed from the run-up flow of the first tsunami wave with a remarkably high water level. However, clear erosional contacts are occasionally recognized in the subunits, which were formed from minor run-up flows after the secondary wave. Unit 2 consists of poorly sorted muddy fine sand and was formed from the return flow due to a drop in water level in the latter half of the tsunami. Unit 3 consists of massive mud settled down from the ponding water after the tsunami.
