Formation processes of tsunami deposits following the 2011 Tohoku-oki earthquake in the estuary of Odaka District, Minamisoma City, Fukushima Prefecture, northeast Japan

Abstract

The 2011 Tohoku-oki tsunami inundated the Idagawa estuarine lowland in Odaka District, Minamisoma City, to a distance of up to 3.2 km 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.

2掘削地点のコアの岩相解釈に基づく福島県南相馬市小高区における完新世エスチュアリー埋積堆積物中の津波堆積物

“…Core depth 0.25-0 m: A single normally graded sand bed (Facies F) eroded and blanketed below the artificial fill and paddy soil (Facies H). This sand bed was deposited by the tsunami triggered by the 2011 off the Pacific coast of Tohoku Earthquake (Oota et al, 2017). The sediments on the sand bed consist of red roughed muddy and sandy sediments.…”

“…The sediments on the sand bed consist of red roughed muddy and sandy sediments. The muddy sediments are considered to have been formed by suspension at the ponding water stage, because the drainage pump was broken and the study area was submerged for over a year after the tsunami (Oota et al, 2017).…”

Understanding flow characteristics from tsunami deposits at Odaka, Joban coast, using a DNN inverse model

“…This study used the samples from Odaka district of Minamisoma City (Figure 1), which is located 25 km north of Fukushima prefecture (Oota et al, 2017;Koff et al, 2012). The tsunami flooded a wide area with the river, which caused major damage to the properties in the Odaka lowland (Koff et al, 2012).…”