4:06 PM - 4:24 PM
[U02-08] Progress in tsunami deposit study after the 2011 Tohoku Earthquake
★Invited Papers
Keywords:2011 Tohoku earthquake, 869 Jogan earthquake, Tsunami deposit
Tsunami deposit has been attracting attention since the 2011 Tohoku earthquake, but it used as one of the methods for paleoseismological study since the late 1980s in Japan. In particular, the tsunami deposits of the 869 Jogan earthquake have been already investigated by Tohoku University and others 30 years ago, and the possibility of a giant tsunami in the near future was indicated based on the recurrence interval. In the 2000s, GSJ/AIST proposed the fault models for the Jogan earthquake as an inter-plate earthquake along the Japan Trench. These data were considered into the long-term evaluation for the subduction zone earthquake along the Japan Trench by the government's Headquarters for Earthquake Research Promotion (HERP), but the 2011 Tohoku Earthquake occurred just before the announcement. Although the information could not be distributed socially, the tsunami inundation area was consequently almost overlapped between the 2011 Tohoku Earthquake and the 869 Jogan earthquake. As a result, the usefulness of the tsunami deposit study for disaster prevention was recognized, and the tsunami deposit survey was conducted everywhere in Japan. The amount of information on tsunami deposits has increased drastically in the past decade.
On the other hand, a number of problems have emerged immediately after the 2011 Tohoku earthquake. To solve them, new analyses have been conducted in the past decade. One of the issues is the accurate assessment of tsunami inundation magnitude, which are two perspectives of micro and macro.
The inundation area of past tsunamis has usually been reconstructed based on the distribution of visible sand layers. However, the 2011 tsunami inundation in the coastal plains reached further inland than the distribution of sandy deposits. In other words, an ordinally geological method using visible sand layers may underestimate the actual inundation area, which also affects the constraint for assuming the source fault. To develop the identification method for tsunami deposits, various analyses such as microfossil, grain size, and organic geochemistry was being applied to the deposits formed by the 2011 tsunami as a modern analog.
The issue from the macroscopic viewpoint is the verification of the maximum class tsunami assumption. After the 2011 Tohoku earthquake, the national and local governments reviewed their tsunami disaster prevention measure, and the maximum class inundation assumptions were established in many areas. However, it is unclear whether such tsunamis actually occurred in the past. It has to examine the existence of tsunami deposits around the inland limit of the assumed inundation area.
In this presentation, I will show the examples of these studies and discuss the future prospects.
On the other hand, a number of problems have emerged immediately after the 2011 Tohoku earthquake. To solve them, new analyses have been conducted in the past decade. One of the issues is the accurate assessment of tsunami inundation magnitude, which are two perspectives of micro and macro.
The inundation area of past tsunamis has usually been reconstructed based on the distribution of visible sand layers. However, the 2011 tsunami inundation in the coastal plains reached further inland than the distribution of sandy deposits. In other words, an ordinally geological method using visible sand layers may underestimate the actual inundation area, which also affects the constraint for assuming the source fault. To develop the identification method for tsunami deposits, various analyses such as microfossil, grain size, and organic geochemistry was being applied to the deposits formed by the 2011 tsunami as a modern analog.
The issue from the macroscopic viewpoint is the verification of the maximum class tsunami assumption. After the 2011 Tohoku earthquake, the national and local governments reviewed their tsunami disaster prevention measure, and the maximum class inundation assumptions were established in many areas. However, it is unclear whether such tsunamis actually occurred in the past. It has to examine the existence of tsunami deposits around the inland limit of the assumed inundation area.
In this presentation, I will show the examples of these studies and discuss the future prospects.