The accumulation of data sets of past tsunamis is the most basic but reliable way to prepare for future tsunamis because the frequency of tsunami occurrence and their magnitude can be estimated by historical records of tsunamis. Investigation of tsunami deposits preserved in geological layers is an effective measure to understand ancient tsunamis that occurred before historical records began. However, the areas containing tsunami deposits can be narrower than the area of tsunami inundation, thus resulting in underestimation of the magnitude of past tsunamis. A field survey was conducted after the 2010 Chile tsunami and 2011 Japan tsunami to investigate the chemical properties of the tsunami-inundated soil to examine the applicability of tsunami inundation surveys considering water-soluble salts in soil. The soil and tsunami deposits collected in the tsunami-inundated areas are rich in water-soluble ions (Na+, Mg2+, Cl-, Br- and SO 4 2-) compared with the samples collected in the non-inundated areas. The analytical result that the ratios of Na+, Mg2+, Br- and SO 4 2- to Cl- are nearly the same in the tsunami deposits and in the tsunami-inundated soil suggests that the deposition of these ions resulting from the tsunami inundation does not depend on whether or not tsunami deposits exist. Discriminant analysis of the tsunami-inundated areas using the ion contents shows the high applicability of these ions to the detection of tsunami inundation during periods when the amount of rainfall is limited. To examine the applicability of this method to palaeotsunamis, the continuous monitoring of water-soluble ions in tsunami-inundated soil is needed as a future study.
Bibliographical noteFunding Information:
We gratefully acknowledge the support of UNESCO for the field survey in Chile and of the organizer of the 2011 Tohoku Earthquake Tsunami Joint Survey Group for the field survey in Japan. The field survey in Japan was conducted with the collaboration of Dr. Tsutomu Sakakiyama, Dr. Gaku Shouji and Dr. Shinya Umeta. We are indebted to the chemical analysis by Arisa Matsuoka and to the grain-size measurement by Toshinori Sasaki and Koichi Okuz-awa. This research was partly funded by JST-JICA’s Science and Technology Research Partnership for Sustainable Development (SATREPS) and the NEDO Industrial Technology Research Grant Program (Project ID: 08E52010a). The manuscript benefitted from constructive reviews from two referees.
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- The 2010 Chile tsunami
- The 2011 Japan tsunami
- Tsunami inundation limit
- tsunami deposit
- water-soluble ions