Geological records of storms, tsunamis and other extreme events

Abstract

The rapidly expanding human occupation of coastal areas results in an ever increasing threat from devastating geo- and meteorological hazards such as earthquakes, tsunamis, storms and floods. Research on the frequency and impact of historical or prehistorical disasters is therefore essential in improving our understanding of their risk so as to be better prepared for future events. The geological record provides the way to understand the prehistorical record of hazard events and is complementary to the short timescale of the historical record. Because of their long time-scales, geological records inform on low-frequency and/or high-consequence disasters that our highly organized modern civilization has never experienced. After the recent major disasters of the 2004 Indian Ocean and 2011 Tohoku Earthquake and Tsunamis, there is an increasing social demand for geohazard research and based on these events major advances in understanding have been made. The international symposium “Tsunami hazards and risks: using geological record” provided a forum for these recent advances on hazard research. The symposium was co-hosted by the Geological Society of Japan and the Geological Society of London, and held at the 121st Annual Meeting of the Geological Society of Japan in Kagoshima in September 2014. This thematic issue was planned in association with the symposium, and includes 6 papers, which we briefly introduce here. Descriptions of modern tsunami deposits are essential in providing more reliable identification of tsunami deposits in prehistorical sedimentary strata, and in the more accurate estimation of inundation areas of past tsunamis. Matsumoto et al. (2016) describe detailed grain size and sedimentary structures of tsunami deposits of the 2011 Tohoku earthquake, and instability of the sedimentary characteristics that probably comes from local topography and also differences in vegetation. They also report on the discordance between inundation distance and inland distribution of the tsunami deposits that can cause considerable underestimation of past tsunami inundation. Geochemical analysis is a powerful technique to identify the source of the sediments, and grows increasingly important in paleoenvironmental, paleotsunami and paleostorm studies. This special issue includes two recent advances in geochemical studies. Shinozaki et al. (2016) measured stable carbon isotope ratios, biomarkers and water-leachable ions in deposits of the 2011 Tohoku tsunami and preexisting surface soils, and examined temporal changes in these geochemical characteristics. They found that concentrations of tsunami-derived water-leachable ions were highest in the soil immediately beneath the tsunami deposit, but were diluted to be undetectable 3 years after the tsunami struck, probably due to rainfall and leaching. Unlike the previous study (Shinozaki et al. 2015), marine biomarkers are not found in this study, implying that more research on biomarker preservation is needed. Chagué-Goff et al. (2016) revealed long-term environmental changes and seawater intrusion events at Lake Tiriara, Mangaia, Cook Islands during the past ca. 3,500 years. They studied a continuous high resolution elemental profile of a peat sequence using aITRAX core scanner, and processed the ITRAX data with principal component analysis to group elements as detrital, organic matter, biogenic and marine origin. Based on geochemical signatures as well as microfossil contents, they propose one cyclone deposit and three probable paleotsunami deposits in the peat sequence. Many boulders on intertidal flat and subaerial lowlands are considered as consequences of tsunami transportation. Coastal boulders have a high potential for being effective recording mediums of past coastal disasters. However, some of them can be moved by single or multiple storms. Though sedimentological and theoretical methods to distinguish between tsunami and storm wave boulders have been proposed, identification of tsunami boulders is often considerably difficult with many uncertainties. Watanabe et al. (2016) developed a numerical scheme to differentiate tsunami and storm wave boulders and applied it on Ishigaki Island, Japan. They estimate the maximum storm wave at the study site from numerical calculations that explains the distribution of storm wave boulders on the reef. On the other hand, coral boulders on the shoreline much exceed the boulder size that could be transported by storm wave at the study site. Because the island was severely damaged by the 1771 Meiwa tsunami, the authors numerically estimate the period and amplitude of the tsunami based on the boulder distribution with the assumption that the boulders were transported by the 1771 Meiwa tsunami. This study provides reasonable hydraulic parameters of past tsunamis, and validates the usefulness of tsunami boulders for disaster reduction. Examples of past environment–society interactions improve our understanding of the impact of future environmental changes on human societies. Goff & Nunn (2016) review social change data on Pacific Ocean islands to obtain insights into environmental changes in the 14th and 15th centuries. Yamada et al. (2016) provide detailed description on erosion and deposition of a flooding event in July 2013 in Japan. Their observations will help not only identifying flooding events in sedimentary strata but also understanding development of flood plains in mountainous countries including Japan.