地層中に存在する古津波堆積物の調査

Abstract

After the Tohoku tsunami, demand increased greatly for geologists with expertise in the geology of tsunami deposits, who could study prehistoric tsunami deposits. This paper reviews, for those new to tsunami geology, how geologists have studied tsunami deposits during the last two decades. Preliminary identification of a paleotsunami deposit is based on the identification of anomalous sand deposits in peat or mud that was deposited in a low-energy environment. Anomalous deposits can then be evaluated using several criteria, such as lateral extent of the deposit, changes in thickness and grain size, sedimentary structures, single or multiple vertical grading, floral and faunal fossils within the deposits, synchroneity of environmental changes, and correlation with historical records of earthquakes. In this process, a multi-proxy approach is used to discount non-tsunami processes such as floods and storms. Identified tsunami deposits are dated using radiocarbon, lead-, cesium-, tephrochronology, and optically stimulated luminescence dating methods. For radiocarbon dating, appropriate selection of materials for analysis is especially crucial for constraining the age of a tsunami. Plant macrofossils or fossil insects within samples from sediments immediately over-or underlying a tsunami deposit will constrain its age well, whereas a bulk peat sample will not. The lateral extent of continuous tsunami deposits allows estimation of a minimum area of tsunami inundation. A meaningful reconstruction of the area of inundation, however, also requires adjustments for changes of the shoreline over the last few thousand years. In studies of paleotsunami deposits, we need to be aware of the uncertainties inherent in geological records. For example, geologic dating at a particular site rarely has the precision to distinguish between a single giant tsunami and a series of small ones.

Post‐depositional alteration of shallow‐marine tsunami‐induced sand layers: A comparison of recent and ancient tsunami deposits, Onagawa Bay, northeastern Japan

“…Parallel laminations are one of the major criteria for the recognition of tsunami deposits (e.g. Sawai, ). However, the core data examined in this study reveal that original sedimentary structures (parallel laminations) in the tsunami deposits were possibly modified by bioturbation, and that the tsunamigenic coarse‐grained deposit (sand) can be distinguished from ordinary background deposits (mud) based mainly on textural differences among the sediments in the semi‐enclosed bay.…”

Paleotsunami researches along the Ryukyu Trench

“…1 , , 1968;, 1983;, 1994;Goto et al, 2010aGoto et al, 1771Goto et al, , 1968Goto et al, 1927Goto et al, 1968Goto et al, , 1981 1. , 2007, 2012Paris et al, 2009Paris et al, , 2010Feldens et al, 2012;Nandasena et al, 2013;Yamada et al, 2014 1 2 , Szczuciński, 2012; 50 cm (Goto et al, 2013), (c) storm boulders (after Goto et al, 2013), and (d) potential sandy tsunami deposits (e.g., Yamamoto, 2008;Geshi, 2009;Hojo et al, 2011;Kawana et al, 2011b;Kugai, 2011;Kobayashi, 2013 1968, 198117711968, 19811980, Bourrouilh-Le and Talandier, 1985Young and Bryant, 19921968, 19811968, 198119902 197250 m 1983210 75 BP 17711968, 1981, 1983, , 198719861987, 1986, 1987;, 1987, 1990, 1987...…”

“…The 1625 AD event may be a localized tsunami at Ishigaki Island (e.g., Araoka et al, 2013). , , 1983;, 199414 C Hughen et al, 2004Hideshima et al, 2001Yoneda et al, 20072000, Suzuki et al, 2008Araoka et al, 2010Araoka et al, 2000 U/Th , 2000; Suzuki et al, 2008;Araoka et al, 2010Araoka et al, , 2013, 2010;, 2011a;, 2012;, 2012;, 2013, 1771, Araoka et al 2013 150-400 …”

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

“…Most previous investigations of tsunami sediments in land areas have been carried out in large strand plains, coastal wetlands, and lakes (Sawai, 2012). And many studies on the 3.11 Tsunami deposits were reported on the Sendai Plain and delta plains in Sanriku coastal bays (e.g., Naruse et al, 2012;Abe et al, 2012).…”