Geochemical characteristics of paleotsunami deposits from the Shizuoka plain on the Pacific coast of middle Japan

摘要

The Pacific coast of Japan has repeatedly suffered earthquakes with magnitudes greater than M-w 8 and accompanying tsunami on coastal areas. Estimating the inundation area of paleotsunami using event deposits in sediment layers can inform and reduce possible damages from future earthquakes and tsunami. In addition to geological and sedimentological analyses, the geochemical signature of sediments can be an effective proxy for identifying event deposits in many cases. However, few geochemical analyses have been applied to study the Pacific coast of middle Japan. Therefore, the geochemical characteristics of paleotsunami deposits (similar to 1000, 3500, and 4000 cal BP) were assessed for core samples from the Shizuoka plain (Oya lowland) on the Pacific coast of middle Japan. In a ternary diagram for (Na2O+CaO)-Al2O3-(Cr+Ni), our data from the paleotsunami deposits from the Shizuoka plain plotted between those of the beach core samples (shoreface and dune deposits) and mud layers in the back marsh deposits. Additionally, vertical and horizontal distributions of titanium normalized values (Na/Ti, Sr/Ti, Ba/Ti, and Cr/Ti atomic ratios) and Si/Al atomic ratios in the cores from the Shizuoka plain provided important clues for discrimination of the paleotsunami deposits from other layers, such as flood deposits from river overflow and mud layers that settled in calm environments. Based on the cluster analysis, the data from the paleotsunami deposits (similar to 3500 cal BP) were discriminated from those of other layers in our case.