Abstract
Japanese historical documents reveal that Mw 8 class earthquakes have occurred every 100–150 years along the Suruga and Nankai troughs since the 684 Hakuho earthquake. These earthquakes have commonly caused large tsunamis with wave heights of up to 10 m in the Japanese coastal area along the Suruga and Nankai troughs. From the perspective of tsunami disaster management, these tsunamis are designated as Level 1 tsunamis and are the basis for the design of coastal protection facilities. A Mw 9.0 earthquake (the 2011 Tohoku-oki earthquake) and a mega-tsunami with wave heights of 10–40 m struck the Pacific coast of the northeastern Japanese mainland on 11 March 2011, and far exceeded pre-disaster predictions of wave height. Based on the lessons learned from the 2011 Tohoku-oki earthquake, the Japanese Government predicted the tsunami heights of the largest-possible tsunami (termed a Level 2 tsunami) that could be generated in the Suruga and Nankai troughs. The difference in wave heights between Level 1 and Level 2 tsunamis exceeds 20 m in some areas, including the southern Izu Peninsula. This study reviews the distribution of prehistorical tsunami deposits and tsunami boulders during the past 4000 years, based on previous studies in the coastal area of Shizuoka Prefecture, Japan. The results show that a tsunami deposit dated at 3400–3300 cal BP can be traced between the Shimizu, Shizuoka and Rokken-gawa lowlands, whereas no geologic evidence related to the corresponding tsunami (the Rokken-gawa–Oya tsunami) was found on the southern Izu Peninsula. Thus, the Rokken-gawa–Oya tsunami is not classified as a Level 2 tsunami.
Highlights
The 2011 off the Pacific coast of Tohoku Earthquake, which is the largest earthquake (Mw 9.0) recorded in Japan, occurred on 11 March 2011 off the Pacific coast of the northeastern Japanese mainland (Fig. 1)
Based on the lessons learned from the 2011 Tohoku-oki earthquake, the Cabinet Office, Government of Japan (2013), has presented information on the two types of tsunamis expected in the future in the Suruga and Nankai troughs, where the Philippine Sea Plate is subducting beneath the Eurasian Plate (Fig. 1) (Goto et al 2014a)
Previous studies have examined the 2011 Tohoku-oki tsunami deposit in many parts of the 900 km long coastline between southwestern Hokkaido and the northern Boso Peninsula (Fig. 1), except for the area near the Fukushima–Daiichi Nuclear Plant, which was severely damaged by the 2011 Tohoku-oki earthquake and the subsequent mega-tsunami
Summary
The 2011 off the Pacific coast of Tohoku Earthquake, which is the largest earthquake (Mw 9.0) recorded in Japan, occurred on 11 March 2011 off the Pacific coast of the northeastern Japanese mainland (Fig. 1). Deposit has been reported from the coastal plain of northern Fukushima Prefecture (Shishikura et al 2010; Sawai et al 2012) These previous studies, which were conducted before the 2011 Tohoku-oki earthquake, demonstrate that tsunami deposits can reveal information about the occurrence and extent of mega-tsunamis over time scales of several thousand years. Based on the lessons learned from the 2011 Tohoku-oki earthquake, the Cabinet Office, Government of Japan (2013), has presented information on the two types of tsunamis expected in the future in the Suruga and Nankai troughs, where the Philippine Sea Plate is subducting beneath the Eurasian Plate (Fig. 1) (Goto et al 2014a). The other type of tsunami is the largest-possible tsunami (Level 2 tsunami) caused by the largest-possible earthquake (Mw 9.1; Cabinet Office, Government of Japan, 2012) that occur along megathrusts in the Nankai Trough (Fig. 2). The sedimentary characteristics and spatial distributions of the tsunami deposits formed by the Tohoku-oki tsunami are used as a reference for determining the deposits that would be generated by a Level 2 tsunami in the coastal area of Shizuoka Prefecture, Japan
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