Identifying possible tsunami deposits on the Shizuoka Plain, Japan and their correlation with earthquake activity over the past 4000 years

Abstract

Three washover sand beds, ranging from 15 to 34 cm in thickness, were recorded in a lagoonal mud sequence on the Shizuoka coast of central Japan, which faces the Suruga Trough. The sand beds were composed of well-sorted and well-rounded beach sand derived by a marine inundation. The basal erosion surface, mud clasts, and presence of both inverse and normal grading suggest that the sand beds formed as a result of high-energy deposition. The sand beds are multilayered, with fine alternations of sand sheets and mud drapes, which are consistent with deposition from a long-period wave train. Radiocarbon dates obtained from the three washover deposits are around ad 1000, 3565-3486 cal. yr BP, and 4000 cal. yr BP. Vertical changes in the diatom assemblages suggest a gradually increasing inflow of seawater up to the second sand bed, followed by a rapid change to freshwater conditions just above the bed. The gradual increase of seawater inflow begins again just above the second sand bed. We conclude that this series of sequential environmental changes indicates interseismic subsidence and coseismic uplift. The study area experienced around 1 m of uplift during the ad 1854 Ansei Tokai earthquake (M w 8.4). In contrast, the area has subsided gradually (c. 6 mm/yr) during the present interseismic period. The youngest sand bed may be correlated with the ad 1096 Eicho earthquake, which caused severe damage along the Shizuoka coast.

A systematic review of geological evidence for Holocene earthquakes and tsunamis along the Nankai-Suruga Trough, Japan

“…Evidence for potential tsunami inundation at Ōike and Suwa Ponds in segment C (Tsuji et al, 2002), Nagaya MotoYashiki in segment D (Takada et al, 2002) and the Ōtagawa Lowlands and Ōya Lowlands (Kitamura et al, 2013a) in segment E support this interpretation (Fig. 4j).…”

“…The lateral extent of the coarse-grained deposits remains uncertain, in particular for the youngest and oldest layers which are each found in only one core. An increase in freshwater diatom species across the middle sand layer is suggestive of coseismic uplift, however the magnitude of this change is not quantified and Kitamura et al (2013a) base their interpretations on a limited number of widely spaced samples with low total diatom count numbers. The youngest sand layer contains a peach seed radiocarbon dated to 790 -930 cal.…”

“…yr BP (AD 1020 -1160), as well as a 6 th century AD pottery fragment. Kitamura et al (2013a) correlate this deposit with the AD 1096 Eichō tsunami. The two older sand layers are bracketed by radiocarbon dates, allowing the development of a sequence model (Supp.…”

“…3, site 64) preserve a sedimentary record exceeding 7300 years (Kitamura and Kobayashi, 2014b;Kitamura et al, 2011Kitamura et al, , 2013a. Seven cores of up to 9 m in length map the stratigraphy at the site.…”

Examination of the largest-possible tsunamis (Level 2) generated along the Nankai and Suruga troughs during the past 4000 years based on studies of tsunami deposits from the 2011 Tohoku-oki tsunami

“…1). In addition, the main morphological elements from marsh through beach ridge to the open Pacific Ocean are similar in both the coastal areas of Shizuoka Prefecture Kitamura et al 2013aKitamura et al , b, 2015aKitamura and Kobayashi 2014a, b;Kitamura and Kawate 2015) and the Tohoku area (e.g., Goto et al 2011;Abe et al 2012;Takashimizu et al 2012;Koiwa et al 2014;Ishimura and Miyauchi 2015). Therefore, 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.…”