4:00 PM - 4:15 PM
[MIS16-09] Paleo-tsunami history on the Joban coast
Tsunami deposits just above the paddy soil of the Middle Yayoi period were identified at the Kutsukata ruin in the Sendai Plain (Matsumoto et al., 2013). The tsunami deposits were dated at356 cal BC to 4 cal AD, and the inundation area inferred from the distribution limit of the sand layer is estimated at 4.2 km, which is comparable to the 869 Jogan and 2011 Tohoku-oki tsunamis. On the other hand, no tsunami deposits in the middle of Yayoi period were identified by earlier tsunami deposit survey from the Joban coast to the Ishinomaki Plain (Sawai et al., 2012). Occurrence of the Yayoi tsunami is still controversial. This study aims to reexamine the Yayoi tsunami deposits in the Joban Coast and to correlate the deposit with those in the Sendai Plain and clarify the extent of the tsunami-affected areas. The study area is Ukedo in Namie Town, Fukushima Prefecture. Sediment samples were retrieved using a handy geoslicer and SCSC borehole drilling. We also collected reports of archaeological sites in the vicinity of the study area and examined relevant giant earthquake and tsunami.
Eight event layers were identified in the sediment cores. Based on the radiocarbon dating results, seven layers were associated with prehistoric events. Some of them were reported earlier by Imaizumi et al. (2008), and three layers between 4300 and 5000 years ago were newly identified. The event layer dated at 371-161 cal BC can be correlated with the Yayoi tsunami deposit in the Sendai Plain. Sedimentological characteristics of the Yayoi event layer suggested that they are likely to have been deposited by a tsunami, rather than a storm surge. No traces of tsunami were known from archaeological sites of the Yayoi period in the northern part of the Joban Coast. However, there are few examples of archaeological sites discovered along the Joban Coast in the Early Period, and there are abundant reports from the Middle and Late Periods, suggesting the existence of a transitional period in the dynamics of archaeological sites. It is too early to say for sure that the transition of archaeological sites was caused by the Yayoi tsunami. Further research is needed to clarify relevance between findings from archaeological and sedimentological observations. So far the possible tsunami deposit in the Yayoi period can be traced from Ishinomaki to the northern part of the Joban Coast, and its southern extent is comparable to that of the Jogan tsunami.
Eight event layers were identified in the sediment cores. Based on the radiocarbon dating results, seven layers were associated with prehistoric events. Some of them were reported earlier by Imaizumi et al. (2008), and three layers between 4300 and 5000 years ago were newly identified. The event layer dated at 371-161 cal BC can be correlated with the Yayoi tsunami deposit in the Sendai Plain. Sedimentological characteristics of the Yayoi event layer suggested that they are likely to have been deposited by a tsunami, rather than a storm surge. No traces of tsunami were known from archaeological sites of the Yayoi period in the northern part of the Joban Coast. However, there are few examples of archaeological sites discovered along the Joban Coast in the Early Period, and there are abundant reports from the Middle and Late Periods, suggesting the existence of a transitional period in the dynamics of archaeological sites. It is too early to say for sure that the transition of archaeological sites was caused by the Yayoi tsunami. Further research is needed to clarify relevance between findings from archaeological and sedimentological observations. So far the possible tsunami deposit in the Yayoi period can be traced from Ishinomaki to the northern part of the Joban Coast, and its southern extent is comparable to that of the Jogan tsunami.