5:15 PM - 7:15 PM
[MIS11-P01] Tsunami-induced erosion and its age in the northern Sendai Plain
Keywords:Paleoearthquake, Japan Trench
Tsunami-induced erosional topography, such as scour pond, has been attracting attention in recent years as a geomorphological record of paleotsunamis. Although majorities can be associated with return flow, run-up flow can also form erosional features around microtopography. If they are buried by sediments and preserved within coastal sedimentary sequences, it be a clue for the age and size of past tsunamis, which contribute to assessment of hazards from earthquakes and tsunamis.
In this study, we investigated the scale of erosion caused by the 2011 Tohoku-oki tsunami and the age of past erosion in the Chochingama marsh on the right bank of the Nanakita River in the northern Sendai Plain. According to existing land classification map, the marsh is located on the beach ridge IIIb with a distance of approximately 400 to 600 m from the present coastline. The marsh surface is covered with a thin sand layer that is considered to have been deposited during the Tohoku-oki tsunami. The sand layer was underlain by muddy marsh sediments with a thickness of several tens of centimeters that cover the sandy deposit of the beach ridge. Based on the topographic change due to the Tohoku-oki tsunami inferred from analysis of pre- and post-tsunami DEMs, and the measured thickness of the Tohoku-oki tsunami sand layer, the tsunami-induced erosion was estimated at 15 to 40 cm with significant local variation. According to a simple theoretical estimation, this corresponds to a flow with a time-averaged Shields’ number of the order of 101 to 102 that lasted for the order of 102 seconds. The oldest radiocarbon age of the sediment sample taken from the base of the marsh deposits was 1494-1640 AD. Since the To-a volcanic tephra, which is dated to 915 or 932 AD, was found just beneath the marsh deposits, it is possible that a remarkable erosion occurred just before the formation of the Chochingama marsh. Possible candidates of the trigger of the erosion are the 1611 Keicho Oshu or the 1454 Kyotoku earthquakes.
In this study, we investigated the scale of erosion caused by the 2011 Tohoku-oki tsunami and the age of past erosion in the Chochingama marsh on the right bank of the Nanakita River in the northern Sendai Plain. According to existing land classification map, the marsh is located on the beach ridge IIIb with a distance of approximately 400 to 600 m from the present coastline. The marsh surface is covered with a thin sand layer that is considered to have been deposited during the Tohoku-oki tsunami. The sand layer was underlain by muddy marsh sediments with a thickness of several tens of centimeters that cover the sandy deposit of the beach ridge. Based on the topographic change due to the Tohoku-oki tsunami inferred from analysis of pre- and post-tsunami DEMs, and the measured thickness of the Tohoku-oki tsunami sand layer, the tsunami-induced erosion was estimated at 15 to 40 cm with significant local variation. According to a simple theoretical estimation, this corresponds to a flow with a time-averaged Shields’ number of the order of 101 to 102 that lasted for the order of 102 seconds. The oldest radiocarbon age of the sediment sample taken from the base of the marsh deposits was 1494-1640 AD. Since the To-a volcanic tephra, which is dated to 915 or 932 AD, was found just beneath the marsh deposits, it is possible that a remarkable erosion occurred just before the formation of the Chochingama marsh. Possible candidates of the trigger of the erosion are the 1611 Keicho Oshu or the 1454 Kyotoku earthquakes.