3:00 PM - 3:15 PM
[MIS16-06] Searching tsunami-related erosional features in Sendai Plain
Sendai Plain is a strand plain that comprise beach ridges and wetlands, which are suitable for formation and preservation of tsunami deposits. Many historic and prehistoric tsunami events have been known from tsunami deposits in the wetlands. However, identification and spatial and temporal correlation of the tsunami events are often controversial due to local variability in deposit formation and preservation potentials and errors in age estimation. In addition, preservation of younger tsunami deposits is greatly affected by human activities such as reclamation. Exploration of other signature, such as tsunami-related erosional feature is needed to establish more complete tsunami history. Combined analysis of tsunami deposits and erosional features is a promising approach for research of past tsunamis with incomplete sedimentary records.
In this study, we used a ground-penetrating radar (GPR) to search for tsunami-related erosional features and previously overlooked deposits in Sendai Plain. The survey was conducted in the areas around Onuma and Minami-Naganuma ponds, which are located between paleo-beach ridges and now apart 1-2 km from the present coastline.
Shore-normal GPR images showed seaward-dipping, nearly continuous reflections. Such a kind of radar facies can be interpreted as internal structure of foreshore and backshore sediments. The GPR data, together with observation of sediment cores, inferred that the beach ridges were continuously formed as a result of progradation of the coastline, and local topographic lows on the ridge were buried by organic silt, which contains some event sand layers and a tephra.
Notable feature detected from the seaward side of Minami-Naganuma is a landward-dipping reflections that cut reflections of the old beach ridge. In addition, a reflection with convex down shape, with a width of about 40 m and a depth of 1.0 m, was observed on the ridge. This suggests an unusual topographic depression is buried within the sediments of the beach ridge. The stratigraphic facies of the depression indicated a wetland environment, and the location of the depression is consistent with a pond described on an map published about 100 years ago. Core samples of the depression did not contain To-a tephra (AD 915 or 932); meanwhile samples from surrounding areas contained the tephra. This can be explained by erosion of the ground surface and formation of the depression after the fall of the To-a tephra. Moreover, seeds obtained from the base of organic silt that fills the depression were dated at AD 1035-1169. Therefore, the depression was formed after AD 915 or 932 and before AD 1035-1169.
As a result of the GPR-aided excavations, three event sand layers (Sand 1 to 3) were found from the seaward side of the Minami-Naganuma and Onuma, with distances from the present coastline ranges 1-2 km. Depositional ages of Sand 1, 2 and 3 were estimated at AD1272-1426, AD1033-1259 and AD130-426, respectively. The depositional age of Sand 2 coincides with the depression near the Minami-Naganuma. Mean grain size and degree of sorting of Sands 1-3 are more similar to beach sediments than to flood sediments, suggesting that they are likely to be of marine origin. The ages of Sand 1 and 2 are not consistent with the ages of known historical tsunamis, but recent geological studies have reported event deposits of similar ages. The age of Sand 3 corresponds to the age of the 4th-5th century tsunami that was reported previously; however, since the sampling locations are quite close to the paleo-coastline, the formation can be explained either by storm or tsunami.
In this study, we used a ground-penetrating radar (GPR) to search for tsunami-related erosional features and previously overlooked deposits in Sendai Plain. The survey was conducted in the areas around Onuma and Minami-Naganuma ponds, which are located between paleo-beach ridges and now apart 1-2 km from the present coastline.
Shore-normal GPR images showed seaward-dipping, nearly continuous reflections. Such a kind of radar facies can be interpreted as internal structure of foreshore and backshore sediments. The GPR data, together with observation of sediment cores, inferred that the beach ridges were continuously formed as a result of progradation of the coastline, and local topographic lows on the ridge were buried by organic silt, which contains some event sand layers and a tephra.
Notable feature detected from the seaward side of Minami-Naganuma is a landward-dipping reflections that cut reflections of the old beach ridge. In addition, a reflection with convex down shape, with a width of about 40 m and a depth of 1.0 m, was observed on the ridge. This suggests an unusual topographic depression is buried within the sediments of the beach ridge. The stratigraphic facies of the depression indicated a wetland environment, and the location of the depression is consistent with a pond described on an map published about 100 years ago. Core samples of the depression did not contain To-a tephra (AD 915 or 932); meanwhile samples from surrounding areas contained the tephra. This can be explained by erosion of the ground surface and formation of the depression after the fall of the To-a tephra. Moreover, seeds obtained from the base of organic silt that fills the depression were dated at AD 1035-1169. Therefore, the depression was formed after AD 915 or 932 and before AD 1035-1169.
As a result of the GPR-aided excavations, three event sand layers (Sand 1 to 3) were found from the seaward side of the Minami-Naganuma and Onuma, with distances from the present coastline ranges 1-2 km. Depositional ages of Sand 1, 2 and 3 were estimated at AD1272-1426, AD1033-1259 and AD130-426, respectively. The depositional age of Sand 2 coincides with the depression near the Minami-Naganuma. Mean grain size and degree of sorting of Sands 1-3 are more similar to beach sediments than to flood sediments, suggesting that they are likely to be of marine origin. The ages of Sand 1 and 2 are not consistent with the ages of known historical tsunamis, but recent geological studies have reported event deposits of similar ages. The age of Sand 3 corresponds to the age of the 4th-5th century tsunami that was reported previously; however, since the sampling locations are quite close to the paleo-coastline, the formation can be explained either by storm or tsunami.