5:15 PM - 6:45 PM
[MIS20-P03] Paleotsunami and flood deposits on the Southern Sanriku Coast and characteristics of estimated tsunami source
After the 2011 Tohoku-oki Earthquake, tsunami deposit researchs have been conducted on the Sanriku Coast, where paleotsunami history was poorly understood. Results of such recent studies were correlated with those from the Sendai and Ishinomaki plains (Ishimura and Miyauchi, 2015; Takata et al., 2016); ages of the tsunami deposits in the northern part of the Sanriku Coast were found to overlap with those of the 869 Jogan, the 1611 Keicho earthquake tsunamis, whose deposits have been found from many locations on the Pacific coast of the Tohoku region. Meanwhile no tsunami deposit has been reported from the south of Onuma, Minamisanriku Town, Miyagi Prefecture (Ishimura and Miyauchi, 2017). The Sanriku Coast is recognized as an area where tsunamis frequently occur, and its southern portion is facing directly to the main rupture area of the Tohoku-oki earthquake. The earthquake is characterized by large slip not only in the deeper portion of the megathrust but also in the shallow, near-trench ruptures. So far, no findings available whether or not similar earthquakes have occurred in the past.
In this study, we conducted tsunami deposit surveys at Kitakami Town in Ishinomaki City in the southern Sanriku Coast. Sediment cores were obtained by using handy geoslicers and SCSC borehole corers. The sediment cores were analyzed in the laboratory to identify event layers and examine their origins and depositional ages. Tsunami sediment transport modeling was performed to assess contributions of the shallow and deep slips to the distribution and source of sandy tsunami deposits.
Among the nine cores, multiple event sand layers and one event clay layer were intercalated within organic silt. The sand layer correlated across eight different sites was named as S1 and the clay layer observed at four different sites was named as M1. Sedimentological features such as erosional contact with the underlying layer, inclusion of rip-up-crust, and landward fining as well as estimated depositional environment and topography suggest a tsunami origin of S1. Radiocarbon dating of S1 suggested the depositional age of S1 after 1730 cal AD, which can be correlated with the 1793 Kansei Miyagi-oki Earthquake Tsunami. Tsunami sediment transport simulations indicated that the sandy tsunami deposits in the study area is formed due to the deep slip of the megathrust, whereas almost no contribution was found for the shallow slip. M1 is characterized by well-developed lamina compared to the background organic silt layers. The XRD analysis of M1 revealed existence of a kaolinite/smectite mixture, which was not detected from the surrounding organic silt. Inclusion of the kaolinite/smectite mixture infers upstream origin of M1, which can be interpreted as a flood deposit.
In this study, we conducted tsunami deposit surveys at Kitakami Town in Ishinomaki City in the southern Sanriku Coast. Sediment cores were obtained by using handy geoslicers and SCSC borehole corers. The sediment cores were analyzed in the laboratory to identify event layers and examine their origins and depositional ages. Tsunami sediment transport modeling was performed to assess contributions of the shallow and deep slips to the distribution and source of sandy tsunami deposits.
Among the nine cores, multiple event sand layers and one event clay layer were intercalated within organic silt. The sand layer correlated across eight different sites was named as S1 and the clay layer observed at four different sites was named as M1. Sedimentological features such as erosional contact with the underlying layer, inclusion of rip-up-crust, and landward fining as well as estimated depositional environment and topography suggest a tsunami origin of S1. Radiocarbon dating of S1 suggested the depositional age of S1 after 1730 cal AD, which can be correlated with the 1793 Kansei Miyagi-oki Earthquake Tsunami. Tsunami sediment transport simulations indicated that the sandy tsunami deposits in the study area is formed due to the deep slip of the megathrust, whereas almost no contribution was found for the shallow slip. M1 is characterized by well-developed lamina compared to the background organic silt layers. The XRD analysis of M1 revealed existence of a kaolinite/smectite mixture, which was not detected from the surrounding organic silt. Inclusion of the kaolinite/smectite mixture infers upstream origin of M1, which can be interpreted as a flood deposit.