10:45 AM - 11:00 AM
[MIS20-05] Tsunami behavior and deposits in Noto Town by the 2024 Noto Peninsula Earthquake
Keywords:Noto Peninsula Earthquake, Tsunami deposit, Tsunami trace, Tsunami boulder, Levee breach, Ostracode
The 2024 Noto Peninsula Earthquake (M7.6) occurred on January 1 in the northern part of the Noto Peninsula. We conducted the field survey on January 27–29, approximately one month after the tsunami. The field survey focused on the tsunami inundation depth, current direction, and tsunami deposits at a lowland in Noto Town, where the tsunami inundated widely. The inundation depth was measured using watermarks preserved on structures and trees, and the direction of rice-plant bents was recorded to reconstruct the current direction. Tsunami deposits were investigated at 82 locations in a lowland by using a shovel and a gauge auger. The sedimentary features of the tsunami deposits were described at each location.
The Nunoura and Kurikawashiri areas face the small and shallow bay (600 × 1000 m and <10 m deep). Most of the Nunoura area, northeast of the Kurikawashiri River, is used for large facilities such as a baseball stadium, while private houses and paddy fields spread in the Kurikawashiri area, southwest of the river. Here, the lowland area up to 550 m wide and 1–3 m in elevation was inundated up to 750 m inland. The measured inundation depth near shore was over 330 cm in the Nunoura area, whereas it was less than 70 cm in the Kurikawashiri area. In the Nunoura area, the maximum thickness was 9.0 cm, and the average was 4.6 cm throughout the area. The landward thinning trend was not observed. In the Kurikawashiri area, the maximum thickness of the tsunami deposits was 16.0 cm, which was the largest in the lowland. In this area, the river levee was estimated to be breached over 23 m in width and 50–70 cm in depth. The measured current directions near the breached levee were spreading from the river to the lowland. These imply that the tsunami overflowing from the river was more dominant than the tsunami from the sea in this area. In addition, preliminary classification of the ostracodes showed that the genera Aurila, Cythere, and Loxoconcha were abundant in the sandy tsunami deposit. These genera are considered to inhabit a sandy bottom and a seagrass bed in a coastal shallow-water zone, indicating that the tsunami deposit was sourced from there. In addition to sandy deposits, at least three boulders were found in the river and the lowland. The boulder transported the farthest inland was located 750 m from the shoreline and was 130 × 110 × 80 cm in size. This boulder was composed of volcaniclastic sediments with clastic rocks of various sizes. The blocks likely collapsed from the outcrop at the mouth of the river due to seismic motion, fell into the river mouth, and were transported inland by the tsunami.
The field survey immediately after the tsunami revealed the distribution and source of tsunami deposits and tsunami behavior. Although still in the preliminary stage, it is likely that tsunami deposits were formed extensively in this study area due to the large supply of sediments from the shallow-water zone and the levee breach caused by the tsunami, which flowed back through the river. In the case of a relatively small tsunami, the formation of extensive tsunami deposits may depend on topographical conditions. Paleotsunami deposit studies have been conducted to reconstruct the past tsunami history at various sites. The results of this study show that understanding the influences of local topography is necessary to predict tsunami behavior and depositional processes in estuarine and inner-bay regions such as the study area, which will need to be fully investigated by numerical simulations before conducting surveys. Indeed, the earthquake motion caused extensive damage to the Noto Peninsula, while the tsunami inundation damage was localized. The tsunami deposit data and behavior obtained in this study area will be helpful for future tsunami inundation and sediment transport calculations. These are expected to contribute to the elucidation of the source area of tsunamis, including submarine landslides.
The Nunoura and Kurikawashiri areas face the small and shallow bay (600 × 1000 m and <10 m deep). Most of the Nunoura area, northeast of the Kurikawashiri River, is used for large facilities such as a baseball stadium, while private houses and paddy fields spread in the Kurikawashiri area, southwest of the river. Here, the lowland area up to 550 m wide and 1–3 m in elevation was inundated up to 750 m inland. The measured inundation depth near shore was over 330 cm in the Nunoura area, whereas it was less than 70 cm in the Kurikawashiri area. In the Nunoura area, the maximum thickness was 9.0 cm, and the average was 4.6 cm throughout the area. The landward thinning trend was not observed. In the Kurikawashiri area, the maximum thickness of the tsunami deposits was 16.0 cm, which was the largest in the lowland. In this area, the river levee was estimated to be breached over 23 m in width and 50–70 cm in depth. The measured current directions near the breached levee were spreading from the river to the lowland. These imply that the tsunami overflowing from the river was more dominant than the tsunami from the sea in this area. In addition, preliminary classification of the ostracodes showed that the genera Aurila, Cythere, and Loxoconcha were abundant in the sandy tsunami deposit. These genera are considered to inhabit a sandy bottom and a seagrass bed in a coastal shallow-water zone, indicating that the tsunami deposit was sourced from there. In addition to sandy deposits, at least three boulders were found in the river and the lowland. The boulder transported the farthest inland was located 750 m from the shoreline and was 130 × 110 × 80 cm in size. This boulder was composed of volcaniclastic sediments with clastic rocks of various sizes. The blocks likely collapsed from the outcrop at the mouth of the river due to seismic motion, fell into the river mouth, and were transported inland by the tsunami.
The field survey immediately after the tsunami revealed the distribution and source of tsunami deposits and tsunami behavior. Although still in the preliminary stage, it is likely that tsunami deposits were formed extensively in this study area due to the large supply of sediments from the shallow-water zone and the levee breach caused by the tsunami, which flowed back through the river. In the case of a relatively small tsunami, the formation of extensive tsunami deposits may depend on topographical conditions. Paleotsunami deposit studies have been conducted to reconstruct the past tsunami history at various sites. The results of this study show that understanding the influences of local topography is necessary to predict tsunami behavior and depositional processes in estuarine and inner-bay regions such as the study area, which will need to be fully investigated by numerical simulations before conducting surveys. Indeed, the earthquake motion caused extensive damage to the Noto Peninsula, while the tsunami inundation damage was localized. The tsunami deposit data and behavior obtained in this study area will be helpful for future tsunami inundation and sediment transport calculations. These are expected to contribute to the elucidation of the source area of tsunamis, including submarine landslides.