5:15 PM - 6:45 PM
[U15-P73] Tsunami deposits formed in Suzu City and Noto Town by the 2024 Noto Peninsula earthquake
Keywords:Noto Peninsula earthquake, Tsunami deposit, Tsunami trace
The 2024 Noto Peninsula earthquake in central Japan triggered a moderate-scale tsunami (i.e., wave height of 4–6 m) that inundated several coastal lowlands up to 750 m inland. This presentation reports the initial results from a field survey conducted immediately after the event. One objective of a post-tsunami survey is to clarify the overall effects of a tsunami and its earthquake. Another important objective of collecting modern examples of tsunami events is to accurately understand the traces of past tsunami events in geologic strata. As more studies have focused on modern tsunami deposits, the complexity of their sedimentological characteristics has become apparent. Most studies on modern tsunami deposits have focused on large-scale trench megathrust earthquakes such as the 2004 Indian Ocean earthquake and 2011 Tohoku-oki earthquake. Moderate-scale tsunamis occur more frequently and are still destructive events that can wash away or flood many houses, but few studies have investigated their modern deposits. To understand the general features and distribution of tsunami deposits, more case studies are required of modern deposits formed by tsunamis of various scales and under diverse topographic conditions.
We conducted a field survey on the distribution and characteristics of tsunami deposits in three areas along the east coast of the Noto Peninsula: Horyu, Uchiura, and Shiromaru. The inundation depth was measured by using watermarks preserved on structures and trees. The bent direction of rice plants was recorded to reconstruct the current direction of the tsunami inundation. The inundation limit was determined by using debris that had been transported the furthest inland by the tsunami as an indicator. Tsunami deposits were investigated by using a shovel and a gauge auger. Visual observation with the naked eye was used to evaluate the thickness, grain size, and sedimentary structure of tsunami deposits at each location.
The Horyu and Shiromaru, where houses are concentrated along the coast, are estimated to have been inundated to 400 m and 300 m from the shoreline, respectively, based on aerial photograph observation. The maximum inundation depths here were 280 cm and 250 cm, respectively. Sandy tsunami deposits with a maximum thickness of 3 cm were observed in Horyu. In Uchiura, where the most significant tsunami inundation and sediment deposition were observed, the tsunami was estimated to have inundated up to about 750 m inland. The tsunami inundation depth measured in the field survey was over 330 cm. Debris and tsunami deposits transported by the tsunami were found outside the estimated inundation area. This indicates that although it is possible to determine the approximate inundation area by aerial photograph observation, it is important to conduct field surveys to accurately determine the inundation area. There are two lowland areas in Uchiura, one on the sea side and the other on the inland side, separated by a river. The reconstructed flow direction indicates that the tsunami entered the seaward lowland directly from the sea, whereas the inland lowland was dominated by the overflow from the river. In the inland lowland, the inland side of the riverbank was significantly eroded at two locations, and tsunami deposits with a maximum thickness of 16.0 cm were observed around there. The thickness of the tsunami deposits in seaside lowland was about 5 cm, indicating that the overflow from the river amplified the sediment deposition in the landside lowland.
Moderate-scale tsunamis may be characterized by a wide inundation area and the formation of tsunami deposits only when the local topographic conditions increase the inundation height. Uchiura, where tsunami deposits were widely distributed, faces a small shallow inner bay, which may have amplified the tsunami wave heights. The presence of a river may also have contributed to the wider inundation area. Future numerical calculations on tsunami inundation and sediment transport are expected to clarify the tsunami behavior in this area. In addition, the distribution characteristics and thickness of the tsunami deposits against the hydraulic quantities of the moderate-scale tsunami were not substantially different from those of mega-tsunamis. Our findings and future studies on this tsunami will contribute to knowledge on tsunami deposit formation and studies on paleotsunamis at various scales.
We conducted a field survey on the distribution and characteristics of tsunami deposits in three areas along the east coast of the Noto Peninsula: Horyu, Uchiura, and Shiromaru. The inundation depth was measured by using watermarks preserved on structures and trees. The bent direction of rice plants was recorded to reconstruct the current direction of the tsunami inundation. The inundation limit was determined by using debris that had been transported the furthest inland by the tsunami as an indicator. Tsunami deposits were investigated by using a shovel and a gauge auger. Visual observation with the naked eye was used to evaluate the thickness, grain size, and sedimentary structure of tsunami deposits at each location.
The Horyu and Shiromaru, where houses are concentrated along the coast, are estimated to have been inundated to 400 m and 300 m from the shoreline, respectively, based on aerial photograph observation. The maximum inundation depths here were 280 cm and 250 cm, respectively. Sandy tsunami deposits with a maximum thickness of 3 cm were observed in Horyu. In Uchiura, where the most significant tsunami inundation and sediment deposition were observed, the tsunami was estimated to have inundated up to about 750 m inland. The tsunami inundation depth measured in the field survey was over 330 cm. Debris and tsunami deposits transported by the tsunami were found outside the estimated inundation area. This indicates that although it is possible to determine the approximate inundation area by aerial photograph observation, it is important to conduct field surveys to accurately determine the inundation area. There are two lowland areas in Uchiura, one on the sea side and the other on the inland side, separated by a river. The reconstructed flow direction indicates that the tsunami entered the seaward lowland directly from the sea, whereas the inland lowland was dominated by the overflow from the river. In the inland lowland, the inland side of the riverbank was significantly eroded at two locations, and tsunami deposits with a maximum thickness of 16.0 cm were observed around there. The thickness of the tsunami deposits in seaside lowland was about 5 cm, indicating that the overflow from the river amplified the sediment deposition in the landside lowland.
Moderate-scale tsunamis may be characterized by a wide inundation area and the formation of tsunami deposits only when the local topographic conditions increase the inundation height. Uchiura, where tsunami deposits were widely distributed, faces a small shallow inner bay, which may have amplified the tsunami wave heights. The presence of a river may also have contributed to the wider inundation area. Future numerical calculations on tsunami inundation and sediment transport are expected to clarify the tsunami behavior in this area. In addition, the distribution characteristics and thickness of the tsunami deposits against the hydraulic quantities of the moderate-scale tsunami were not substantially different from those of mega-tsunamis. Our findings and future studies on this tsunami will contribute to knowledge on tsunami deposit formation and studies on paleotsunamis at various scales.