5:15 PM - 6:45 PM
[MIS20-P01] Spatial distribution of the 17th century tsunami deposits in the western Mukawa city, Hokkaido, northern Japan
Keywords:tsunami deposits, grain size distribution, spatial distribution
The 17th century tsunami deposits were distributed from the coastal lowlands in the eastern part of the Iburi Coast of Hokkaido (Takashimizu et al., 2007). The deposits were considered to be 1611 Keicho-Sanriku earthquake-induced tsunami deposits (Nishimura et al., JpGU 2023). However, the spatial height distribution, thickness, and grain size distribution of tsunami deposits are not clear. This is a problem when considering the size of a tsunami. This information is significant as fundamental data for tsunami disaster prevention and mitigation, and it is necessary to obtain its exact characteristics. Therefore, the spatial distribution of features of tsunami deposits in the coastal lowlands of western Mukawa Town was investigated.
Geological surveys using geoslicer instrument were conducted at 33 sites in the coastal lowlands distributed from the Irishikabetsu River to the right-side of the Mukawa River, which is approximately 4 km along the coastline and 1.5 km inland. The study area is flat farmland and residential, with low elevation in the west (approximately 4 m) and high elevation in the east (approximately 9 m). The survey sites were as scattered as possible, ranging from several hundred meters to several tens of meters. Tsunami deposits were identified at 27 sites, excluding six sites. The height distribution of the tsunami deposits was lower in the western area (3.5–6.0 m) and higher in the eastern area (4.8–6.9 m). Although a spatial distribution such as our method does not allow strict reconstruction of geographical features and height of river channels and/or dunes, it provides an approximate height distribution and is important for studying the behavior of paleo-tsunamis. The thickness of the tsunami deposits ranged from several to ten odd centimeters, with a maximum thickness of 33 cm at the seaward site. The inland thinning trends of the thickness are not simple and inconsistent. The results may be a general feature of tsunami deposits on coastal lowlands and should be studied in detail in the future. The grain size distribution was characterized by a bimodal distribution in the western area, whereas the unimodal distribution was dominant in the eastern area. The mode grain size of the fine fraction in all samples was very fine-grained sand and stable (3.125–3.375 phi), while the coarse fraction varied from coarse- to fine-grained sands (0.375–2.125 phi). In addition to the variability in the mode grain size of coarse fractions, the 90th percentile particle size (D90) was also variable. Inland fining trends are recognized as the relationships between distance from coastline and D90 grain size, and distance from coastline and mode grain size of the coarse fraction. These features indicate that the coarse fractions in the tsunami decreased during the tsunami run-up on the coastal lowlands and finally disappeared. Instead, a very fine-grained sand fraction became dominant during the tsunami. We will continue to study the spatial features of tsunami deposits to contribute to future tsunami inundation simulations and hazard mapping. In addition to the characteristics of these spatial distributions, we will continue to study parameters that are truly effective for tsunami inundation simulation and hazard mapping.
Geological surveys using geoslicer instrument were conducted at 33 sites in the coastal lowlands distributed from the Irishikabetsu River to the right-side of the Mukawa River, which is approximately 4 km along the coastline and 1.5 km inland. The study area is flat farmland and residential, with low elevation in the west (approximately 4 m) and high elevation in the east (approximately 9 m). The survey sites were as scattered as possible, ranging from several hundred meters to several tens of meters. Tsunami deposits were identified at 27 sites, excluding six sites. The height distribution of the tsunami deposits was lower in the western area (3.5–6.0 m) and higher in the eastern area (4.8–6.9 m). Although a spatial distribution such as our method does not allow strict reconstruction of geographical features and height of river channels and/or dunes, it provides an approximate height distribution and is important for studying the behavior of paleo-tsunamis. The thickness of the tsunami deposits ranged from several to ten odd centimeters, with a maximum thickness of 33 cm at the seaward site. The inland thinning trends of the thickness are not simple and inconsistent. The results may be a general feature of tsunami deposits on coastal lowlands and should be studied in detail in the future. The grain size distribution was characterized by a bimodal distribution in the western area, whereas the unimodal distribution was dominant in the eastern area. The mode grain size of the fine fraction in all samples was very fine-grained sand and stable (3.125–3.375 phi), while the coarse fraction varied from coarse- to fine-grained sands (0.375–2.125 phi). In addition to the variability in the mode grain size of coarse fractions, the 90th percentile particle size (D90) was also variable. Inland fining trends are recognized as the relationships between distance from coastline and D90 grain size, and distance from coastline and mode grain size of the coarse fraction. These features indicate that the coarse fractions in the tsunami decreased during the tsunami run-up on the coastal lowlands and finally disappeared. Instead, a very fine-grained sand fraction became dominant during the tsunami. We will continue to study the spatial features of tsunami deposits to contribute to future tsunami inundation simulations and hazard mapping. In addition to the characteristics of these spatial distributions, we will continue to study parameters that are truly effective for tsunami inundation simulation and hazard mapping.