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[G05-P18] Damage in the Oita Bungoono Geopark caused by the earthquake occurred on January 22, 2022 in the Hyuga-nada region, Japan
Keywords:Oita Bungoono Geopark, Hyuga-nada, earthquake, damage, topography
A magnitude 6.6 (provisional value) earthquake with the epicenter of Hyuga-nada occurred at 1:08 am on January 22, 2022, causing shaking with a maximum seismic intensity of 5+ in Oita and northern Miyazaki prefectures. According to the Japan Meteorological Agency, the depth of the epicenter was 45 km (provisional value), and the focal mechanism was a normal fault type with a tension axis in the WNW-ESE direction. The seismic intensity at each observation point in the Oita Bungoono Geopark, which is about 50 to 80 km away from the epicenter, was a seismic intensity of 4, however, A tremor with a seismic intensity of 5+ in Oita City, Ogimachi and Naoiri Town in Taketa City, and Takachiho Town in Miyazaki Prefecture, which are farther from the epicenter. Shaking with a seismic intensity of 5- was observed in Aso City and Takamori Town in Kumamoto Prefecture, and damage due to liquefaction was confirmed in the coastal area of Oita City. Looking at the seismic intensity distribution estimated by the Japan Meteorological Agency, it seems that the range of seismic intensity 5- or higher is distributed so as to surround Bungoono City.
In this earthquake, no noticeable damage was confirmed at each site of the Oita Bungoono Geopark, but damage to the roof tiles of private houses and cracks on the walls occurred in the area. In addition, stone structures such as tombstones were also found to have fallen or damaged, such as the stone Gorinto tower and stone lanterns placed at the top of the burial mound at the Michinoue Kofun in Miemachi.
Looking at the distribution of damage, relatively much damage can be seen in the eastern part of the area, such as Inukaimachi and Miemachi, which are closer to the epicenter and have a large population, in the geopark area, however the damage is concentrated in some areas. One of the topographical commonalities of these areas is that they are often on the edge of the plateau. At the Oita Bungoono Geopark, the plateau where the dissected sedimentary surface of the Aso-4 pyroclastic flow about 90,000 years ago occupies a wide area, and most of the damaged areas are on the edge of such plateau or just above a cliff on the edge of the plateau. The plateau is composed of Aso-4 pyroclastic flow deposits, but at least a few meters above it is a relatively fragile, non-welded or weakly welded deposit rich in pumice. It can be pointed out that the damage may have been concentrated due to the amplification of the seismic motion, coupled with the topographical effect of the top of the cliffs on the edge of the plateau. The above-mentioned Michinoue Kofun is also located on the cliff at the tip of the plateau, and it is possible that the topographical effect was further enhanced by the construction of a burial mound with a height of 6 m.
In this earthquake, although the seismic intensity of all the observation points in the geopark area was seismic intensity 4, although it was local, damage equivalent to seismic intensity 5- to 5+ was actually caused. Therefore, it seems necessary to add more detailed studies in the future and reflect them in regional disaster prevention.
In this earthquake, no noticeable damage was confirmed at each site of the Oita Bungoono Geopark, but damage to the roof tiles of private houses and cracks on the walls occurred in the area. In addition, stone structures such as tombstones were also found to have fallen or damaged, such as the stone Gorinto tower and stone lanterns placed at the top of the burial mound at the Michinoue Kofun in Miemachi.
Looking at the distribution of damage, relatively much damage can be seen in the eastern part of the area, such as Inukaimachi and Miemachi, which are closer to the epicenter and have a large population, in the geopark area, however the damage is concentrated in some areas. One of the topographical commonalities of these areas is that they are often on the edge of the plateau. At the Oita Bungoono Geopark, the plateau where the dissected sedimentary surface of the Aso-4 pyroclastic flow about 90,000 years ago occupies a wide area, and most of the damaged areas are on the edge of such plateau or just above a cliff on the edge of the plateau. The plateau is composed of Aso-4 pyroclastic flow deposits, but at least a few meters above it is a relatively fragile, non-welded or weakly welded deposit rich in pumice. It can be pointed out that the damage may have been concentrated due to the amplification of the seismic motion, coupled with the topographical effect of the top of the cliffs on the edge of the plateau. The above-mentioned Michinoue Kofun is also located on the cliff at the tip of the plateau, and it is possible that the topographical effect was further enhanced by the construction of a burial mound with a height of 6 m.
In this earthquake, although the seismic intensity of all the observation points in the geopark area was seismic intensity 4, although it was local, damage equivalent to seismic intensity 5- to 5+ was actually caused. Therefore, it seems necessary to add more detailed studies in the future and reflect them in regional disaster prevention.