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[SVC27-P05] Mapping risk induced by pyroclastic density currents in Nasudake region (2)
Keywords:Nasudake, pyroclastic density current, risk map
Most of the hazard maps prepared for volcanoes in Japan show the maximum hazard reach, which cannot be read from the hazard maps. For pyroclastic flows flowing at high speed down to the ground surface, the location of the starting point of the flow may contribute to the reachable area, depending on the amount of eruption. It is also pointed out that a power law relationship exists between eruption volume and frequency (e.g., Nakada, 2015; Sandri et al., 2016). Kohno and Takarada (2023, Volcanological Society of Japan Fall Meeting) attempted to create a pyroclastic flow risk map by adding an uncertainty assessment of the crater. However, the hazard assessment only shows the results with fixed eruptive volume and basal friction angle, and the evaluation of simulation input parameter uncertainties is still open for further study. For the eruptive volume and bottom friction angle, Ogburn and Calder (2017) showed that the logarithmic values of the two are inversely correlated. In this paper, we report the results of a pyroclastic flow arrival risk mapping study that includes uncertainties in the location and volume of eruptions, considering the relationship between eruptive volume and basal friction angle.
For hazard assessment (Hazard), as in Kohno and Takarada (2023), a vent opening map was created from volcanic structure information available from geological maps, and simulations were conducted from the center point of each mesh where the vent opening probability value≠ 0. The geological information is based on the geological map of Nasu Volcano provided by the Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST). Simulations are based on Titan2D (Pitman et al., 2003; Patra et al., 2005). The eruptive volume is based on the definition of VEI (Newhall and Self, 1982), 1.0 × 10-3 - 1 km3 (equivalent to VEI= 2 - 4). The basal friction angle was determined from the eruptive volume using the Ogburn and Calder equation, and the initial velocity of the flow was fixed at 40 m/s.
We applied the method of deriving the probability distribution of eruption (VEI) estimated from the frequency distribution by eruption size, such as Sandri et al. and Shimizu and Tanabe. The eruption history of Mt. Nasudake is taken from the "Catalog of eruptive events during the last 10,000 years in Japan" of the AIST.
The multiplication of the estimated probability values for the vent opening and the eruptive volume (opening probability value x volume probability value) is the weight for the envelope output from the simulation results, and the hazard assessment is the sum of all pyroclastic flow reach areas to which this weight has been applied.
Four types of social infrastructure around Mt. Nasudake were used for exposure assessment (Exposure): building distribution, population, roads and railroads, and land use classification. The exposure distributions were transformed into a relative importance distribution with values ranging from 0 to 1 using the method of Del Negro et al. (2020) and integrated by weighted linear combination. The risk value is calculated by (Risk=Hazard×Exposure×Vulnerability; UNESCO, 1972). The Vulnerability of each exposure target in this presentation is assumed to be 1. This means the target will be damaged if a pyroclastic flow reaches it.
Most pyroclastic flows flowed to the west of Mt. Chausudake, an active volcano on Mt. Nasudake, while those to the east were not as frequent as those to the west, but they flowed along well-developed streams. The area shown to be at high risk was centered on the hot spring resort area at the southeastern foot of Mt. Chausudake. As Nakamura et al. (2021) pointed out, this may be because a trunk road has been constructed along the stream, and hot spring inns are located along the well-developed road.
For hazard assessment (Hazard), as in Kohno and Takarada (2023), a vent opening map was created from volcanic structure information available from geological maps, and simulations were conducted from the center point of each mesh where the vent opening probability value≠ 0. The geological information is based on the geological map of Nasu Volcano provided by the Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST). Simulations are based on Titan2D (Pitman et al., 2003; Patra et al., 2005). The eruptive volume is based on the definition of VEI (Newhall and Self, 1982), 1.0 × 10-3 - 1 km3 (equivalent to VEI= 2 - 4). The basal friction angle was determined from the eruptive volume using the Ogburn and Calder equation, and the initial velocity of the flow was fixed at 40 m/s.
We applied the method of deriving the probability distribution of eruption (VEI) estimated from the frequency distribution by eruption size, such as Sandri et al. and Shimizu and Tanabe. The eruption history of Mt. Nasudake is taken from the "Catalog of eruptive events during the last 10,000 years in Japan" of the AIST.
The multiplication of the estimated probability values for the vent opening and the eruptive volume (opening probability value x volume probability value) is the weight for the envelope output from the simulation results, and the hazard assessment is the sum of all pyroclastic flow reach areas to which this weight has been applied.
Four types of social infrastructure around Mt. Nasudake were used for exposure assessment (Exposure): building distribution, population, roads and railroads, and land use classification. The exposure distributions were transformed into a relative importance distribution with values ranging from 0 to 1 using the method of Del Negro et al. (2020) and integrated by weighted linear combination. The risk value is calculated by (Risk=Hazard×Exposure×Vulnerability; UNESCO, 1972). The Vulnerability of each exposure target in this presentation is assumed to be 1. This means the target will be damaged if a pyroclastic flow reaches it.
Most pyroclastic flows flowed to the west of Mt. Chausudake, an active volcano on Mt. Nasudake, while those to the east were not as frequent as those to the west, but they flowed along well-developed streams. The area shown to be at high risk was centered on the hot spring resort area at the southeastern foot of Mt. Chausudake. As Nakamura et al. (2021) pointed out, this may be because a trunk road has been constructed along the stream, and hot spring inns are located along the well-developed road.