5:15 PM - 7:15 PM
[HDS07-P13] Reports on the geophysical observations of the slush flows that occurred on the northwestern slopes of Mt. Fuji on April 9, 2024.
Keywords:snow avalanche, seismic observation, Mt. Fuji
On April 10, 2024, various media outlets reported that soil and gravel had flowed onto the road near the first and fourth stations of the Fuji-Subaru Line, which connects the northern foot of Mt. Fuji with the fifth station of the Yoshida route. To investigate the behavior and impact of the event, Mount Fuji Research Institute (MFRI) conducted a waveform analysis using the seismic data around Mt. Fuji and the field survey around the road. This presentation focuses on when and where the avalanches occurred and presents the results of the seismic analyses. A separate report, titled “Reports on the damage and caused by the slush flows that occurred on the northwestern slopes of Mt. Fuji on April 9, 2024,” discusses the findings of the field survey, including the extent of infrastructure damage and the possible causes of the avalanche.
The locations of the stations are shown in Figure 1. There were 14 active seismic stations around Mt. Fuji, which were managed by the Japan Meteorological Agency (JMA), the National Research Institute for Earth Science and Disaster Resilience (NIED), Earthquake Research Institute, the University of Tokyo (ERI), and MFRI. In addition to the seismic stations, we obtained the data from our gravity station at the fourth station of the Fuji-Subaru Line, near the location where the deposits had flowed onto the road. We also collected the meteorological data from the source including JMA (AMeDAS stations), the Yamanashi Prefecture Road Corporation, and the Mt. Fuji Sabo Office of the Chubu Regional Development Bureau, Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) (Fuji Sabo). We analyzed all the day of April 9, 2024 with a focus on around 10 AM, when the avalanches was reported by a monitoring camera operated by Fuji Sabo.
As a basic analysis, we examined the raw waveforms, high-pass filtered waveforms at 2 Hz, and the amplitude variations. Additionally, we applied the amplitude source location (ASL) technique (Perez-Guillen et al. 2018) to the signals using the seismic station network. To correct the site amplification, we used the coda spectrum referencing the M6.0 earthquake that occurred at (N37.7, E141.9) at 12:16 on April 4, 2024.
At noon on the day before the hazard (April 8, 2024), the temperature at the summit of Mt. Fuji rose from -5 ℃ to 0 ℃. This high-temperature condition persisted until 3 PM the next day. Rain began at 9 PM on April 8, and its intensity increased, reaching a precipitation rate of 10 mm/hour at 1 AM on the day of the hazard (April 9, 2024). From 6 AM, the background seismic noise level significantly increased. At 8:20 AM, when rainfall was approaching its peak with the precipitation of 30 mm/hour, Fuji Sabo reported the debris avalanche flowing on Osawa river. At 9:14 AM, Fuji Sabo’s wire sensor detected the flow in the Osawa river. Around 10 AM, rainfall reached its peak intensity. At 10:10 AM, the gravimeter recorded a saturated signal, and Fuji Sabo reported a relatively large flow in the Osawa river. After that, the rainfall began to decrease and ceased by noon on the day (April 9).
Although the seismic noise level increased as the rainfall intensified, we were able to detect seismic events. By cross-referencing the earthquakes reported in the JMA Unified Catalogue, we identified nine relatively large signals on 05:30, 08:07, 09:11, 09:36, 09:55, 10:21, 10:28, 11:47, and 15:18 on April 9, 2024. The largest signal was at 09:55 AM, and its seismic amplitude distribution indicated that the event was around the Osawa and the fourth stop of the Fuji-Subaru Line. The ASL analysis showed that the seismic source moved from the summit side to the downstream of Osawa or Namesawa. Although the location error range requires further investigation, the 09:55 AM signal is a possible source of the road failure. However, as mentioned above, the gravity observation data and monitoring camera indicate that the largest event occurred around 10:10 AM, when no apparent seismic signal was identified. Further investigation is needed to clarify the actual arrival time of the hazard.
The locations of the stations are shown in Figure 1. There were 14 active seismic stations around Mt. Fuji, which were managed by the Japan Meteorological Agency (JMA), the National Research Institute for Earth Science and Disaster Resilience (NIED), Earthquake Research Institute, the University of Tokyo (ERI), and MFRI. In addition to the seismic stations, we obtained the data from our gravity station at the fourth station of the Fuji-Subaru Line, near the location where the deposits had flowed onto the road. We also collected the meteorological data from the source including JMA (AMeDAS stations), the Yamanashi Prefecture Road Corporation, and the Mt. Fuji Sabo Office of the Chubu Regional Development Bureau, Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) (Fuji Sabo). We analyzed all the day of April 9, 2024 with a focus on around 10 AM, when the avalanches was reported by a monitoring camera operated by Fuji Sabo.
As a basic analysis, we examined the raw waveforms, high-pass filtered waveforms at 2 Hz, and the amplitude variations. Additionally, we applied the amplitude source location (ASL) technique (Perez-Guillen et al. 2018) to the signals using the seismic station network. To correct the site amplification, we used the coda spectrum referencing the M6.0 earthquake that occurred at (N37.7, E141.9) at 12:16 on April 4, 2024.
At noon on the day before the hazard (April 8, 2024), the temperature at the summit of Mt. Fuji rose from -5 ℃ to 0 ℃. This high-temperature condition persisted until 3 PM the next day. Rain began at 9 PM on April 8, and its intensity increased, reaching a precipitation rate of 10 mm/hour at 1 AM on the day of the hazard (April 9, 2024). From 6 AM, the background seismic noise level significantly increased. At 8:20 AM, when rainfall was approaching its peak with the precipitation of 30 mm/hour, Fuji Sabo reported the debris avalanche flowing on Osawa river. At 9:14 AM, Fuji Sabo’s wire sensor detected the flow in the Osawa river. Around 10 AM, rainfall reached its peak intensity. At 10:10 AM, the gravimeter recorded a saturated signal, and Fuji Sabo reported a relatively large flow in the Osawa river. After that, the rainfall began to decrease and ceased by noon on the day (April 9).
Although the seismic noise level increased as the rainfall intensified, we were able to detect seismic events. By cross-referencing the earthquakes reported in the JMA Unified Catalogue, we identified nine relatively large signals on 05:30, 08:07, 09:11, 09:36, 09:55, 10:21, 10:28, 11:47, and 15:18 on April 9, 2024. The largest signal was at 09:55 AM, and its seismic amplitude distribution indicated that the event was around the Osawa and the fourth stop of the Fuji-Subaru Line. The ASL analysis showed that the seismic source moved from the summit side to the downstream of Osawa or Namesawa. Although the location error range requires further investigation, the 09:55 AM signal is a possible source of the road failure. However, as mentioned above, the gravity observation data and monitoring camera indicate that the largest event occurred around 10:10 AM, when no apparent seismic signal was identified. Further investigation is needed to clarify the actual arrival time of the hazard.