9:15 AM - 9:30 AM
[SVC32-11] Seismicity of Mt. Ontake in 2024–2025
Keywords:Mt. Ontake, Seismicity
1. Introduction
Mt. Ontake has shown persistent seismicity in the summit region since the 2014 eruption, with elevated seismicity from late Feb to early Mar 2022. Nagoya University has improved its monitoring capability by using 10 seismometers in the summit region since Nov 2017, in addition to existing stations in the surrounding region. We improved the monitoring system in Jul 2024 to detect the events more completely. The seismicity has increased since Dec 2024, resulting in an eruption warning level of 2 on Jan 16, 2025. In this talk, we present the seismicity of Mt. Ontake, including the unrest in 2024–2025, and its implications for understanding the activity of this volcano.
2. Method
We monitored the seismicity of Mt. Ontake using the WIN system with manual picks of arrival times. Before Jun 2024, we created an earthquake catalog using real-time data from the surrounding region, and a complementary catalog including delayed data from the summit region for only events that showed automatic hypocenters near the summit. After Jul 2024, we waited for data from the summit and created a single earthquake catalog based on all available data. We detected 243 earthquakes in the summit region from Jul to Nov 2024 by the latter approach, which would have been only 83 by the previous approach.
To monitor low-frequency (LF) seismicity, we calculated the median peak frequency at the stations within 5 km from the summit; for each station, we computed the peak frequency of the vertical waveform in a time window after 1-s before the P-wave arrival time during which the number ratio of samples beyond the background noise level was > 0.3.
3. Results
The hypocenters of earthquakes in the summit region of Mt. Ontake showed clusters at ~ 1900 and 900 m above sea level when the velocity model of Maeda and Watanabe (2023, EPS) was used; they were close to the top and bottom surfaces of an old (0.78–0.39 Ma) eruptive deposit (OED) layer, respectively. The shallower cluster was dominant in the overall seismicity, whereas seismicity in the unrest periods of 2022 and 2024–2025 occurred mainly in the deeper cluster. The seismicity abruptly migrated to the shallower cluster after the largest (M2.3) earthquake on Jan 21, 2025, which was associated with a tremor and tilt change. LF (< 5 Hz) earthquakes occupied only 12% of the total seismicity in Jul–Nov 2024, which increased to 23% in Dec 2024 (the early period of the unrest) and decreased to 14% in Jan 2025.
4. Discussion
Kato et al. (2015, EPS) showed a depth difference of ~ 1 km between the pre- and post-eruptive seismicity at Mt. Ontake in 2014, although the absolute depths were uncertain because of the absence of stations in the summit region. If the post-eruptive seismicity was near the top of the OED as in 2017, the pre-eruptive seismicity was likely near the bottom of the OED. Maeda and Watanabe (2023) postulated that a low-permeability OED prevents the upward migration of fluids from a low-temperature magma reservoir below. The resultant fluid accumulation near the bottom of the OED may explain the 2014 pre-eruptive seismicity and the 2022 and 2024–2025 unrests near this depth. The abundance of LF events in Dec 2024 suggests an intrusion of compressible, relatively gas-rich fluids. The sources of the 2014 pre-eruptive tilt change and the largest earthquakes in 2022 and 2024–2025 were located in the OED, suggesting that fluids that accumulated below the OED ultimately broke the OED. The resultant fluid migration through the OED may explain the shallower seismicity after the 2014 eruption and the largest earthquake in 2025.
5. Acknowledgments
We used arrival times picked by Junko Sumida and Eri Hibino, and seismograms from the Japan Meteorological Agency, Nagano and Gifu prefectures, and the National Research Institute for Earth Science and Disaster Resilience. This study was supported by the Third Earthquake and Volcano Hazards Observation and Research Program.
Mt. Ontake has shown persistent seismicity in the summit region since the 2014 eruption, with elevated seismicity from late Feb to early Mar 2022. Nagoya University has improved its monitoring capability by using 10 seismometers in the summit region since Nov 2017, in addition to existing stations in the surrounding region. We improved the monitoring system in Jul 2024 to detect the events more completely. The seismicity has increased since Dec 2024, resulting in an eruption warning level of 2 on Jan 16, 2025. In this talk, we present the seismicity of Mt. Ontake, including the unrest in 2024–2025, and its implications for understanding the activity of this volcano.
2. Method
We monitored the seismicity of Mt. Ontake using the WIN system with manual picks of arrival times. Before Jun 2024, we created an earthquake catalog using real-time data from the surrounding region, and a complementary catalog including delayed data from the summit region for only events that showed automatic hypocenters near the summit. After Jul 2024, we waited for data from the summit and created a single earthquake catalog based on all available data. We detected 243 earthquakes in the summit region from Jul to Nov 2024 by the latter approach, which would have been only 83 by the previous approach.
To monitor low-frequency (LF) seismicity, we calculated the median peak frequency at the stations within 5 km from the summit; for each station, we computed the peak frequency of the vertical waveform in a time window after 1-s before the P-wave arrival time during which the number ratio of samples beyond the background noise level was > 0.3.
3. Results
The hypocenters of earthquakes in the summit region of Mt. Ontake showed clusters at ~ 1900 and 900 m above sea level when the velocity model of Maeda and Watanabe (2023, EPS) was used; they were close to the top and bottom surfaces of an old (0.78–0.39 Ma) eruptive deposit (OED) layer, respectively. The shallower cluster was dominant in the overall seismicity, whereas seismicity in the unrest periods of 2022 and 2024–2025 occurred mainly in the deeper cluster. The seismicity abruptly migrated to the shallower cluster after the largest (M2.3) earthquake on Jan 21, 2025, which was associated with a tremor and tilt change. LF (< 5 Hz) earthquakes occupied only 12% of the total seismicity in Jul–Nov 2024, which increased to 23% in Dec 2024 (the early period of the unrest) and decreased to 14% in Jan 2025.
4. Discussion
Kato et al. (2015, EPS) showed a depth difference of ~ 1 km between the pre- and post-eruptive seismicity at Mt. Ontake in 2014, although the absolute depths were uncertain because of the absence of stations in the summit region. If the post-eruptive seismicity was near the top of the OED as in 2017, the pre-eruptive seismicity was likely near the bottom of the OED. Maeda and Watanabe (2023) postulated that a low-permeability OED prevents the upward migration of fluids from a low-temperature magma reservoir below. The resultant fluid accumulation near the bottom of the OED may explain the 2014 pre-eruptive seismicity and the 2022 and 2024–2025 unrests near this depth. The abundance of LF events in Dec 2024 suggests an intrusion of compressible, relatively gas-rich fluids. The sources of the 2014 pre-eruptive tilt change and the largest earthquakes in 2022 and 2024–2025 were located in the OED, suggesting that fluids that accumulated below the OED ultimately broke the OED. The resultant fluid migration through the OED may explain the shallower seismicity after the 2014 eruption and the largest earthquake in 2025.
5. Acknowledgments
We used arrival times picked by Junko Sumida and Eri Hibino, and seismograms from the Japan Meteorological Agency, Nagano and Gifu prefectures, and the National Research Institute for Earth Science and Disaster Resilience. This study was supported by the Third Earthquake and Volcano Hazards Observation and Research Program.