11:00 〜 11:15
[HTT13-02] Relationship between precipitation just above the lava dome and displacement of the dome at Unzen Fugendake
★Invited Papers
キーワード：溶岩ドーム、雲仙普賢岳、GBSAR、モニタリング、溶岩ドーム崩壊
Introduction
Lava domes pose a significant threat to settlements and infrastructure located at the foot of mountains, and rainfall is considered a major destabilizing factor (Kelfoun,2021). However, quantifying the relationship between rainfall and dome motion has been challenging due to volcano-specific limitations, such as the frequency and location of rainfall data measurements (Carn,2004; Yamasato,1998; Matthews,2002). To address this research gap, this study investigated the relationship between precipitation and dome movement at the Unzen volcano in Japan.
Research location and Method
Unzen Fugendake is a historically active volcano composed mainly of andesite and dacite (Otani,2004; Hoshizumi,1999). The 1991-1995 eruption formed a lava dome on the east side of Fugendake, which has remained stable without major collapse since the end of the eruption but continues to slide downward. To observe micro lava dome variations, ground-based synthetic aperture radar (GBSAR) was used with 48-hour period displacement data from five dome blocks. The displacement was based on the position of the domes 48 hours earlier, and displacement was defined as the distance moved during the 48 hours. In this study, local rainfall at the summit of Mt. Unzen was observed using the Ministry of Land, Infrastructure, Transport, and Tourism's XRAIN, a rainfall observation system that combines high resolution with real-time performance and can observe rainfall at one-hour intervals. In this study, the hourly rainfall just above the dome was averaged to calculate 48 hours (2 days) of rainfall to match the GBSAR data set.
Result and Discussion
After comparing approximately 30 months of time series data from the two datasets, four distinct event sets with rainfall exceeding 150 mm/48 hours were recorded during the study period, and three of these events were repeated within several weeks with distinct displacements in the displacement of the domes. The lower-positioned domes had larger amplitude changes, and the series of changes lasted longer. However, another rainfall event of over 150 mm was detected at the end of March 2020, but it did not show a clear response in displacement, which we attribute to the precipitation that occurred before and after the high rainfall event.
The comparison of rainfall and dome displacement indicates that rainfall events associated with "Background Rain" are effective triggers for dome displacement. The delay in the temporal response from rainfall to displacement is different from the almost immediate relationship between rainfall and dome collapse at active volcanoes that generate pyroclastic density flows (Carn,2004). Our results indicate that lava domes of quiescent volcanoes exhibit different response times to rainfall and that it is important to consider the slow response of domes and extend the warning time over time. This is essential for the application of hazard and disaster risk studies.
Conclusion
In conclusion, the dome of Unzen-Fugendake is reacting to a complex of high-intensity and background rainfall, and so over several month periods, suggesting that the vadose zone and the groundwater movement may play an important control-ling role. Furthermore, this time lag suggests that hazards and disaster risk alerts need to be adapted to account for those delayed changes. The next step in this research will now be to model the time lag of the dome to rainfall events.
References
1. Kelfoun, et al., Bull Volcanol 2021, 83 (2)
2. Carn, et al., Journal of Volcanology and Geothermal Research 2004, 131 (3)
3. Yamasato, et al., Pap. Met. Geophys. 1998, 48 (3)
4. Matthews, et al., Geophysical Research Letters 2002, 29 (13)
5. Otani, et al., Report of Hydrographic and Oceanographic Reserchies No.40 March 2004.
6. Hoshizumi, et al., Journal of Volcanology and Geothermal Research Volume 89, Issues 1-4, April 1999
Lava domes pose a significant threat to settlements and infrastructure located at the foot of mountains, and rainfall is considered a major destabilizing factor (Kelfoun,2021). However, quantifying the relationship between rainfall and dome motion has been challenging due to volcano-specific limitations, such as the frequency and location of rainfall data measurements (Carn,2004; Yamasato,1998; Matthews,2002). To address this research gap, this study investigated the relationship between precipitation and dome movement at the Unzen volcano in Japan.
Research location and Method
Unzen Fugendake is a historically active volcano composed mainly of andesite and dacite (Otani,2004; Hoshizumi,1999). The 1991-1995 eruption formed a lava dome on the east side of Fugendake, which has remained stable without major collapse since the end of the eruption but continues to slide downward. To observe micro lava dome variations, ground-based synthetic aperture radar (GBSAR) was used with 48-hour period displacement data from five dome blocks. The displacement was based on the position of the domes 48 hours earlier, and displacement was defined as the distance moved during the 48 hours. In this study, local rainfall at the summit of Mt. Unzen was observed using the Ministry of Land, Infrastructure, Transport, and Tourism's XRAIN, a rainfall observation system that combines high resolution with real-time performance and can observe rainfall at one-hour intervals. In this study, the hourly rainfall just above the dome was averaged to calculate 48 hours (2 days) of rainfall to match the GBSAR data set.
Result and Discussion
After comparing approximately 30 months of time series data from the two datasets, four distinct event sets with rainfall exceeding 150 mm/48 hours were recorded during the study period, and three of these events were repeated within several weeks with distinct displacements in the displacement of the domes. The lower-positioned domes had larger amplitude changes, and the series of changes lasted longer. However, another rainfall event of over 150 mm was detected at the end of March 2020, but it did not show a clear response in displacement, which we attribute to the precipitation that occurred before and after the high rainfall event.
The comparison of rainfall and dome displacement indicates that rainfall events associated with "Background Rain" are effective triggers for dome displacement. The delay in the temporal response from rainfall to displacement is different from the almost immediate relationship between rainfall and dome collapse at active volcanoes that generate pyroclastic density flows (Carn,2004). Our results indicate that lava domes of quiescent volcanoes exhibit different response times to rainfall and that it is important to consider the slow response of domes and extend the warning time over time. This is essential for the application of hazard and disaster risk studies.
Conclusion
In conclusion, the dome of Unzen-Fugendake is reacting to a complex of high-intensity and background rainfall, and so over several month periods, suggesting that the vadose zone and the groundwater movement may play an important control-ling role. Furthermore, this time lag suggests that hazards and disaster risk alerts need to be adapted to account for those delayed changes. The next step in this research will now be to model the time lag of the dome to rainfall events.
References
1. Kelfoun, et al., Bull Volcanol 2021, 83 (2)
2. Carn, et al., Journal of Volcanology and Geothermal Research 2004, 131 (3)
3. Yamasato, et al., Pap. Met. Geophys. 1998, 48 (3)
4. Matthews, et al., Geophysical Research Letters 2002, 29 (13)
5. Otani, et al., Report of Hydrographic and Oceanographic Reserchies No.40 March 2004.
6. Hoshizumi, et al., Journal of Volcanology and Geothermal Research Volume 89, Issues 1-4, April 1999