09:15 〜 09:30
[SVC30-02] The 2021 Eruption of Nyiragongo Volcano (Democratic Republic of Congo): Uncovering the Complex Interplay between Tectonism and Volcanism
キーワード：b-value, ETAS model, Rate-and-state friction law, Coulomb stress change, Maximum likelihood, Replenishment method
Nyiragongo volcano is part of the Virunga Volcanic Province (VVP), a complex tectonic setting with eight large volcanic edifices at the borders of the Democratic Republic of Congo (DRC), Rwanda, and Uganda along the western branch of the East African Rift System (EARS). This volcano has both scientific and societal significance. It serves as an ideal site to study volcanism in the extensional setting of continental break-up associated with EARS, highlighting interactions between rift faults, magmatism, and pre-existing basement structures. Furthermore, it poses a serious threat to the densely populated regions of Goma city (D.R.C.), Gisenyi town (Rwanda), and the surrounding villages, which together house over one million inhabitants.
Not much is known about how long the Nyiragongo has been erupting. However, its eruptive style alternates between periods of a permanent lava lake or intermittent eruptions filling the crater, sink collapses, and quiet phases (e.g., Pouclet and Bram, 2021; Bantidi and Mavonga, 2022). The first documented flank eruption at Nyiragongo, including eruptive fissures and lava flow paths, occurred on January 10, 1977, killing at least 70 people. On January 17, 2002, claiming at least 150 lives (Hamaguchi, 2003; Mavonga et al., 2010). After a 20-year repose, Nyiragongo erupted again on May 22, 2021, claiming over 30 lives and causing substantial economic loss. Despite extensive studies, the triggering mechanism of this most recent eruption remains unclear.
Here, we analyze earthquakes beneath the volcano's summit using existing catalogs to examine the b-value variations in the Gutenberg–Richter law, which reflects the ratio of small to large earthquakes and is inversely correlated with differential stress. Our goal is to track the magma system dynamics leading up to the 2021 eruption and identify its catalyst. We observe notable differences in the b-value estimates during the pre- and syn-eruptive phases. First, the b-value consistently increases over 20% above the background level (bref = 0.94) for about two months, peaking at 1.55 two days before the eruption, and then drops to 59% below the bref during the eruption. In the six hours after the eruption, b-values fluctuate around 90% of the bref. These elevated b-values likely reflect the activation of small cracks from the influx of magma in the vicinity of the volcano. Additionally, Coulomb stress changes calculated by combining the Epidemic-Type Aftershock Sequence model with the rate-and-state model, reveal a stress increase of approximately 10.5 MPa just before the eruption. To address missing data, the replenishing method was applied, successfully reproducing b-values and demonstrating its reliability for analyzing volcanic seismicity in regions with sparse networks. Our findings underscore the importance of incorporating systematic b-value time-series analysis into regular volcano monitoring programs to improve forecasting capabilities.
Not much is known about how long the Nyiragongo has been erupting. However, its eruptive style alternates between periods of a permanent lava lake or intermittent eruptions filling the crater, sink collapses, and quiet phases (e.g., Pouclet and Bram, 2021; Bantidi and Mavonga, 2022). The first documented flank eruption at Nyiragongo, including eruptive fissures and lava flow paths, occurred on January 10, 1977, killing at least 70 people. On January 17, 2002, claiming at least 150 lives (Hamaguchi, 2003; Mavonga et al., 2010). After a 20-year repose, Nyiragongo erupted again on May 22, 2021, claiming over 30 lives and causing substantial economic loss. Despite extensive studies, the triggering mechanism of this most recent eruption remains unclear.
Here, we analyze earthquakes beneath the volcano's summit using existing catalogs to examine the b-value variations in the Gutenberg–Richter law, which reflects the ratio of small to large earthquakes and is inversely correlated with differential stress. Our goal is to track the magma system dynamics leading up to the 2021 eruption and identify its catalyst. We observe notable differences in the b-value estimates during the pre- and syn-eruptive phases. First, the b-value consistently increases over 20% above the background level (bref = 0.94) for about two months, peaking at 1.55 two days before the eruption, and then drops to 59% below the bref during the eruption. In the six hours after the eruption, b-values fluctuate around 90% of the bref. These elevated b-values likely reflect the activation of small cracks from the influx of magma in the vicinity of the volcano. Additionally, Coulomb stress changes calculated by combining the Epidemic-Type Aftershock Sequence model with the rate-and-state model, reveal a stress increase of approximately 10.5 MPa just before the eruption. To address missing data, the replenishing method was applied, successfully reproducing b-values and demonstrating its reliability for analyzing volcanic seismicity in regions with sparse networks. Our findings underscore the importance of incorporating systematic b-value time-series analysis into regular volcano monitoring programs to improve forecasting capabilities.