What Triggered the 2021 eruption at Nyiragongo volcano (D.R. Congo)? Unraveling the Complex Interplay between tectonism and magmatism

Abstract

After a 20-year repose, Nyiragongo volcano in the eastern Democratic Republic of the Congo experienced a flank eruption on May 22, 2021, claiming over 30 lives and causing substantial economic loss. Despite extensive studies, the eruption's triggering mechanism remains unclear. In this study, we analyze earthquakes beneath the volcano's summit to examine the b-value variations in the Gutenberg–Richter law, which reflects the ratio of small to large earthquakes and inversely correlates with differential stress. Our goal is to track magma system dynamics leading to the eruption and identify its catalyst. We observe notable differences in b-value estimates during the pre- and syn-eruptive phases. First, the b-value consistently increases over 20 % above the background level ($b_{\mathrm{ref}}$ = 0.94) for a few days, peaking at 1.55 two days before the eruption, then drops to 59 % below the $b_{\mathrm{ref}}$ during the eruption. In the six hours following the eruption, b-values fluctuate around 90 % of the $b_{\mathrm{ref}}$. 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 (ETAS) model with the rate-and-state model reveal a stress increase of approximately 10.5 MPa just before the eruption. To address the issue of missing events caused by seismic network density incompleteness and short-term aftershock incompleteness, we created a replenished catalog. Analyzing this catalog, we successfully reproduced b-values, 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 capability.