Assessing Premonitory Phases of Volcanic Eruptions in Near Real-Time: Implications for 'Volcano Traffic Light Alert Systems’

Volcanic eruptions often result from the failure of rocks surrounding magma pathways. A decrease in b -value typically precedes such failures, indicating increased stress and eruption likelihood. Here, we analyze b -value time series of seismic data from 8 well-monitored volcanoes across various tectonic regions: Eyjafjallajökull, Fagradalsfjall, Grímsvötn, and Holuhraun (Bárðarbunga) in Iceland; Mauna Loa in Hawaii (USA); and Kirishima, Ontake, and Usu in Japan. We aim to assess priming timescales and potential improvements in near-real-time forecasting. We define red light as instances when the b -value drops exceed 10% below background level ( bₜₕ ) and examine whether eruptions occur within 10 days. Of the 25 identified cases, 40% are followed by eruption, whereas 60% are not. Among the latter, 71% involve red lights without subsequent eruptions but are mostly linked to large earthquakes, and 29% show no drop before the eruption. Despite the relatively low percentages, our forecasting success rate is similar to those reported in studies using InSAR analysis, GNSS deformation, or thermal anomalies. This suggests that monitoring changes in b -value can serve as a valuable indicator of impending eruptions. However, to improve reliability, we recommend integrating b -value analysis with other geophysical observables to better distinguish volcanic unrest from imminent eruptions.