In recent years, Iceland’s Reykjanes Peninsula has experienced intense earthquake swarms and repeated volcanic eruptions, including the 2021 Fagradalsfjall eruption and the 2023 crisis near Grindavík that forced thousands of residents to evacuate. Our study, published in Geophysical Journal International, shows how clusters of earthquakes can be used to monitor the underground processes that precede eruptions. Using data from 15 seismic stations of the REYKJANET network operated by the Czech Academy of Sciences, we applied the “in-situ Vp/Vs method”, which measures the ratio of P to S wave velocities within dense swarms of earthquake clusters. This provides a sensitive way to track short-term changes in the physical state of the crust as magma intrudes.
The results are striking: before each eruption, the Vp/Vs ratio dropped significantly, reaching its lowest value just before the first eruption at Fagradalsfjall in 2021—the volcano’s first in 7,000 years. Our analysis suggests that the main cause of this drop is the infiltration of supercritical fluids such as CO₂ into the fracture system, which reduces the P-wave velocity. Crack formation during dyke intrusion or stress accommodation may also contribute to the anomalies, acting together with the fluids. By introducing the concept of “change points”, we were able to pinpoint where and when these shifts occurred along the volcanic dyke, finding a strong match with the eventual eruption site.
Temporal Vp/Vs variations can serve as valuable indicators for forecasting upcoming eruptions, providing an effective early-warning signal for public safety. When combined with online networks such as REYKJANET, this method has a potential for efficient, near real-time volcanic monitoring with minimal computational demands—making it a powerful tool for both understanding and tracking volcanic activity.
Figure 1. Seismic swarms on the Reykjanes Peninsula (2017–2023) with eruption sites, volcanic systems and lava fields (top), and temporal evolution of seismicity in Fagradalsfjall with swarm-specific Vp/Vs ratios (triangles) and eruption onsets (icons, bottom).
Masihi A., Fischer T. (2025): In-situ Vp/Vs ratio variations in seismic swarms as indicator of magmatic processes: Fagradalsfjall volcanic activity, SW Iceland. Geophysical Journal International, 241(1). (DOI)
