In-Situ Vp/Vs ratio variations in seismic swarms as indicator of magmatic processes: Fagradalsfjall volcanic activity, SW Iceland

Abstract

In this study, we applied the 'in-situ Vp/Vs method' to monitor variations of seismic velocity ratio (Vp/Vs) within swarms, providing insights into eruption processes. This method, particularly effective in volcanic regions, estimates Vp/Vs by comparing P- and S-wave arrival times of closely located earthquake pairs, reducing errors from unknown crustal velocity variations and is well-suited for detecting rapid changes associated with volcanic swarms. Our study focused on seismic swarms on the Reykjanes Peninsula, south-west Iceland where, swarms have been frequent since 2017 and led up to eruptions in 2021, 2022 and 2023. We analysed the entire period (2017-2023) as well as the 2021 swarm separately using data from over 40 000 seismic events recorded by the REYKJANET network. We observed significant decrease in the Vp/Vs ratio before major pre-eruption swarms, compared to the background Vp/Vs value of 1.78. From the 2020 swarm, we observed a lower Vp/Vs of 1.72, but the lowest estimated value was 1.70, associated with the 2021 pre-eruption swarm that preceded Fagradalsfjall's first eruption after 7000 yr. Reduced Vp/Vs ratios were also noted before the 2022 and 2023 eruptions, suggesting supercritical fluids in the crust during these stages. We also introduce the concept of 'change points' to interpret Vp/Vs variations along the dyke. Change points denote specific locations or times of significant Vp/Vs shifts, potentially indicating subsurface changes such as fluid influx or new fracturing from magma intrusion. Identifying these points allows us to pinpoint key moments when the system undergoes substantial changes, offering insights into eruption timing and location. Focusing on 2021 pre-eruption swarm, interestingly the spatial change point found in a location very close to the eruption site. Temporal analysis identified two main change points: the first corresponding with initial activity in the northern dyke and the second with a shift to the southern segment, ultimately leading to eruption. These points mark stages in magma progression, with each showing an initial rapid Vp/Vs drop that could indicate CO2-rich fluid infiltration, followed by an increase as magma enters. The in-situ Vp/Vs method's sensitivity to changes in seismic properties makes it a powerful tool for real-time volcanic monitoring. By detecting critical Vp/Vs changes with minimal computational demand, this method has potential for integration with online seismic networks, providing an effective early warning system for volcanic hazards.