Investigating magmatic processes on the Reykjanes Peninsula, Iceland, with local earthquake tomography

The Reykjanes Peninsula in SW Iceland has experienced repeated volcanic eruptions since 2021, eliciting active interest in its magmatic architecture. Here, we present new 3-D local earthquake tomographic models of P- and S-wave velocity and V P /V S ratio of the crust beneath the active volcanic systems, providing context for the ongoing magmatic unrest on the Reykjanes Peninsula. Our models are derived from manual P- and S-wave arrival time picks from ∼ 1200 local earthquakes recorded throughout 2020–2024. Synthetic tests demonstrate the robustness of recovered model features in the 2–6 km depth range. We recover a low-velocity region at 4-6 km depth beneath the Svartsengi geothermal field, which we interpret as the reservoir supplying magma to the 2023–2025 eruptions, in agreement with geodetic modelling and petrological studies. Reduced velocities and elevated V P /V S are also imaged below the site of ongoing ground deformation in the Krýsuvík volcanic system at ∼ 5 km depth; however, the relative lack of constraints on structure at depth in this region precludes robust interpretations. Reduced velocities aligned with the inferred plate boundary are recovered at the brittle–ductile transition depth in the Fagradalsfjall volcanic system. A low V S , high V P /V S anomaly is imaged at 7–12 km depth beneath the Fagradalsfjall eruption site, constrained by a single cluster of deep, long-period events. We interpret this as the potential signature of magma stalling at mid-crustal depths during ascent from a deeper reservoir. Through synthetic inversions, we show that low-velocity features in the lower crust are poorly constrained with the available earthquake distribution, highlighting the importance of careful resolution analysis in local earthquake tomography.