Magma Ascent and Accumulation Drives Surface Deformation and Ring Fault Seismicity Preceding the 2024 Eruption of Fernandina Volcano, Galápagos Islands

Abstract At caldera volcanoes, ring faults can play a key role in controlling the dynamics of unrest and patterns of associated surface deformation and seismicity. Here we show that the March 2024 eruption of Fernandina volcano was preceded by a 2.5‐year episode of magma accumulation, driving surface uplift and asymmetric seismicity on a ring fault system. Analyses of data from a local hybrid seismic network‐array, and satellite‐derived deformation measurements, reveal: (a) that rates of earthquakes increased with surface uplift from November 2021 until the end of April 2023; (b) subsequent unrest involved short‐duration uplift and seismicity pulses; and (c) a final unrest pulse included a deep Ml 4.4 earthquake, followed 20 hr later by the initiation of an intense, laterally‐migrating seismic swarm. This swarm culminated with the eruption onset on a set of circumferential fissures on the SE caldera rim. Spatial patterns of pre‐eruptive surface deformation are consistent with pressurization of shallow sources located ∼2 km below the caldera floor (1 km below sea level), whereas co‐eruptive subsidence also involves contraction of a deeper geodetic source at 4 km below sea level. Earthquake source locations indicate asymmetric ring fault activity close to the deeper deformation source. The data provide no evidence for shallow movement on the ring fault. These observations suggest a complex interplay between the magmatic plumbing and ring fault systems, and reveal important differences in behavior with previous eruptions at Fernandina and elsewhere in the Galápagos.