Long-period microseismicity reveals cryptic earthquake-triggered fluid activity can facilitate caldera eruptions

Spatiotemporal correlations between moderate-to-large earthquakes and volcanic eruptions indicate that earthquake-induced stress changes can trigger eruptions. Triggering is rarely instantaneous, with time-dependent processes inferred to mediate magma ascent to the surface, such as fracture propagation or fluid pressurization. While various processes have been proposed, observational constraints on the specific triggering mechanisms remain limited. Here we integrate an earthquake catalogue, generated by machine-learning and template-matching techniques to enhance microseismicity detections, with geodetic data to show that seismic triggering of the 2018 eruption of Sierra Negra Volcano, Galápagos Islands, was facilitated by a cryptic phase of fluid activity. Following 13 years of near-continuous magmatic inflation, totalling > 6.5 m, stress changes induced by a moment magnitude 5.4 intra-caldera earthquake on 26 June 2018 did not immediately trigger magma release from the pressurized magma reservoir. Instead, long-period earthquake swarms revealed post-seismic fluid activity along reservoir-bounding faults in the northwestern caldera, locally promoting edifice failure and facilitating magma intrusion that initiated 8 hours later. These observations demonstrate that even when a pressurized reservoir experiences significant stress perturbation, cascading processes could potentially be essential to initiate volcanic eruptions.