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Abstract:
Large basaltic volcanoes offer natural laboratories to study the fundamental processes of crustal deformation and seismogenesis. High rates of magma flux in to and/or out of shallow reservoirs can result in strain rates and stress regimes that seldom occur in purely tectonic settings. Eruption cycles at Sierra Negra, a basaltic caldera volcano in the western Galápagos Islands, Ecuador, are often associated with particularly high amplitudes and rates of deformation, and high rates of seismicity. Spatiotemporal patterns of surface deformation at Sierra Negra can largely be explained by magma flux in and out of a shallow sill within a laterally continuous viscoelastic medium. However, seismic strain is localized almost exclusively on an intra-caldera ‘trapdoor’ fault system. Here we describe the seismicity and deformation that occurred before, during, and after the 2018 eruption at Sierra Negra, documenting the activation and stress evolution of the trapdoor fault system. During the 13 years and 6.5 m of inflation before the eruption, increasing seismicity rates with both time and total uplift, and progressively degreasing Gutenberg-Richter b-values. The stress conditions on the trapdoor fault reversed during the co-eruptive deflation. On resumption of inflation, the number of earthquakes per unit of uplift was very low, and the b-value high, reflecting the newly relaxed stress state of the fault system.
