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What triggers caldera ring-fault subsidence at Ambrym volcano? Insights from the 2015 dike intrusion and eruption
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  • Tara Shreve,
  • Raphaël Grandin,
  • Delphine Smittarello,
  • Valérie Cayol,
  • Virginie Pinel,
  • Marie Boichu,
  • Yu Morishita
Tara Shreve
Institut de Physique du Globe de Paris, Institut de Physique du Globe de Paris

Corresponding Author:[email protected]

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Raphaël Grandin
Institut de Physique du Globe de Paris, Institut de Physique du Globe de Paris
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Delphine Smittarello
University Grenoble Alpes, University Savoie Mont Blanc, University Grenoble Alpes, University Savoie Mont Blanc
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Valérie Cayol
Laboratoire Magmas et Volcans, Laboratoire Magmas et Volcans
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Virginie Pinel
Institut de Recherche pour le Développement, Institut de Recherche pour le Développement
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Marie Boichu
Université de Lille, Université de Lille
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Yu Morishita
Geospatial Information Authority of Japan,Hokkaido University, Geospatial Information Authority of Japan,Hokkaido University
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Abstract

Surface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate pre-existing, caldera ring-faults. Ring-fault reactivation, however, is rarely observed during moderate-sized eruptions. On February 21st, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 meter of co-eruptive uplift, as measured by InSAR, SAR correlation, and Multiple Aperture Interferometry (MAI). Here we show that an average of ∼40 cm of slip occurred on a normal caldera ring-fault during this moderate-sized (VEI < 3) event, which intruded a volume of ∼24 million cubic meters and erupted ∼9.3 million cubic meters of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym’s main craters. After estimating magma compressibility, we provide an upper-bound on the critical fraction (f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate- sized dike intrusions.
Jun 2021Published in Journal of Geophysical Research: Solid Earth volume 126 issue 6. 10.1029/2020JB020277