What Triggers Caldera Ring‐Fault Subsidence at Ambrym Volcano? Insights From the 2015 Dike Intrusion and Eruption

T Shreve,R Grandin,Y Morishita,V Pinel,V Cayol,M Boichu,D Smittarello

doi:10.1029/2020jb020277

Abstract

AbstractSurface 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 preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. 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 × 106 m3 and erupted ∼9.3 × 106 m3 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.

Highlights

All calderas host collapse structures, instrumentally recorded caldera ringfault activation is rare
At Sierra Negra, which hosts the largest caldera in the Galápagos (9 km maximum diameter), there is no geological evidence of a catastrophic caldera collapse (e.g., lithic breccias and ignimbrite deposits (Druitt & Bacon, 1986)) in the literature, despite voluminous historical eruptions (Reynolds et al, 1995; Munro & Rowland, 1996)
Using the 3D Boundary Element Method, we conclude that the stress transfer from a depressurizing reservoir most likely promoted ring-faulting

Summary

Introduction

All calderas host collapse structures, instrumentally recorded caldera ringfault activation is rare. A handful of extreme cases of caldera ring-fault activation—catastrophic caldera collapse—have been observed historically (Neal et al, 2018; Gudmundsson et al, 2016; Peltier et al, 2009; Geshi et al, 2002). Many of these spectacular events included noncoherent, or piecemeal, collapse, where upwards propagating faults break up the collapsing piston (e.g., Bárðarbunga (Ágústsdóttir et al, 2019), Kılauea At Sierra Negra, which hosts the largest caldera in the Galápagos (9 km maximum diameter), there is no geological evidence of a catastrophic caldera collapse (e.g., lithic breccias and ignimbrite deposits (Druitt & Bacon, 1986)) in the literature, despite voluminous historical eruptions (Reynolds et al, 1995; Munro & Rowland, 1996)

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