What triggers caldera ring-fault subsidence at Ambrym volcano? Insights from the 2015 dike intrusion and eruption

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 ∼24×10 6 m 3 and erupted ∼9.3×10 6 m 3 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 moderatesized dike intrusions.

Reservoir depressurization driven by passive gas emissions at Ambrym volcano

“…( 2021)), gas exsolution may occur at deeper levels, ∼1 km. This is still too shallow to exsolve bubbles in a reservoir residing at 2-4 km depth, as estimated for the shallowest levels of Ambrym's magmatic system (Shreve et al, 2019(Shreve et al, , 2021. Geodetic inversions estimate a reservoir depth of ∼2 km beneath Ambrym's lava lakes.…”

“…Previous estimates of Ambrym's shallow magmatic reservoir have ranged from 0.5 (Allard et al, 2015) to 9.5 km 3 (Shreve et al, 2021), with no estimates in the range of >12 km 3 . This may imply one of the following:…”

“…InSAR measures ground displacements after a VEI 2 eruption in February 2015, when an intra-caldera vent opened and fed a lava flow (Figure 1a) (Hamling & Kilgour, 2020;Shreve et al, 2021). A time series is processed with the software MintPy (Yunjun et al, 2019) using images acquired by the European Space Agency's satellite constellation Sentinel-1 (S1) (Figure 1a,b and Section 2.3).…”

“…We multilook ascending S1 interferograms (Track 81) 36 times in range and 12 times in azimuth, filter with a power spectrum filtering strength of 0.3 and unwrap with SNAPHU (Chen & Zebker, 2001). The ascending ALOS-2 interferograms (Track 101) are multilooked 8 times in range and 16 times in azimuth, fil-tered with a power spectrum filtering strength of 0.2, and unwrapped with a coherencebased NSBAS module ("MPD") described in Shreve et al (2021). A 12 m TanDEM-X Global DEM is used to remove the topographic phase contribution (Wessel, 2016).…”

“…Seismicity declined and eruptive activity stabilized by the end of November 2017 (Global Volcanism Program, 2017b). Using the Defvolc 3D Mixed Boundary Element Method (BEM) code (Cayol & Cornet, 1997;Fukushima et al, 2005) and Neighborhood Algorithm (Sambridge, 1999b(Sambridge, , 1999a) (See Shreve et al (2021) for a summary of the BEM and Text S2 for a description of the inversion scheme and uncertainty estimation), we perform a joint inversion of the Sentinel-1 velocity map and ALOS-2 ascending interferogram (scaled to obtain displacement in cm year -1 ). See Text S3 for a detailed description of data preparation for the geodetic modeling, which includes downsampling, estimating the data covariance matrix, and creating the topography mesh.…”