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 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.