Magma-fluid interactions beneath the Akutan Volcano in Aleutian Arc
based on the results of local earthquake tomography
Abstract
Akutan Island hosts a volcano considered as one of most active in the
Aleutians Arc. Besides the regular eruption activity, Akutan in 1996
experienced a remarkable seismic unrest with earthquakes reaching the
magnitude of M 5.3. We build a new tomography model including the 3D
distributions of the , and ratio based on arrival time data from more
than 4000 local earthquakes. In this model, we reveal a columnar anomaly
of high ratio with the top boundary at a depth of 5-6 km below surface,
which represents a steady conduit feeding the Akutan volcano. At a depth
of ~4 km, the deep conduit is split in two branches, one
of which ascend to the summit area and another one directs to a fumarole
field at the flank of the volcano. This structure explains distinct
geochemical features of emitted gases in these two areas. In the upper
part of the velocity model, the highly heterogeneous structures are
associated with interactions of shallow magmatic sources, meteoric and
magmatic fluids, as well as degassing. Besides the prominent anomaly
associated with the shallow magma reservoir beneath the caldera and the
active cone, we observe several areas with high ratio, some of which are
interpreted as shallow magma storages, and some as zones of meteoric
water penetration. We propose a scenario of abrupt fluid ejection from
the deep magma conduit that led to the seismic unrest and strong ground
deformations in the Akutan area in 1996.