Text S1 Effect of a structural discontinuity on the deformation pattern
We explore the principal effect a of structural discontinuity on surface deformation in
order to test ground deformation perturbation as a possible scenario to explain the large
subsidence recorded during the period of deflation at the east flank of SHV. We
implemented a fault into the model domain that cuts the flat surface in a lateral distance
of 3 km to the source centre and extends downwards to a depth of 3 km. We applied dip
angles in the range between 56° towards (reverse faulting) and 37° away from the
source (normal faulting). Accounting for faults with smaller dip angles seems
unreasonable given the fact that the anomaly is found on the vertical rather than on the
horizontal component of the data (Fig. S1a-b). Except for the structural discontinuity,
which is defined via very low rigidity values (E = 0.1 Pa), the medium is assigned with
homogeneous property conditions (E = 30 GPa). We investigated the effect of a fault on
the deformation pattern exemplarily for a prolate pressure source that is centred in
12 km below sea level.
Our results show that a structural discontinuity affects the predicted deformation pattern
simultaneously on the horizontal and the vertical component (Fig. S1c-d). The most
markedly changes are found at the intersection of the fault with the surface, where a
sudden decrease of vertical deformation occurs, while simultaneously the amplitude of
horizontal deformation escalates. Recorded data from Montserrat, however, show no
differences in the amplitude of horizontal deformation at the east flank of SHV
compared to all other areas on the island. In contrast, the observed vertical deformation
at the east flank is markedly increasing with distance to the vent (see Fig. S1a-b).
Comparing the recorded data with the predicted results from our model with a structural
discontinuity in the deformation field, we infer that the data can not be explained by the
presence of a fault east of the SHV vent.
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A2 Influence of topography and lateral heterogeneities
Differences in elevation of the cGPS site locations with respect to their radial distance
from the source centre can be accounted for in a first approximation when applying a
volcanic slope of 2° (0.4 km height, 11 km length). We implemented the simplified
topography into our model in order to compare the results with the predicted
deformation for the flat surface model and thus, to have a control on the influence of
first-order topographic effects.
A 3-D image of the P-seismic velocity to a depth of 5 km beneath Montserrat [Shalev et
al., 2008] revealed a zone of dense material with E = 60 GPa that is located beneath the
centre of SHV. This zone is of oblate geometry with a radius of approximately 2 km,
extending between 1.5 and 3 km depth. We performed a model that accounts for this
observation as well as for the compliant material in the shallow magma chamber (sphere
with a radius of 1 km, centred at 6 km depth) that is constrained to E = 18 GPa. Our
results allow us to quantify the influence of second-order heterogeneities on ground
deformation.
We exemplarily tested the influence of topography and lateral heterogeneities on ground
deformation applying a slightly prolate magma chamber that is centred at 12 km below
sea level (i.e., source parameters inferred from analyses of ground inflation data). We
further applied crustal heterogeneity as deduced from the P-wave velocity profile.
We found that accounting for topography does not affect the vertical deformation
pattern, but yields an amplification of horizontal deformation, with an increase of the
ratio Urmax/Uzmax of 0.05 (Fig. A2). The additional application of lateral
heterogeneities results in a very minor increase of (i) the wavelength of the vertical
deformation as well as (ii) the amplitude of horizontal deformation (increase of
Urmax/Uzmax by 0.02; Fig. A2). The differences in the predicted deformation pattern
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correspond to a pressure source with a slightly less eccentric geometry, when
additionally applying topography and second-order heterogeneities as opposed to flatsurface models with first-order mechanical heterogeneities only. However, the
difference in eccentricity a/c of the sources inferred from both model approaches is
about 0.1 and has thus only minor impact on the results, given the uncertainty in the
data.
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