Figure 8 . Longitudinal deviatoric stress fields for geometry
conditions F2 (panels a and c , ice front in 2000) and
F3 (panels b and d , ice front in 2013). The background
image in panel c is the Envisat ASAR image acquired on August
03, 2004, showing the rift causing the northern calving event. The
background image in panel d is the Landsat-8 OLI panchromatic
image acquired on January 11, 2014, showing the expansion of R2 to Gipps
Ice Rise.
5.4 Impact of rift-induced weakening (rift scenarios)
The modeling results of rift scenarios indicate the R2-induced weakening
near Gipps Ice Rise was critical to the propagation of R1. If the
weakening effect of R2 is not considered, the southern rift tip (‘A’) of
R1 would be in a compressive longitudinal condition (Figure 8d). Once
the R2-induced weakening is incorporated (Figure 9b), the southern rift
tip (‘A’) would be in a tensile longitudinal condition that is favorable
for rift growth. This suggests that R1 would be less likely to propagate
towards the south if the ice shelf were not damaged by R2 near the ice
rise. Based on satellite images, R1 almost reached R2 in 2010 (Figure
2c), which further weakened the area. The continued weakening can
further enhance the longitudinal stretching over an expanded area for
both rift scenarios (Figures 9e, h), indicating that this weakening
could promote the subsequent northward propagation of R1.
We also analyzed the first principal stresses (Figure 10). The first
principal stress is the maximum value of stress that is normal to the
plane in which the shear stress is zero. Therefore, it indicates the
magnitude and orientation of the maximum tensile stress. Fractures tend
to open along the direction of the first principal stress, and the
opening rate would be maximized if the flow direction is along the first
principal axis (e.g., Benn et al., 2007; Colgan et al., 2016; Kulessa et
al., 2014). The angle between the first principal axis and flow
direction (stress-flow angle) can be used to evaluate rift propagation
and ice shelf stability (Kulessa et al., 2014). A small stress-flow
angle would be more likely to promote rift propagation than a large
angle. The rift-induced weakening would modify the stress-flow angles
(Figures 10a–c) along the trajectory of R1 to more favorable conditions
for rift propagation. The magnitude of first principal stresses would
also increase in response to the rift-induced weakening (Figure 10d-f).
The northern rift tip (‘B’) of R1 reached an area with stress-flow
angles of less than 10 degrees after January 2016, which could explain
the rapid propagation of R1 afterwards. The path of rift tip ‘B’ growth
was perpendicular to the flow direction. Once the rift tip was in the
area where the first principal axis was aligned with the flow direction,
the rift growth was unstoppable until it reached to the area where the
stress-flow angles were greater than 30 degrees. Therefore, the
rift-induced weakening could increase both longitudinal stresses and
first principal stresses and modify the stress-flow angles nearby,
resulting in favorable stress conditions for the rift (R1) propagation
towards the north. The opening of R2 therefore preconditioned the R1
growth and the 2017 calving event.
The modeling results also indicate the rift-induced weakening caused the
localized flow accelerations at the downstream portion of southern
Larsen C. The flow velocity increases near Gipps Ice Rise as compared to
the non-rift scenario are shown in Figure 9d. With the further enhanced
weakening, the acceleration increases both upstream and downstream of
the rifted location (Figure 9g). This is consistent with the observed
spatial pattern of velocity increase (Figure 4). The rift development
can also reduce the ice shelf backstress (Figures 9c, f, i), although
the magnitude and extent of backstress loss depend on how much weakening
is caused by rifts. With enhanced weakening, the backstress loss can
extend upstream and potentially reach the grounding line. This suggests
that rift propagation needs to be considered for evaluating the
buttressing change of ice shelves.