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.