Figure 2 . Development of two rifts that led to the 2017 calving event. a . Long and transverse fractures formed between Kenyon Peninsula and Gipps Ice Rise, delineated from the 1963 DISP imagery.b . The locations of R1 and R2 on the 1988 Landsat imagery. The cyan line corresponds to L2 in panel a . c . The locations of R1 and R2 on the 2010 PALSAR imagery, showing the widening of R2 and the propagation of R1 (from points A’ to A and B’ to B).d . The propagation of R1 from 2010 to 2017. e . The widening of R2 between 1988 and 2017 following the 1986 calving event that occurred at the southern ice front.
4.3 Temporal variability of flow velocity during 1963–2020
The earliest flow velocities were derived from the 1963 DISP imagery and the Landsat imagery acquired in 1986 and 1988. Since the available images for 1986 cannot fully cover Larsen C, we used the 1988 imagery to fill the data gaps. We also derived the 1986–1988 flow velocities for the area where both 1986 and 1988 images are available. Due to the limited resolution of the DISP imagery and the long-time span with the Landsat data, the flow velocities for this period (1963–1986/1988) are relatively sparse, and are distributed over the areas with large visible surface features (Figure 3a). Nevertheless, these velocity measurements still provide important baseline information for examining the long-term changes in ice flow. Figure 3b shows the temporal variability of flow velocity over five areas, including the upstream ice shelf (Area 1), the northern downstream portion (Area 2), the ice front near Bawden Ice Rise (Area 3), the central ice shelf (Area 4), and the southern downstream portion (Area 5). Before 1990, there was no significant velocity change across Larsen C. On average, the flow velocities after 2000 were higher compared to the velocities before 1990. Area 1 shows a gradual velocity increase between 2000 and 2016. Both Area 3 and Area 5 exhibit a step change in velocity (~50 m/year) between the 2000–2002 and 2008–2010 periods. The acceleration of Area 3 was related to the northern calving near Bawden Ice Rise in 2004/2005. The acceleration of Area 5 was concurrent with the rift propagation of R1 during 2006–2010. The 2017 calving event did not trigger an immediate flow speedup across the ice shelf.