Modelling of the ice-shelf rifting around Hemmen Ice Rise, prior to the calving of iceberg A38.

The calving of icebergs from the Ronne Ice Shelf is controlled mainly by the propagation of rifts originating from Hemmen Ice Rise (HIR) and d'Orville Coast. Using interferometric radar images collected by ERS-1, ERS-2 and Radarsat-1, we observe the evolution of rifts near Hemmen Ice Rise, prior to the large calving event of October 13th 1998, which resulted in the formation of iceberg A38. Using LEFM models combined with INSAR, Larour (in press 2003) showed that the propagation of those rifts is linked to their opening rate. The opening rate is inferred from the velocity field on each flank of a rift. Therefore, an accurate modelling of the ice-shelf flow is required to predict the evolution of the rifts prior to calving. Forward modelling of the ice-shelf flow by MacAyeal and others (1998) showed that softening along the ice-shelf margins and the presence of melange in between the rifts are key factors influencing the ice flow around Hemmen Ice Rise. Here we use a control method based on MacAyeal (1992) and Rommelaere and others (1997) to infer the distribution of the flow parameter and ice thickness on Ronne Ice Shelf to best match a velocity map from INSAR. The residual misfit between model and INSAR is reduced +-30m/a on the ice shelf. We then apply this parameter selection to run an ice-shelf flow model, coupled with LEFM, to predict the evolution of rifts. The results compare well with satellite observation. We thank the California Institude of Technology for making this study possible.