Tide-induced Flexure of an Antarctic Ice-Shelf as seen by a Coupled Seismic-GPS Instrument

Ice-shelves are known to be the keystone to better understand the impact of the climate change on ice-sheets. Indeed they are the most vulnerable pieces of the ice-sheet system as they are in direct contact with the ocean. Until now most of the studies carried out to better understand the ice-shelf dynamics use single geophysical methods only. In early 2014, we conducted a field study using 4 experimental coupled seismic-geodetic instruments installed for a month at the transition zone between an ice-rise promontory and the Roi Baudoin ice-shelf in East-Antarctica. These relatively low-cost, low-weight and low-consumption instruments originally designed for monitoring soil deformation in small (max 1 km) and dense (every 100 m) deployment were used here to detect continuously and simultaneously ice cracks-induced seismicity and surface ice displacements over a 100 km2 ice covered area. We found that while the horizontal displacement is found to be linear in space and time (with a maximum of 35 cm/day), strong periodicity is observed for vertical displacement correlating almost perfectly with regional tide height models and tide gauge observations a few hundreds of km away. Such a periodic vertical displacement (with a maximum of 1.5 m in 5 hours) can be associated with periodicity in the local seismicity which shows the largest amplitudes for the grounding line area explained by the tide-induced flexure of the ice-shelf. In addition fewer seismic events of larger amplitude are recorded on the 4 sites and are localized at the front of the ice-rise promontory. Due to their low frequency content and their occurence at the start of spring tides, they may be attributed to basal instabilities triggered by renewed infiltration of large amount of ocean water. Preliminary results from a complementary experiment carried out in early 2015 will also be presented from the analysis of 15 similar instruments installed further upstream on the main flow of the same tributary ice stream.