Sea ice strain, stress, and fracture activity at kilometer scales

Abstract

Through-thickness fractures including cracks, leads, and pressure ridges divide sea ice into individual plates and plate assemblies. While they remain intact, plates deform via continuous strain as they interact. Radar interferometry can identify active fractures at plate assembly boundaries and measure the continuous strains between them at high spatial resolution and spatial scales from meters to kilometers. However, interferograms are only sensitive to the one­ dimensional component of surface strain parallel to a radar’s lines of sight. Working with coauthors, I develop a novel analytical inverse model to estimate two-dimensional, horizontal surface motion from the one-dimensional information provided by interferograms over areas of ice experiencing spatially uniform strain. Model results accurately capture thermal strain in sheltered landfast ice and realistically estimate rigid displacements in drifting ice. In areas of non-uniform strain, we combine one-dimensional interferometric strain measurements with field observations from a sea ice camp in the Beaufort Sea to investigate relationships between strain, stress, and fracture activity. We calculate the first published estimates of the effective elastic modulus, E*, and effective Poisson’s ratio, v*, of in situ drifting sea ice under natural loading rates. We estimate E* within the range typically used in sea ice models but estimate v> > 0.5, larger than typically assumed and indicative of anisotropy in sea ice Poisson response at low strain rates. Finally, we synthesize interferometric records of strain and fracture to identify an approximately 1 km radius of influence of impact forces resulting from contact across active fractures. We also identify apparent fracture reactivation after multi-day quiescent periods, indicating prior fractures may remain weaker than surrounding ice for such periods. Together, this work outlines both new observations and new tools for future researchers to utilize in studies of sea ice mechanics and dynamics at intermediate scales in areas of high-concentration winter pack ice.