Relating Crevassing to Non-Linear Strain in the Floating Part of Jakobshavn Isbrae, West Greenland

Jakobshavn Isbrae is a major ice stream that drains the west-central Greenland ice sheet and becomes afloat in Jakobshavn Isfiord (69degreesN, 49degreesW), where it has maintained the world's fastest-known sustained velocity and calving rate (7 km a(-1)) for at least four decades. The floating portion is approximately 12 km long and 6 km wide. Surface elevations and motion vectors were determined photogrammetrically for about 500 crevasses on the floating ice, and adjacent grounded ice, using aerial photographs obtained 2 weeks apart in July 1985. Surface strain rates were computed from a mesh of 399 quadrilateral elements having velocity measurements at each corner. It is shown that heavy crevassing of floating ice invalidates the assumptions of linear strain theory that (i) surface strain in the floating ice is homogeneous in both space and time, (ii) the squares and products of strain components are nil, and (iii) first- and second-order rotation components are small compared to strain components. Therefore, strain rates and rotation rates were also computed using non-linear strain theory. The percentage difference between computed linear and non-linear second invariants of strain rate per element were greatest (mostly in the range 40-70%) where crevassing is greatest. Isopleths of strain rate parallel and transverse to flow and elevation isopleths relate crevassing to known and inferred pinning points.

Fear Reduction Processes in Imaginal and In Vivo Flooding: A Comment on James' Review

The research comparing imaginal and in vivo exposure in the treatment of clinically significant fear, recently reviewed by James (1986), is reexamined from the perspective of bioinformational theory and the concept of emotional processing. Fear is assumed to be stored in long term memory as a network of propositionally-coded information, which has to be processed if treatment is to be successful. Emotional processing is indicated by activation of fear responses and their habituation within and across treatment sessions. Consistent with the theory, our review indicates that successful treatment via imaginal and in vivo exposure is indeed related to activation and habituation of fear responses; interference with processing has a negative impact upon fear reduction, regardless of the specific treatment techniques employed. Furthermore, some apparently discrepant findings in the available research literature can be understood in terms of the theories cited. These ideas provide a useful perspective from which to plan future research efforts and to advance our understanding of the processes underlying reduction of pathological fear.