Conference Abstract: Lecture format preferred The Slab Avalanche as a Self-Organized Critical Phenomenon David McClung Professor and NSERC - Canadian Mountain Holidays Chair in Snow and Avalanche Science Department of Geography University of British Columbia Vancouver, B.C. V6T 1Z2 Tel: 604-822-9157; FAX: 604-822-6150 e-mail: email@example.com Abstract: Alpine snow causing slab avalanches fails in shear as a pressure sensitve, dilatant strain-softening material. This behaviour is similar to soils responsible for some landslides, such as over-consolidated clay, and rock joints which are responsible for earthquakes. Avalanches, landslides and earthquakes undergo a transition in time scales when they occur: rapid fractures are preceded by a much longer period of development before failure. In this sense, they represent some characteristics of a critical system. During the 1990's, self-organized ciriticality (SOC) as formulated by Per Bak and others was explored by countless papers, with perhaps the most quoted papers in all the scientific literature. Avalanche phenomena were used initally as the proto-types with a combination of modelling and lab experiments. In order to obey SOC, basically 3 criteria must be satisfied for a phenomenon: 1. it must fit the definition of a critical system; 2. size scaling for events must be scale invariant; and 3. the arrival frequency must be fractal. In this paper, I present field data on slab fracture properties and time arrival of slab avalanches to explore these 3 criteria for the snow slab. The results represent the first time that all three criteria for SOC have been examined entirely from field data for avalanche phenomena without resorting to theoretical modelling. The primary results are: 1. the c lassical definition of a critical system does not fit for slab avalanches; 2. slab avalanches are not scale invariant with respect to the fundamental size scaling parameter (slab thickness) derived from fracture mechanics. Instead, the slab thickness follows a log-normal probability density function; 3. time arrival of slab avalanches in the frequency domain is not fractal as shown by calculation of power spectra. Instead, the frequency dependence appears to decay exponenitally The combined results suggest that if the slab avalanche is to be described as a critical system similar to SOC, modification of the definitions proposed for SOC is required. The results have fundamental implications for definition of critical systems and forecasting for natural events including avalanches, landslides and earthquakes.