Dry snow slab avalanches release by propagating fractures. The first fracture occurs in shear within a thin weak layer underneath a planar slab which eventually fails in tension perpendicular to the weak layer at some distance up-slope. Thus fracture properties of both the weak layer (in shear) and the slab (in tension) are important for determining the character of the slab avalanche including its volume and, hence, destructive potential. In this paper, the fundamental fracture properties relevant during slab avalanche release are evaluated for both weak layer shear and slab tensile fracture from field measurements. Data from nearly 300 slab avalanches are used to estimate tensile and shear fracture toughness. Two important practical results come from the analysis. The first is that, on average, slab tensile toughness is about 5--7 times the weak layer shear toughness with a range of about 2--15. This is of immense practical importance since it allows estimates of slab dimensions (length) and width from measured estimates of the slab depth, D. The second important result is that a length scale is provided for a highly stressed (tensile stress) boundary layer of about 20 cm from the weak layer up through the body of the slab over which the tensile crack should first form. This estimate gives an important scale for field evaluation of snow slab instability for measurements from snow profiles. For example, it gives a guideline for estimating important hardness changes between the weak layer and the slab which have been shown from field data to be associated with human triggering of snow slabs. We expect that our results will provide a framework for analysis of rock avalanches and flake-type landslides which fail on weak interfaces under slabs.