| Finite element analysis of the two-dimensional distribution of stress around a single fracture in a layered medium characterizes regions of reduced crack-driving stress around open extension or internally pressurized fractures (i.e., driving-stress shadows). Fractures are inhibited from forming in a driving-stress shadow. Under a far-field tension, a driving-stress shadow is wider in beds with higher Young's moduli (E), suggesting fracture spacing should be wider in these beds. This contradicts the observation that more closely spaced fractures occur in beds with higher E, but is explained by the relationship between E and fracture stress under far-field tension. For fluid-driven fractures, a driving-stress shadow is wider in beds with higher E, resulting in more widely spaced fractures in these beds. The spacing of fluid-driven fractures decreases with increasing pore pressure.;In a given tectonic environment, the stratigraphic distribution and orientation of fractures is controlled by the contrast in Poisson ratio, fracture toughness, poroelastic constant and flaw size in adjacent rock types. In older, uplifted basins where rock properties do not change dramatically with depth, fractures are naturally partitioned into rock types with lower Young's moduli, poroelastic constants and fracture toughnesses. In young, actively subsiding basins where rock properties change with depth, fracture distribution is highly dependent on the poroelastic constant. When the poroelastic constant is greater than 0.6 all fractures will initiate horizontally and will be partitioned into carbonates. For poroelastic constants less than 0.3, all fractures will initiate vertically and will be weakly partitioned into rocks with the lowest poroelastic constants.;Use of a modified ring test specimen alleviates some of the problems currently causing the wide scatter of ice fracture toughness values. Modified ring tests performed on synthetic ice Ih and firn from the GISP2 site yield mean fracture toughness values of 146.3 and 105.7 kPa... |
