The shapes, major axis orientations and displacement patterns of fault surfaces

Displacement contour diagrams constructed using seismic reflection data and coal-mine plans are analysed to establish the factors determining the dimensions, shapes and displacement patterns of normal faults. For blind isolated normal faults in layered sequences the average aspect ratio is 2.15, with sub-horizontal major axes. Earthquake slip-surface aspect ratios range from 0.5 to 3.5 and are independent of slip orientation. The principal control on the shape of blind isolated faults is mechanical anisotropy associated with rock layering, resulting in layer-parallel elongation of fault surface ellipses. Faults that intersect the free surface and/or interact with nearby faults have aspect ratios ranging from 0.5 to 8.4, and are referred to as restricted. Restriction of fault growth has various effects including: (i) reduced curvature of the tip-line and of displacement contours; and (ii) increased displacement gradients in the restricted region. Many faults are restricted at more than one place on their tip-line loop and so have highly irregular shapes and displacement patterns. Subsequent linkage of interacting faults produces combined faults with aspect ratios within the normal range for unrestricted faults. Lateral interaction between faults does not necessarily lead to a change in the power-law exponent of the fault population.