Abstract -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 0.5-3.5 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. Many faults intersect the free surface and/or
interact with nearby faults and have aspect ratios ranging from 0.5-8.4
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 the 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.
Journal of Structural Geology 18, 235 -248, 1996.