The estimation of fault slip from map data: The separation-pitch diagram

Although an essential feature of faults is that they produce a relative movement of two blocks, the total or net slip is known only for a very small percentage of faults. This note proposes a diagram for the graphical determination of net slip from the strike separation of markers recorded on geological maps, with or without knowledge of the net slip direction. The strike separation of one marker allows the drawing of a solutions line on the diagram, depicting the range of slip vectors compatible with the measured separation. The net slip vector can be obtained by either the intersection of two or more solutions lines, or from a single solutions line if the direction of slip is independently known. The diagram gives a visual appreciation of the effects of data uncertainty on the estimate of net slip, and is a useful device for the representation of the kinematic characteristics of a collection of faults. The use of the diagram is illustrated with examples from the South Wales Coalfield and the Moine Thrust zone. ► A new technique is presented for estimating the net slip of a fault from separation data. ► The method is simple and involves plotting a graph which visually illustrates how the available separation data constrains the net slip. ► The new graph gives a visual appreciation of the effects of data uncertainty on the solution.