Fault slip evolution determined from crack-seal veins in pull-aparts and their implications for general slip models

Most observations of fault slip history are limited to a short period of a faultʹs lifetime (usually less than five major earthquake cycles) and are therefore not representative of long-term slip behaviour. A new approach to determine more complete slip histories on small inactive faults is outlined, where calcite-filled veins produced by individual slip events are measured. Two normal faults are described from the south Bristol Channel area, near Kilve, north Somerset, U.K. Fault 1 had a variable slip evolution, with variable slip magnitudes through time, but a smooth and fairly constant cumulative increase in displacement. Fault 2 also has a variable slip history but with temporary repetition of similar-sized (characteristic) slip events which were limited to a maximum of eight successive increments. Each set of temporary characteristic events was followed by a period of variable slip. On both faults there were no significant changes in slip increments immediately preceding or following the larger slip events. The average amount of cumulative slip per event at different localities was either constant, accelerated, or decelerated during the fault growth history. The two faults show a slip pattern where the total displacement along the fault varies smoothly, and magnitude of slip increment at any point is highly variable through time. The data from Fault 2 suggests that the central area, where displacement was a maximum, resulted from larger slip for each event rather than more frequent events of similar magnitude, whereas Fault 1 suggests more frequent slip events occurred in the centre of the fault to produce the displacement maximum.