Constraining slip rates and spacings for active normal faults

Numerous observations of extensional provinces indicate that neighbouring f aults commonly slip at different rates and, moreover, may be active over di fferent time intervals. These published observations include variations in slip rate measured along-strike of a fault array or fault zone, as well as significant across-strike differences in the timing and rates of movement o n faults that have a similar orientation with respect to the regional stres s field. Here we review published examples from the western USA, the North Sea, and central Greece, and present new data from the Italian Apennines th at support the idea that such variations are systematic and thus to some ex tent predictable. The basis for the prediction is that: (1) the way in whic h a fault grows is fundamentally controlled by the ratio of maximum displac ement to length, and (2) the regional strain rate must remain approximately constant through time. We show how data on fault lengths and displacements can be used to model the observed patterns of long-term slip rate where me asured values are sparse. Specifically, we estimate the magnitude of spatia l variation in slip rate along-strike and relate it to the across-strike sp acing between active faults. (C) 2001 Elsevier Science Ltd. All rights rese rved.