Spatial and temporal variations in growth rates along active normal fault Systems: an example from the Lazio-Abruzzo Apennines, central Italy

The geometry, kinematics and rates of active extension in Lazio–Abruzzo, Italian Apennines, have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems. We present fault map traces, throws, throw-rates and slip-directions for 17 parallel, en e´chelon or end-on active normal faults whose 20– 40 km lengths combine to form a soft-linked fault array ca. 155 km in length and ca. 55 km across strike. Throw-rates derived from observations of faulted late-glacial features and Holocene soils show that both maximum throw-rates and throw-rate gradients are greater on centrally-located faults along the strike of the array; total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike. When summed across strike, throw-rates are increasingly high towards the centre of the array relative to summed throws. We interpret the above to suggest that throw-rates have changed in the recent past (ca. 0.7 Ma) from spatially-random fault growth rates (initiating at 2.5–3.3 Ma) to growth rates that are greater on centrally-located faults. We interpret this as evidence for fault interaction producing throw-rate variations that drive throw profile readjustment on these crustal scale soft-linked faults. The results are used to discuss seismic hazards in the region, which are quantified in a second paper in this issue.