Millennial Slip-Rates Variability of Along-Strike Active Faults in the Italian Southern Apennines Revealed by Cosmogenic ³⁶Cl Dating of Fault Scarps

We present slip versus time histories derived from in situ ³⁶Cl cosmogenic dating for three active normal faults in the southern Apennines, Italy. In this region the total extensional strain is accommodated by either a small number of faults located across strike from each other or, in places, a single fault where no other active faults exist across strike. We investigate how strain-rates on individual faults vary through time in the context of the overall geometry of the fault system. The ³⁶Cl results confirm that the San Gregorio Magno, Auletta, and Vallo di Diano faults were active in the Holocene, with each fault exhibiting alternating periods of relatively rapid and slow, or even absence of, slip. During periods of rapid slip, lasting a few millennia, the faults accumulate up to ∼5 m of slip, which we interpret as earthquake clusters. At other times, the faults exhibit no slip for time periods lasting multiple millennia. The fluctuations in slip-rates reveal the migration of activity between faults and out-of-phase behavior. Such fluctuations have important consequences for tectonic evolution and crustal rheology, and in particular for hazard estimation because they introduce considerable variability and hence uncertainty in earthquake probability calculations.