Characteristics and modelling of slip-rate variability and temporal earthquake clustering across a distributed network of active normal faults constrained by in situ 36Cl cosmogenic dating of fault scarp exhumation, central Italy

We present a compilation of new and unpublished in situ ³⁶Cl cosmogenic isotope data recording the exhumation of 27 active normal fault planes by earthquake slip for the central Apennines, Italy. We do this to constrain the characteristics of slip-rate variability and temporal earthquake clustering and anticlustering across the entire extending orogen, and to assess why it occurs. From the ³⁶Cl observations we report(1) the long-term slip-rates averaged since 20 ka, (2) the percentage of sites with clusters and anticlusters in each 1 kyrs time-slice back to 20 ka, (3) regional maps showing cluster locations for every 1 kyr time-slice, (4) cluster and anticluster durations, and (5) the amounts of slip and slip-rates averaged over the duration of clusters and anticlusters. To study why this slip-rate variability has developed we conduct modelling of stress interactions between faults and underlying shear-zones, and between neighbouring fault/shear-zone structures. We show that the measured slip-rate variability and temporal clustering can be replicated by a model where the transfer of differential stress between faults and their underlying shear-zones, and between neighbouring fault/shear-zone structures, produces changes in strain-rates on underlying viscous shear-zones which drive periods of rapid or reduced slip-rate on their overlying faults. We suggest that stress and hence strain-rate increase on an underlying shear-zone produced by coseismic slip on its overlying fault is the mechanism that initiates an earthquake cluster. Clusters progress because the increased strain-rate on the shear-zone reloads the overlying fault producing a positive feedback loop. The clusters also produce stress reduction on fault/shear-zones located across strike, initiating anticlusters in those locations. The durations of anticlusters will be set by the summed coseismic and interseismic stress changes through time, because although these shear-zones develop relatively low viscous strain-rates, eventually they will load their overlying fault to failure initiating a new cluster. These interactions cause the locations of clusters and anticlusters to migrate across and along-strike within the fault system. Such constraints on the processes producing clustering and anticlustering should allow observations of these phenomena to be included in probabilistic seismic hazard assessments (PSHA), and interpretations of the rheology of deforming continental crust.