Structural controls on simultaneous earthquake clustering and normal fault synchronization

• 36 Cl cosmogenic dating show clustered activity of neighbouring along-strike faults. • Mutual stress changes between shear-zones prompt simultaneous earthquake clusters. • Fault synchronization is driven by the synchronization of their viscous shear-zones. • Low spacing and minimal strike changes favours simultaneous earthquake clustering. Slip-rate fluctuations over multiple seismic cycles are a key factor to consider regarding the behaviour of active faults, as they are associated with clustering of earthquakes in space and time and may alter the earthquake recurrence on neighbouring faults. However, processes that produce slip-rate fluctuations are yet to be fully defined. This paper tests whether the interaction between neighbouring along-strike brittle faults/viscous shear-zones can produce slip-rate fluctuations associated with simultaneous earthquake clustering and fault synchronization. To achieve this, we study nine normal faults/shear zones of the Central Apennines fault system (Italy), organised in six different arrangements of along-strike pairs with different values of fault spacing and strike variation. We combine cosmogenic 36 Cl dating of tectonically exhumed fault planes and modelling of the mutual differential stress changes within the fault/shear-zone pairs. Our results reveal a mechanism for the occurrence of simultaneous earthquake clusters, based on the synchronization of high driving stresses for the viscous shear-zones underneath the brittle faults, that is strongly controlled by the spacing and strike changes between faults/shear-zone pairs. In settings with low along-strike spacing and minimal strike change between neighbouring faults/shear-zones, earthquake clusters cause positive differential stress variations on neighbouring shear-zones of sufficient magnitude to induce positive slip-rate variations on their overlying brittle faults. This in turn produces positive feedback that sustains the occurrence of earthquake clusters that will continue to positively load the neighbouring shear zones. This positive feedback mechanism contributes to the understanding of fault dynamics at multiple timescales and to seismic hazard assessments.