Orogen-scale uplift drives episodic behaviour of earthquake faults

Many areas of the Earth’s crust deform by distributed extensional faulting and the occurrence of earthquakes reflects complex fault interactions. Geodetic data may indicate a simpler picture of continuum deformation over decades but relating this behaviour to earthquake occurrence over centuries, given numerous potentially active faults, remains a global problem in hazard assessment. We address this challenge for an array of seismogenic faults in the central Italian Apennines, where regional surface uplift and crustal extension are associated with devastating earthquakes. We present a new strategy to constrain fault slip-rates since ~18 ka using systematic variations in the concentration of cosmogenic 36Cl on bedrock scarps and compare these rates to those inferred from geodesy. The 36Cl data reveal that individual faults typically accumulate meters of displacement relatively rapidly over several thousand years, separated by similar intervals when slip-rates are much lower, and activity shifts between faults across strike. Our rates agree with continuum deformation rates when averaged over long spatial or temporal scales (104 yr; 102 km) but over shorter timescales most of the deformation may be accommodated by < 30% of the total fault population. We attribute these shifts in activity to temporal variations in the mechanical work of faulting.