Earthquake clustering controlled by shear zone interaction: insights from cosmogenic dating and stress interactions for normal faults in central Italy

Observations show that active seismogenic faults undergo periods of rapid slip containing clusters of potentially damaging earthquakes, separated by anticlusters with slower or no slip. This confounds our attempts to mitigate seismic hazard, because the greater the aperiodicity, the greater the uncertainty in recurrence intervals. The causes of clustering/anti-clustering are debated, but we provide an explanation where fault and shear zone interactions induce changes in differential stress and strain-rate within viscous shear zones underlying active brittle faults. We do this by linking Coulomb stress modelling and experimental quartz flow laws, with observations of clustering from ³⁶Cl cosmogenic dating of fault scarps that are 15 ± 3 kyrs old. We propose that strain-rate changes within the shear zones in turn drive slip-rate changes on the overlying brittle faults, causing clustered/anticlustered slip. We measure the timescales and magnitudes of slip-rate changes using ³⁶Cl cosmogenic data from six neighbouring active normal faults, the inverted slip histories are in good agreement with independent historical and paleoseismic catalogues. These slip histories are used as inputs for differential stress and strain-rate calculations. We calculate changes in differential stress of -4 and -2.8 MPa for two faults studied in detail, which corresponds to a change in strain rate of 34% and 19% respectively, calculated using a quartz flow law. The magnitudes and timings of brittle fault slip-rate changes implied by our calculations are comparable to those measured using ³⁶Cl analyses. Our work provides a quantified explanation for earthquake clustering on normal fault and provides insights on how multiple faults share tectonic deformation. These results provide new insights into the controls on continental deformation and seismic hazard that extend and complement existing findings from studying historical seismicity and geodesy.