Dual control of fault intersections on stop-start rupture in the 2016 Central Italy seismic sequence

Large continental earthquakes necessarily involve failure of multiple faults or segments. But these same critically-stressed systems sometimes fail in drawn-out sequences of smaller earthquakes over days or years instead. These two modes of failure have vastly different implications for seismic hazard, but it is not known why fault systems sometimes fail in one mode or the other, or what controls the termination and re-initiation of slip in protracted seismic sequences.
Our understanding of these issues has been severely limited by a paucity of high-resolution observations of modern seismic sequences, but a series of three Mw>6 earthquakes from August to November 2016 in Central Italy represents a uniquely well-observed example. Here we exploit a wealth of geodetic, seismological and field data to determine the causative fault geometry at depth and the spatio-temporal evolution of slip throughout the sequence.
The earthquakes ruptured the major SW-dipping Laga-Vettore normal fault system, along with subsidiary oblique and antithetic faults that intersect this main structure. Our results suggest these intersecting faults controlled the extent and termination of rupture in all three events, as they closely bound the extent of major slip for each earthquake. In addition, aftershocks following the first Mw 6.2 earthquake migrate northwards along these same fault intersections, reaching the hypocentre of the second Mw 6.1 earthquake at the time of its nucleation. The temporal evolution of this migration is consistent with fluid diffusion and we suggest that fluids channelled along high-permeability fault intersections triggered the second earthquake, determining the timing of rupture re-initiation. This may also explain why the sequence ‘jumped’ over the fault region of the third and largest Mw 6.6 earthquake, which ultimately ruptured the region between the first two events.
Fault intersections therefore appear to have exercised a dual control on the stop-start rupture throughout this earthquake sequence, first preventing rupture in a single large earthquake, and second determining the temporal evolution of failure over the following two months. This strong structural control in seismic sequences may be common, and we suggest that future efforts should focus on investigating its prevalence.