Fault Planes, Fault Zone Structure and Detachment Fragmentation Resolved With High‐Precision Aftershock Locations of the 2016–2017 Central Italy Sequence.

Three devastating earthquakes of MW ≥ 5.9 activated a complex system of high‐angle normal, antithetic, and sub‐horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross‐correlation based double‐difference location of nearly 400,000 aftershocks illuminate complex, fine‐scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequent MW ∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub‐horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution. Plain Language Summary: In 2016–2017, a sequence of three devastating earthquakes with magnitudes ∼6 near the towns of Amatrice, Visso, and Norcia in central Italy triggered hundreds of thousands of aftershocks. We compute high‐precision locations for these aftershocks that have the power to illuminate the fine‐scale structure of the complex system of intersecting and interacting faults that were activate during the sequence. The normal faults that ruptured during the three mainshocks are capped at ∼9 km depth by a sub‐horizontal detachment zone. The normal faults intersect the detachment at an acute angle, causing complex fault zone structures and fragmentation of the detachment where they intersect. We observe a deep reaching seismic barrier that we conclude may play a role in controlling rupture extent and evolution of large earthquakes along this section of the Apennines. In general, fault complexity here is higher than in similar sequences in 2009 near L'Aquila to the south and in 1997 near Colfiorito to the north. Key Points: Precise locations of 390,334 aftershocks of the 2016–2017 Central Italy sequence illuminate the complex normal fault systemA system of high‐angle bookshelf faults accommodates slip at the intersection between normal and detachment faultsA deep‐reaching seismic barrier is identified north of the Sibillini Thrust that may control the ruptures of large earthquakes [ABSTRACT FROM AUTHOR]