Fine-Scale Structure of the 2016-2017 Central Italy Seismic Sequence From Data Recorded at the Italian National Network

We explore the three-dimensional structure of the 2016-2017 Central Italy sequence using similar to 34,000 M-L >= 1.5 earthquakes that occurred between August 2016 and January 2018. We applied cross-correlation and double-difference location methods to waveform and parametric data routinely produced at the Italian National Institute of Geophysics and Volcanology. The sequence activated an 80 km long system of normal faults and near-horizontal detachment faults through the M-W 6.0 Amatrice, the M-W 5.9 Visso, and the M-W 6.5 Norcia mainshocks and aftershocks. The system has an average strike of N155 degrees E and dips 38 degrees-55 degrees southwestward and is segmented into 15-30 km long faults individually activated by the cascade of M-W >= 5.0 shocks. The two main normal fault segments, Mt. Vettore-Mt. Bove to the North and Mt. della Laga to the South, are separated by an NNE-SSW-trending lateral ramp of the Sibillini thrust, a regional structure inherited from the previous compressional tectonic phase putting into contact diverse lithologies with different seismicity patterns. Space-time reconstruction of the fault system supports a composite rupture scenario previously proposed for the M-W 6.5 Norcia earthquake, where the rupture possibly propagated also along an oblique portion of the Sibillini thrust. This dissected set of normal fault segments is bounded at 8-10 km depth by a continuous 2 km thick seismicity layer of extensional nature slightly dipping eastward and interpreted as a shear zone. All three mainshocks in the sequence nucleated along the high-angle planes at significant distance from the shear zone, thus complicating the interpretation of the mechanisms driving strain partitioning between these structures.