The Unlocking Process Leading to the 2016 Central Italy Seismic Sequence

Approximately 23,000 well-located earthquakes from 2009 to 2016 are used as templates to recover seismic activity preceding the 2016 Central Italy seismic sequence. The resulting spatiotemporal pattern is analyzed by additional ∼91,000 newly detected events. In the 8 years before the sequence onset, seismicity (ML ≤ 3.7) develops at the hangingwall of the 2016 normal faults and along a sub-horizontal shear zone, bounding the active extensional system at depth. This activity, mainly organized in foreshock-mainshock and swarm-like clusters, migrates toward the nucleation area of the first Mw 6.0 mainshock of the sequence (24th of August in Amatrice). We propose an unlocking process based on variable temporal clustering of the seismicity, including repeaters, identifying fault portions with different degrees of coupling. Such a progressive localization of the seismic activity at the fault edges induces a weakening of the locked patch of the Amatrice mainshock.