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 (M L ≤ 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 M w 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.
Plain Language SummaryWe exploit a high-resolution seismic catalog to describe the activity preceding the 2016 Central Italy sequence. Newly retrieved events are analyzed in space and time to characterize the earthquake preparatory phase leading to the first mainshock of the sequence. Our 8-year-long observations show that most seismic activity involves structures surrounding the nucleation and rupture zone. Interesting seismicity patterns are found along an almost horizontal discontinuity below the upper crustal normal faults and at their northern and southern edges. We highlight migrations, clustering, and progressive seismicity localization close to the first mainshock of the sequence, unveiling a complex preparatory phase.