The Unlocking Process Leading to the 2016 Central Italy Seismic Sequence

• Published in: Geophysical research letters 2023-03, Vol.50 (5), p.n/a
• Main Authors: Sugan, M., Campanella, S., Chiaraluce, L., Michele, M., Vuan, A.
• Format: Article
• Language: English
• Subjects: Aftershocks; Clustering; Earthquakes; Fault detection; Fault lines; foreshocks; Localization; Nucleation; Pattern analysis; Seismic activity; Seismicity; seismicity localization; Shear zone; slow slip; swarms; template matching; Templates; unlocking
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL101838

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. Plain Language Summary We 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. Key Points Eight‐year seismic data scrutinized by template matching provide ∼4 times more events before the Mw 6.0 Amatrice mainshock Fore‐mainshocks and swarm‐like clusters persisted before the 2016 Amatrice mainshock at the northern and southern fault edges An unlocking process by progressive seismicity localization weakened the 2016 main fault volume peripheral area