Seismic stress perturbation and triggering patterns induced by the 2016 Central Italy earthquake sequences

Numerous shallow earthquakes, including 24th August Amatrice, 26th October Visso, and 30th October Norcia earthquakes, ruptured the segments of Mount Vettore-Gorzano fault system in the central Apennines (Italy) in 2016. In order to investigate the stress perturbation and triggering patterns among the earthquake sequences, we introduce a more realistic nonplanar coseismic fault geometry model, which improve the rupture model by assimilating relocated aftershocks and the GPS observations. We adopt the seismic slip inversion program of the steepest descent method (SDM) to create the detailed coseismic rupture models and optimize Coulomb Failure Stress model by varying the coefficient of friction and received fault parameters. The results indicate that the nonplanar fault geometry model is more reflective of the deep slip of the coseismic rupture than planar model. As evidenced by the coseismic Coulomb stress changes caused by the three mainshocks at different depth slices, the stress loading mainly distributes on the active fault zones and the stress changes can well explain the spatial distribution of aftershocks. The first large Amatrice mainshock accelerates the occurrence of the Mw 5.9 Visso and Mw 6.6 Norcia earthquakes, with the positive stress changes at the hypocenter exceeding the stress triggering threshold (0.010×106 Pa) and up to 0.015×106 and 0.257×106 Pa, respectively. Furthermore, the Mw 5.9 Visso earthquake as well encourages the occurrence of the Mw 6.6 Norcia event with the increased stress changes of 0.052×106 Pa on the hypocenter. It is concluded that the stress transfer and accumulation play crucial roles on the linkage triggering mechanism among the mainshock-mainshock and mainshock-aftershocks. Noteworthily, the cumulative stress changes on the southwest segment of the Norcia Fault (NF), the southeast parts of the Montereale Fault System (MFS) and Mount Gorzano Fault (MGF) of the main regions are up to (1.5∼3.5) ×106 Pa. The cumulative stress changes have not been released sufficiently by aftershocks, which may increase the seismic hazard in those regions.