The 2016-2017 Central Italy Earthquake Sequence: How the Largest Events Initiated

Abstract:

The 2016-2017 central Italy seismic sequence activated 70 km of a complex network of faults in the central Apennine mountains, beginning with the August 24 2016 Mw 6.0 Amatrice event. It was followed by the Mw 5.9 Visso earthquake on October 26 before culminating in the Mw 6.5 Norcia event on October 30. We analyze the progression of this sequence using a catalog of ~900,000 events built using a deep-neural-network-based phase picker combined with high-precision relative relocation. The Amatrice event occurred without foreshocks or even notable activity near its hypocenter in the preceding months. In the two weeks prior, the closest event to its hypocenter was a M0 event 5km away, and there were fewer than 20 events within the region delineated by its aftershocks. Aftershock activity decayed in number and intensity for the next two months before a Mw 5.4 event struck on October 26. This event was not preceded by any clear acceleration of seismic activity. Two hours later, the Visso event initiated at the edge of the aftershock zone of this Mw 5.4 foreshock. Four days later, the Norcia event initiated at the edge of the aftershock zones of the Visso event and its Mw 5.4 foreshock. We found no repeating foreshocks around the hypocentral regions of the Mw 5.4, Visso, and Norcia events. Based on the location uncertainty, magnitude of completeness, and rupture dimension estimates, M2 repeating events would have been detected. Therefore, while aseismic afterslip is not unexpected, we found no evidence of accelerating slip preceding the largest events. Our observations support the cascade model, where one earthquake loads its peripheral regions, spawning nucleations of which sometimes one initiates the rupture of a large magnitude event.