M 6.6 - 5 km ESE of Preci, Italy

Tectonic Summary

The October 30, 2016 M 6.6 earthquake north of Norcia, Italy, occurred as the result of shallow normal faulting on a NW-SE oriented fault in the Central Apennines. The Apennines is a mountain range that runs from the Gulf of Taranto in the south to the southern edge of the Po basin in northern Italy. Geologically, the Apennines is largely an accretionary wedge formed as a consequence of subduction. This region is tectonically and geologically complex, involving subduction of the Adria micro-plate beneath Eurasia and the Apennines from east to west, continental collision between the Eurasia and Nubia (Africa) plates building the Alpine mountain belt further to the north, and the opening of the Tyrrhenian basin to the west (the latter of which is in turn related to Adria subduction and eastward trench migration). The evolution of this system has caused the expression of all different tectonic styles acting at the same time in a broad region surrounding Italy and the central Mediterranean. The October 30, 2016 normal faulting earthquake is an intraplate event, an expression of the east-west extensional tectonics that now dominate along the Apennine belt.

The October 30, 2016 event is the largest event in an on-going sequence of damaging earthquakes that include:

- the August 24, 2016, M 6.2 central Italy (Amatrice) earthquake which caused approximately 300 fatalities, and severely damaged the town of Amatrice.

- The October 26, 2016, M 6.1 central Italy earthquake, which, at the time of writing, is not known to have caused any fatalities. This M 6.1 event was preceded by several hours by a M 5.5 earthquake.

Since the August 24, 2016 M 6.2 earthquake, the USGS has reported 44 events of M 4.0 and larger, including a M 5.6 earthquake within an hour of the August 24 shock, and two large events on October 26 – an M 5.5 event at 17:10 UTC, and the M 6.1 earthquake at 19:18 UTC. Both October 26 events are at the northern end of the aftershock sequence of the M 6.2 August 24 earthquake; the October 26, M 6.1 event is about 30 km northwest of the August 24, M 6.2 earthquake, and thus may reflect triggered failure of an adjacent fault or fault segment. The October 30, M 6.6 event is between the two prior largest earthquakes, approximately 10 km southeast of the October 26, M 6.1 event. While related to the previous seismicity, the larger size of this earthquake implies it has ruptured an independent fault patch that had not slipped as part of preceding earthquakes. As of 11:30 UTC on October 30, the M 6.6 earthquake has been followed by three M4+ aftershocks.

The central Apennine region has experienced several significant earthquakes in recorded history. The largest instrumentally recorded earthquake within 100 km of the 2016 events was the January 13, 1915 M6.7 earthquake, which was nearly 90 km to the south-southeast of the October 26, 2016 event, near Avezzano. The 1915 earthquake killed approximately 32,000 people. In September 1997, a Mw 6.0 earthquake 35 km west-northwest of the October 30, 2016 event killed 11, injured over 100 and destroyed approximately 80,000 homes in the Marche and Umbria regions. This 1997 event was part of a series of earthquakes known as the Umbria-Marche seismic sequence, which included eight events of magnitude greater than M5.0 in a two-month period between September and November of that year, including the events that substantially damaged the Basilica of St Francis in Assisi. In April 2009, a Mw 6.3 earthquake 60 km to the south-southeast of the October 30, 2016 event, near the town of L’Aquila, killed at least 295, injured over 1,000 and left 55,000 or more homeless. The L’Aquila earthquake resulted in significant landsliding in the local area, and was also followed by a vigorous aftershock sequence, including 5 other events of M 5.0 or larger. The location of the 2016 earthquake sequence is predominantly in a gap between the aftershock sequences of the 1997 and 2009 events; the October 26, 2016 M 5.5 and M 6.1 events overlap with the southern end of the 1997 sequence. The October 30, M 6.6 event is to the southwest of the M 6.1 event 3 days earlier; further studies will be necessary to determine how the fault strands that ruptured in each event relate to one another. This is the largest earthquake in Italy since the 1980 Mw 6.9 Irpinia earthquake in southern Italy.

This is a complex sequence of related earthquakes, occurring on more than one fault segment in central Italy. We can expect aftershocks to continue for weeks and possibly months. We cannot rule out the possibility of similar sized or larger events, though the probability of a larger event is low.

The largest earthquake in a sequence is generally referred to as the mainshock but each large earthquake will have associated aftershocks.

Further research into this ongoing sequence will more clearly determine how each event relates to other earthquakes, and how the sequence as a whole developed in space and time.