Past seismic sequences
This region in Italy is renowned for its seismic sequences and there have been more than 10 damaging earthquake sequences with magnitudes above 4.7 since the 13th century.
In September 1997, a M6.0 earthquake killed 11 people, injured over 100 and destroyed approximately 80,000 homes. It occurred 25 km northwest of the 24 August M6.2 Amatrice event and was part of the Umbria – Marche seismic sequence, which included eight events of M ≥ 5.0 in a two-month period between September and November of that year (USGS).
In April 2009, a M6.3 earthquake occurred 72 km to the southeast of the 26 October M6.1 event near the town of L’Aquila. The quake killed 308, injured over 1,000 and left 55,000 homeless and was also followed by a vigorous aftershock sequence, including five other events of M ≥ 5.0 (USGS).
Linking the sequences
The 1997, 2009 and 2016 sequences are linked but exactly how is still not certain. Seismologists are excited by such sequences of related quakes of medium magnitude that are ongoing over many months. They prompt questions concerning cause and effect of event triggers and distinction between aftershocks and foreshocks. They are also far less common than long stretches of seismic calm punctuated by large quakes and the following smaller and less frequent aftershocks.
Did the 1997 sequence trigger the sequence in 2009? Studies suggest that the 2009 sequence probably triggered the 2016 sequence. Serpelloni et al. (2012) calculated the Coulomb stress transferred to the region by the 2009 L’Aquila quake. This brought the fault that subsequently ruptured during the 24 August M6.0 Amatrice quake closer to failure - albeit only by a small amount (about 0.05 to 0.10 bar). As a general guide, stress increases of at least 0.1 bar are associated with aftershocks and an increased probability of future main-shocks, so the stress change was small but positive.
The fault systems in the central Apennines are relatively young at less than one million years old, short and structurally complex, and contrast with those that have simple continuous features. Therefore, earthquakes in the region are not large by global standards (often M<6.8), but are often shallow and occur in sequences or clusters as stress is transferred from fault to fault.
Filling the gap?
The location of the 2016 earthquake sequence lies predominantly in a gap between the aftershock sequences of the September 1997 M6.0 Umbria – Marche event and the April 2009 M6.3 L’Aquila event (Figure 3). However, this is such a tectonically complex region that it is difficult to know if the seismic gap has been filled by the recent seismic sequence.
The gap between the 1997 and 2009 shocks has a history of moderate quakes so it is not aseismic. Galli, Galadini and Calzoni (2005) found evidence of repeated surface ruptures in the past 20,000 years along the region’s Norcia fault zone. It is still not known whether the 30 October M6.5 quake was the largest and final quake of the 2016 sequence. Collection of additional geophysical and geodetic data from the recent events should reveal whether stress has been transferred elsewhere, and historical data can help determine whether seismic gaps might remain. However, forecasting where and when another M6 quake will strike remains uncertain.
What we do know is that with such uncertainty, we can turn to probability and the most recent probabilistic seismic hazard map of Italy performed excellently. The recent quakes have all struck in the most seismically active part of Italy and the observed shaking was at roughly the maximum level forecast. These expectations are plotted on a probabilistic seismic hazard map that shows the potential hazard from earthquakes within a region based on the combined views of geologists and seismologists. It is probabilistic in the sense that it considers uncertainties in the size and location of earthquakes and the resulting ground motions that can shake a particular site.
Figure 3: Seismicity of the Central Apennines since 1985.
Source:INGVterremoti.com (January16, 2017)
Seismic activity in the Apennine region is a complex sequence of related earthquakes, occurring on more than one fault segment. Aftershocks will continue for months and the possibility of similar sized or larger events cannot be ruled out, although the probability of a larger event is low. Analysis of satellite data suggests that deeper parts of the Vettore-Bove fault remain locked (i.e. could still rupture) and so seismic risk remains. Analysis of the seismic data suggests that the Gorzano fault has been stressed by the 2016 events. It was also stressed by the L’Aquila quake in 2009.
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