Ross Stein, Volkan Sevilgen, and David Jacobson, Temblor (Updated: 1 November 2016)
The Central Italy earthquakes are unfolding into an extended progressive sequence. Certainly since 24 August 2016, and perhaps even since the 2009 M=6.2 L’Aquila event, earthquakes have been rupturing in a chaotic northward-propagating falling domino pattern.
The shock strikes an evacuated region, sparing thousands of lives
If there is any silver lining to this relentless and traumatic sequence, it is that the Amatrice earthquake and its aftershocks led to the widespread evacuation of the region, and so today’s M=6.6 earthquake miraculously took no lives (as reported at the time of this post). Had the same quake struck without being preceded by numerous smaller quakes, the death toll would likely have been in the thousands. A sobering comparison is the 1915 M=6.7 Avezzano earthquake 100 km to the southeast, which killed 32,000 people, and the 1980 M=6.9 Irpinia shock, 250 km to the southeast, which killed 2,483 people. Of course, if all the world’s quakes behaved this way, we would be living on a kinder, gentler planet. But far more common are long stretches of seismic quiet punctuated by large quakes followed by smaller and less frequent ones. These quakes catch us all unawares, and so are lethal.
Smoking Gun: Classic Active Faults
Although many of Italy’s extensional faults are so young that there is frustratingly little evidence for geologists to map at the surface. They are called ‘blind faults,’ because the geologists are blind to their presence. But the faults near Norcia are an exception: They are remarkably clear and well developed features. Although uncertain, these faults probably last ruptured in 1703. One sees a landscape (below) very similar to the U.S. Basin and Range province of Nevada and Utah, which is also strewn with active extensional faults.
Stressed faults continue to rupture, and they in turn change the stress
Thanks to insight from Michael Blanpied at the USGS and Francesca Cinti at the INGV, we now know that it was the Vettore-Bove Fault that ruptured. Photographs taken after the quake revealed a surface rupture 12 km (7.5 mi) east of Norcia (See photo below). This surface rupture was not surprising given that 50-100 cm of slip is reasonable for a M=6.6 earthquake. It is also possible that from this shock, stress has been transferred southwest onto the Norica Fault, which is now in play. One might ask why the epicenter lies 12-13 km west of the Vettore-Bove fault, and we don’t have a good answer. The epicenter should lie perhaps 5-6 km to the west, since the fault surface is itself inclined (‘dipping to’) the west, but not as far as it would appear right now.
In our judgment, the Norcia, Gorzano, Capitignano Faults could now be the next domino pieces to fall.
Sources:
F. Galadini and P. Galli (2000), Active Tectonics in the Central Apennines (Italy) – Input Data for Seismic Hazard Assessment, Natural Hazards, 22, 225–270.
DISS Working Group and INGV, Database of Individual Seismic Sources (v.3)— A compilation of potential sources for earthquakes of M≥5.5 in Italy and the surrounding regions, http://diss.rm.ingv.it/dissGM/
INGV, USGS (ANSS and PAGER), and EMSC