A L'Aquila trigger, seismic gaps and poor construction: What we've learned from the August earthquake in Italy

By Ross Stein and Volkan Sevilgen

On Aug. 24, a magnitude-6.2 earthquake struck a central Apennine normal fault near Rieti, Italy, about 100 kilometers northeast of Rome. At least 296 people died, most in the nearby town of Amatrice, and hundreds more were injured. Thousands in several mountain towns east of Rome were left homeless and roads and bridges were damaged or destroyed. Aftershocks continued to rattle the region, where a month later people were still sifting through rubble amid damaged buildings that threatened to crumble at any time. The destruction was severe. “The town isn’t here anymore,” said Sergio Pirozzi, the mayor of Amatrice, according to news reports.

As seismologists, we are trained to take a close look at what happened, and what it might mean for the future of this region. Here are what struck us, in the days after the quake, as the most important findings and issues.
 

Was the mainshock triggered by the 2009 L’Aquila earthquake?

The mainshock occurred only 40 kilometers from L’Aquila, where a magnitude-6.3 earthquake struck in April 2009 and killed more than 300 people. There is a long history of earthquakes along the central Apennine normal faults; the Adria Microplate dives beneath the Eurasian Plate where the Apennine Mountains rise up and the Eurasian and African plates collide, creating compression and thrusting at depth, and extension and normal faults at the surface. Prior to L’Aquila, the last major seismicity in the area was a series of magnitude-5 to -6 quakes in Umbria and Marche on Sept. 26, 1997, which killed 11 people and injured more than 100. The people here are familiar with earthquakes; more than 10 damaging earthquake sequences with magnitudes above 4.7 have hit this region since the 13th century alone.

Was this recent earthquake then triggered by the L’Aquila quake? The answer is a qualified yes. In a 2012 study in Geophysical Journal International, Enrico Serpelloni of the National Institute of Geophysics and Volcanology (INGV) and colleagues calculated the Coulomb stress transfer to the region surrounding the 2009 quake. The fault that ruptured during the Aug. 24, 2016, mainshock was brought closer to failure by the L’Aquila quake, but only by about 0.05 to 0.10 bar. Generally, stress increases of at least 0.1 bar are associated with aftershocks and an increased probability of future mainshocks. So the effect is small but positive.
 

Was there a precursory swarm?

A political and legal struggle arose after the 2009 L’Aquila earthquake over whether the population was adequately warned about a potential earthquake. The broad region — but not L’Aquila itself — experienced a seismic swarm for a month or so before the mainshock. This prompted questions about whether the swarm indicated anything about future earthquakes; after some public discourse, a public official said the swarm didn’t indicate that any earthquakes were imminent (which, of course, they couldn’t say one way or another, since earthquakes are not predictable) — a statement that led to the public official and six scientists being charged with manslaughter.

During the trial, the prosecutor argued that the politicians and scientists had not adequately warned that a swarm could portend a mainshock. The resulting manslaughter convictions were, fortunately and appropriately, overturned, but only years later. So it stands to reason that with the August 2016 quake, the world would wonder two things: Was there a precursory swarm again? And would anyone be charged with anything?

   A radar image of the ground deformation caused by the earthquake. Credit: JAXA.

The data show that, at least at the level of magnitude 2 or larger, the August 2016 quake was not preceded by a swarm. In fact, there was only one magnitude-2.2 shock near the future rupture zone during the 30 days before the mainshock struck. There is, however, a 200-kilometer-long zone of seismicity with small quakes less than magnitude 2 that extends northwest from L’Aquila, and its shock rate has been steady for a decade.

The absence of a precursory swarm is not a surprise: As the scientists argued during the trial, swarms seldom occur except at volcanic sites or along creeping faults, and only rarely are they harbingers of future mainshocks. In this quake, the gods answered the lawyers: Earthquakes cannot be predicted, at least not yet.

As of press time, no one had been charged with anything regarding this earthquake, but charges may come against building owners.
 

How does this compare to previous seismicity in the area?

As with precursory swarms, progressive earthquake sequences are uncommon. However, over a period of 19 days in 1703, three large earthquakes struck this region in a 36-kilometer southeastward progression, as Emanuela Guidoboni of the University of Bologna and Gianluca Valensise of INGV reported in Earthquakes and Structures in 2015. The epicenters were near Norcia (approximately magnitude 6.7 on Jan. 14, 1703), Montereale (approximately magnitude 6.2 on Jan. 16, 1703) and L’Aquila (approximately magnitude 6.7 on Feb. 2, 1703), rupturing all of the known active faults between Norcia and L’Aquila. About 10,000 people died during this 19 days of terror, although casualty numbers remain uncertain.

Could this happen again? All one can say is that because this has happened in the past, and because multiple ruptures and very large aftershocks are historically common here, it could happen again. So, one should neither assume nor assure anyone that the 2016 earthquake sequence is over.
 

What about seismic gaps?

Another intriguing feature of this part of the Apennines is that there appear to be 10- to 20-kilometer-long gaps between the Aug. 24, 2016, quake and the 1997 Umbria-Marche quake to the northwest, and a 20- to 25-kilometer-long gap that extends toward the site of the 2009 L’Aquila shock to the southeast. (Seismic gaps are simply gaps in the rupture zones of past earthquakes; the presence of a gap has not proven to be predictive of future events.)

The extent of the current rupture is best assessed not by the aftershocks, but by the ground deformation. That’s because aftershocks can strike far from the rupture location in areas where the stress has been increased by the mainshock. One can see from an interferogram — a radar image of ground deformation showing differences from about a year before the mainshock to the day after the mainshock — that the deformation created by the August 2016 earthquake covers an area about 20 kilometers long. The yellow area to the west is uplifted, while the central fringes or contours indicate subsidence.

The gaps between this shock and the sites of the 1997 and 2009 shock have a history of moderate or large quakes — they certainly are not aseismic. Paolo Galli of the Department of Civil Protection in Rome and colleagues reported in Tectonophysics in 2005 evidence of repeated surface ruptures in the past 20,000 years along the Norcia Fault Zone that extends through this region. Therefore, another magnitude-6 quake is still possible — but by no means is it necessarily imminent.
 

How good was the national seismic hazard map?

One can only assess a probabilistic hazard map by looking at hundreds of large quakes, but if we had to judge it by this one alone, it was excellent. The magnitude-6.2 quake struck in the most seismically active part of Italy, and the observed shaking was at about the maximum level forecast by the map. The map is based on the modern and ancient record of earthquakes, on the identification of active faults, and on the crustal strain measured by GPS receivers — the same constituents of the USGS seismic hazard map for the United States.
 

Why was it so damaging?

Why the quake was so damaging is an embarrassing but essential question for Italy. To calibrate the U.S. Geological Survey (USGS) PAGER (Prompt Assessment of Global Earthquakes for Response) system that forecasts casualties and economic consequences of earthquakes immediately after they occur, Kishor Jaiswal and David Wald of USGS and Keith Porter of the University of Colorado Boulder analyzed quake deaths worldwide from 1973 to 2009. Shockingly, they found that if 10,000 people were subjected to violent shaking (Modified Mercalli Intensity IX) in Iran, 3,000 would die; in Italy, 150; and in California, just three. 

In this quake, about 10,000 people were exposed to violent shaking (Modified Mercalli Intensity IX) and the current death toll is, sadly, consistent with their estimate. None of these disparities are caused by differences in shaking; all are due to construction. The entire Abruzzo region was destroyed in 1703, so many buildings date from the early 1700s. Buildings more than a century old are typically made of heavy, unreinforced masonry or stones, and cannot resist the side-to-side shear forces exerted by even a modest quake like this one.

The highest recorded ground acceleration during this quake was in Norcia, 19 kilometers from the epicenter. The strong shaking lasted only for three to four seconds and peaked at a relatively modest 0.375 g (we are all accelerated downward at 1 g; if we were accelerated upward at 1 g, we would lift off the ground). Well-built wood frame homes should come through this level of shaking unscathed. On the basis of damage, it is possible — but doubtful — that the shaking was much stronger at Amatrice, the town that was almost leveled. The mainshock lies between Norcia and Amatrice, so the seismic waves traveled in both directions toward these two towns and so should be comparable. Sadly, the Norcia record is a strong indictment of the seismic resilience of the buildings, especially in Amatrice.
 

What’s to be done?

First, earthquake early-warning systems, which can warn people of impending shaking if the earthquake is large but distant, would probably not have helped for this small quake; the distances from the epicenter to the towns are so short that the shaking might have beaten the electronic warning to people’s cellphones and public sirens. However, a very dense seismic network would have afforded several seconds of warning to Norcia and Amatrice, enough time to “drop, cover, and hold on,” but not to get out of a building.

Second, and more importantly, is the issue of building construction. It is difficult and expensive — but nevertheless possible — to retrofit old, or even ancient, buildings. At the very least, schools and public buildings need to conform to a much higher standard. Homeowners should investigate retrofitting or moving to new homes; one can also insure against earthquakes for financial protection.

After the L’Aquila quake (from which thousands of people are still in “temporary” shelters), authorities noted the need for earthquake-resistant construction and retrofitting throughout the region. Probably little of that occurred before the earthquake, but public outcry over the lack of progress is not surprising; effective retrofit would have saved lives and towns. One local prosecutor told La Repubblica that any contractors who had reinforced buildings “on the cheap” may have contributed to the death toll from this quake and could face criminal charges. But, as the 2009 L’Aquila earthquake showed, more prosecution is not the answer — more preparation is.

As this issue went to press in late October, three large, damaging earthquakes struck the Apennines region of central Italy. The first two quakes, measuring magnitude 5.5 and 6.1, struck near Visso, causing significant damage, but no deaths. The third, a magnitude-6.6 quake, struck near Norcia and also caused significant damage but no deaths. According to the authors, the quakes were likely triggered by the August quake, which increased stress on several nearby faults, including the ones that ruptured. Read more about the October quakes at Temblor.net.

A version of this story was published on Temblor.net in August.