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expert reaction to earthquake in Amatrice, Italy

There are reports that a magnitude 6.2 earthquake has struck central Italy leaving many dead and missing.

 

Dr Richard Walters, Lecturer in Earth Sciences, Durham University, said:

“The Apennine mountains in Central Italy have the highest seismic hazard in Western Europe and earthquakes of this magnitude are common. The region has a long history of damaging earthquakes, including the 2009 L’Aquila earthquake, which was very similar in type and size. There is always a risk of further aftershocks following an earthquake, and although aftershocks are usually smaller, they can be of similar magnitude.

“There is currently no way of predicting earthquakes, but we do know that Central Italy has very high seismic hazard due to its tectonic setting. The Apennine mountain belt which runs down the spine of Italy is gradually being stretched in a NE-SW direction by tectonic forces at a rate of around 3 mm per year. This slow stretching causes stress to build up in the Earth’s crust, which is then released in earthquakes just like this one. Whilst we cannot know the exact timing of these earthquakes, we do know that they are a common occurrence in this region, and will continue to strike Central Italy in future.”

 

Prof. Ian Main, Professor of Seismology and Rock Physics and Director of Research, School of Geosciences, University of Edinburgh, said:

Is there a risk of further aftershocks?

“Yes, these are likely to diminish in number and severity as time goes on.”

Could they be as big as the original?

“Exceptionally, yes.  Modern operational forecasting models for earthquake populations take this small but non-zero probability into account – see the International Commission  on Earthquake Forecastng for civil protection (ICEF) report at http://www.geos.ed.ac.uk/homes/imain/igmpapers/LAquila.pdf

Is it likely that everything that would fall down now has, so the remainder is relatively safe?

“Not necessarily – weakened buildings can be brought down in a smaller aftershock.  The main determinant of the vulnerability of a building is the extent to which it has been designed to withstand earthquake shaking by structural engineers, and the degree of policing of the building codes developed from long-term hazard maps.  In the area affected it has been reported that there is a mixture of building types – some constructed to withstand earthquakes and many, often historical buildings, that are not.”

What system is in place to predict earthquakes in Italy?

“The national institute of geophysics and volcanology (INGV) run a national network of sensors which are combined with historical and geological data to estimate the long-term probability of ground shaking due to earthquakes – a hazard map (see Fig 1.2 of the ICEF report cited above). The magnitude 6.2 – Norcia, Italy earthquake occurred in an area of high long term hazard.

“Sadly, and despite a huge effort to find reliable precursors, individual earthquakes still cannot be predicted reliably in advance with current understanding – see also the ICEF report and Susan Hough’s accessible account at http://press.princeton.edu/titles/8990.html. This could be because the signals are too small to be seen, or could be an inherent property of the complex and non-linear dynamics involved in the statistical mechanics of earthquakes (see the Nature website debate on this topic – “Is the reliable prediction of individual earthquakes a realistic scientific goal?” at http://www.nature.com/nature/debates/earthquake/equake_frameset.html)

“In between these extremes operational forecasts of earthquake probability based on the tendency of earthquakes to cluster – for example during aftershock sequences or swarms – can provide significant probability gains of 100-1000 above the long-term hazard level for the occurrence of damaging earthquakes, though the absolute probability remains low (less than 1% in a week – see also ICEF report). Nevertheless these can aid civil protection and humanitarian organisations in risk management while dealing with crises during ongoing aftershock sequences. The INGV run such operational forecasts to provide such information (see examples in the presentation by Marzocchi et al., http://www.reaktproject.eu/reakt_presentations/EGU-2014_Marzocchi_OEF.pdf).”

Was this one predicted? Should it have been? Just how good are we at predicting earthquakes now?

“No – we can’t predict individual earthquakes with current knowledge.”

How common are earthquakes in this area and why? If this one was a surprise, do we know why it happened?

“The earthquake location, magnitude and fault orientation is consistent with the current seismo-tectonic model and hazard map for Italy. It is not a surprise.”

What protection is in place now for buildings to minimise quake damage – are they only applicable to new build, or can historic towns be retrofitted?

“This is the best question – earthquakes don’t kill people, buildings do. However much we research earthquake forecasting the front line of defence is to get the buildings right.  We know how to do this, and there is a lot of good practice in Italy in earthquake engineering.  Nevertheless there is a large legacy of mainly historical masonry buildings that remain vulnerable.  These can be retrofitted at some cost, but budgets are rarely large enough to do this for all vulnerable structures – requiring a targeted approach to buildings where the consequences of failure would be greatest, starting for example with schools and hospitals.”

 

Prof. Sandy Steacy, Head of School, Physical Sciences, The University of Adelaide, Australia, said:

Is there a risk of further aftershocks? Could they be as big as the original? Is it likely that everything that would fall down now has, so the remainder is relatively safe?

“Aftershocks are likely to continue for several weeks. These earthquakes cause disproportionate damage for their size because they shake structures weakened in previous events; hence they are particularly dangerous for emergency service workers attempting to rescue survivors from partially collapsed buildings. The largest aftershock so far has been a magnitude 5.5 and it is unlikely that any larger ones will occur. However, there is small chance that the magnitude 6.2 Norcia earthquake will trigger another earthquake of a similar size.”

What system is in place to predict earthquakes in Italy? Was this one predicted? Should it have been? Just how good are we at predicting earthquakes now?

“Earthquakes cannot be predicted. By ‘prediction’, earthquake scientists mean a forecast of the time, location, and magnitude of an impending event. However, for every large damaging earthquake there are thousands of smaller ones and hence a successful prediction would require that we know in advance of an earthquake not only that it will occur but also how big it will be. Our best understanding of the physics is that an earthquake’s size depends on the detailed stress state of the fault on which it occurs and we have no way of measuring this.”

How common are earthquakes in this area and why? If this one was a surprise, do we know why it happened?

“This earthquake occurred in the central Apennines, one of the most seismically active parts of Italy. Earthquakes in this area take place on extensional faults that form part of the complex tectonic system of the region. The most damaging recent earthquake in the vicinity was the 2009 magnitude 6.3 L’Aquila  event which killed about 300 people and left 55,000 homeless.”

What protection is in place now for buildings to minimise quake damage – are they only applicable to new build, or can historic towns be retrofitted?

“Buildings can be retrofitted to better withstand earthquake shaking although the cost can be significant.”

 

Prof. Bill McGuire, Emeritus Professor of Geophysical & Climate Hazards, UCL, said:

“Aftershocks are pretty much a certainty. In some cases these may be comparable, or even larger, than the original. It is not possible, however, to determine if this will be the case here. Note: even if the aftershocks are smaller, they may still be able to cause significant damage and further collapse to buildings weakened in the original shock.

“Earthquakes CANNOT be predicted. There is, however, a seismic hazard map for Italy, based upon the pattern of historical seismic activity and average return periods of quakes on different faults. This is an area of Italy that has had significant earthquakes in the past, so this event is not a surprise. Just to reiterate, we currently have no capability at all for making useful predictions of when and where an earthquake will occur.

“Earthquakes are quite common in this part of central Italy, and the last occurred at L’Aquila in 2009, killing close to 300 people. Although the region is not on a tectonic plate boundary, there are many active faults in the region, accumulating strain and releasing it periodically in moderate earthquakes of around magnitude 6. Even larger ones are also possible. After the L’Aquila experience, the current quake should be no surprise at all.

“In theory, buildings should be constructed to withstand expected shaking. In the case of L’Aquila, and in today’s quake, however, much of the building stock is old. Ideally, this can be retrofitted to withstand expected quakes. This is expensive, however, and I doubt if it has been undertaken to any significant degree. It always comes down to money and a will to get this done, which is not always evident.”

 

Dr Joanna Faure Walker, Lecturer, UCL Institute for Risk and Disaster Reduction, said:

Is there a risk of further aftershocks? Could they be as big as the original? Is it likely that everything that would fall down now has, so the remainder is relatively safe?

“There is almost always a risk of aftershocks.  Aftershocks are smaller than the main event (usually they have a maximum size of 1 magnitude less than the main event).  However, in addition to aftershocks, one earthquake may trigger another earthquake on a neighbouring fault which can be as large as (or in some cases larger) than the original earthquake.  This would not be called an ‘aftershock’ but could take place shortly after the main event.  Such sequences of event have been seen in the historical record in Italy e.g. in 1349 and more recently during the 1997 Umbria-Marche sequence.

“Regarding safety of buildings, this is an unknown and residents should listen to formal advice from trained engineers.  Some buildings may have been weakened by the earthquake but are still standing so further aftershock activity could cause further damage. However, it might also be true to say that some of the more seismically resistant buildings have withstood the main event and therefore may have shown they are less likely to collapse following aftershocks.

What system is in place to predict earthquakes in Italy? Was this one predicted? Should it have been? Just how good are we at predicting earthquakes now?

“Earthquakes cannot be predicted. However, the INGV (Istituto Nazionale di Geofisica e Vulcanologia) and other civil protection and scientific agencies work hard to produce hazard maps showing where events are more likely to occur. We still do not know enough about the fundamental processes involved to predict such events.”

How common are earthquakes in this area and why? If this one was a surprise, do we know why it happened?

“Earthquakes are common in the Apennines. There have been around 20-30 large damaging earthquakes over the last 700yrs in the central Apennines. The extension rate across the Apennines is up to 3mm per year over the last 15,000yrs (Faure Walker et al., 2010) and these rates are also seen in Geodetic (GPS) records and similar to when we analyse the historical record of past events over the last 700yrs.

“The Apennines are an area of active continental extension (like Greece and the Basin and Range). There are many active faults in the region capable of having large magnitude (>moment magnitude 5.5) earthquakes. However, regions of extension cannot have earthquakes as large as regions of compression (moment magnitude 8-9). Regions of compression include subduction zone areas (e.g. Japan, Chile, Alaska). Extension areas cannot have large magnitude 9 events because rocks are stronger in compression than in extension so can store more stress that can be released in earthquakes.

What protection is in place now for buildings to minimise quake damage – are they only applicable to new build, or can historic towns be retrofitted?

“Following the 2009 earthquake, many towns went through a process of retrofitting buildings, with priority given to buildings such as schools. It is possible to retrofit old buildings, but some retrofitting techniques are not appropriate as they have high visual impact.”

 

Prof. David Rothery, Professor of Planetary Geosciences, Open University, said:

“The magnitude 6.2 earthquake that struck Amatrice last night (01.36 GMT) was similar to the magnitude 6.3 quake that hit L’Aquila in April 2009. Both occurred at a shallow depth (which exacerbates the shaking at the surface) and resulted from local extensional faulting in this tectonically complicated region. However, unlike the L’Aquila quake, which was preceded by swarms of smaller quakes and led to claims (unjustified in my view) that the eventual big quake should have been predicted, this one appears to have struck out of the blue as is more usually the case.

“It is too early to assess the full nature of the damage. I would expect older buildings and bridges to have suffered worse than modern structures, which should have been constructed according to well-known seismic resilience codes.

“Aftershocks are continuing. The biggest aftershocks have been magnitude 4. These were in the two hours immediately after the initial earthquake. Currently aftershocks sit in the M2-M3 range. They can be expected to generally become weaker, but the occasional stronger aftershock cannot be ruled out.  People close to the epicentre of a weak aftershock may feel it more strongly than the original earthquake, though the shaking will not last for so long. Aftershocks can bring down buildings already weakened, and put at risk those trying to rescue people tapped in rubble. Journalists might like to visit http://www.centrometeoitaliano.it/terremoti-in-tempo-reale-italia-mondo-ingv/

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