Italy caught in Earth's pincer

Finale Emilia Torre dei Modenesi: The shattered clock tower of Finale Emilia

It is a haunting image. The severed clock tower in the town of Finale Emilia near Bologna is sure to be shown over and over again whenever anyone wants to illustrate the destructive consequences of earthquakes.

Not that Italy needs reminders.

The two big tremors (magnitudes 6.0 and 5.8) of recent days in the Emilia Romagna region have now claimed the lives of more than 20 people and pushed thousands more out of their homes.

The southern European country is seismically very active, and its most tragic events bear comparison with the most catastrophic anywhere in the world.

In its less prepared history, quakes have claimed many tens of thousands of lives.

In 1908, it is thought as many 70,000 were killed when a 7.2-magnitude tremor flattened the Sicilian city of Messina. An associated tsunami heaped more misery on an already desperate situation.

Experts say quakes have influenced everything in Italy from the distribution of the population and adaptation of architecture to the dialect spoken in different parts of the nation. And it is an influence that has been extremely well documented.

Whenever a quake has damaged a church, the local people have insisted on it being repaired or re-built, and that act has always been recorded, written down in tomes that have since become a rich source of information for scientists trying to understand Italy's seismic history.

Push-pull

On the big scale, the nation's problems can be seen in the context of the great collision between the African and Eurasian tectonic plates; but when it comes down to the specifics of many quakes, the details are far more complicated.

Set against Africa's march northward at about 2cm a year, Italy is also being pulled and pushed in some complex motions.

A century of major Italian quakes

  • 2009: L'Aquila (M6.3) - 295 deaths
  • 1980: Irpinia (M6.5) - 3,000 deaths
  • 1976: Friuli (M6.5) - 1,000 deaths
  • 1968: Belice (M6.5) - 231 deaths
  • 1930: Irpinia (M6.5) - 1,404 deaths
  • 1920: Toscana (M6.4) - 171 deaths
  • 1919: Mugello (M6.3) - 100 deaths
  • 1915: Avezzano (M7.0 - 32,610 deaths
  • 1908: Messina (M7.2) - 70,000 deaths

The Tyrrhenian Basin, or Sea, which lies to the west of the country, between the mainland and Sardinia/Corsica, is slowly opening up.

Scientists say this is contributing to extension, or "pull-apart", along the Apennines, the belt of mountains that runs down through central Italy.

And to the east, in the Adriatic, there is some evidence that the Earth's crust continues to move under (subducting) Italy, although there is some debate about this. GPS data indicates this region, too, is shifting to the northeast.

Aside from the terrible loss of life and destruction to property in recent days, the two big quakes in the Emilia Romagna region have drawn attention because they were in the north of the country.

That is not to say quakes are unheard of in this part of the country. Far from it. But further south, you see on the landscape the very clear expression of all the contortion that is taking place inside the Earth. It is writ large for example in the great limestone scarps of the Apennines.

Massive undertaking
Map of Italy

In contrast, in the huge, flat sedimentary basin that surrounds the River Po, this evidence is much less obvious.

"You may remember the L'Aquila earthquake in 2009 in the Apennines," says Dr John Elliott from the Centre for the Observation and Modelling of Earthquakes and Tectonics (Comet) at Oxford University.

"That was a result of extension. What we've seen in recent days has been quite different and is the result of closure - it's called thrust faulting. And what can often happen with this type of faulting - particularly when small faults are involved - is that it can be blind.

"That is, in a deep sedimentary basin as we have here, you get folds in the sediments but the fault doesn't break through to the surface. It's a hidden risk, if you like."

Italy is a modern country and it is very well prepared for earthquakes. Even so, the shaking associated with strong, shallow tremors will test many buildings.

Dr Susanne Sargeant from the British Geological Survey commented: "Yes, the seismic hazard is well understood, but the volume of the building stock that still needs to be strengthened is a massive undertaking.

"A lot of the older structures would benefit from strengthening measures - we know they work - but getting them implemented in every single building is quite a challenge.

"Couple this with the fact that sedimentary basins tend to amplify ground motions, you might end up seeing really quite strong levels of shaking."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter

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