By Jonathan Amos
Science reporter, BBC News
A series of foreshocks preceded the L'Aquila quake, but scientists say it was almost impossible to predict the main event
Powerful tremors are an ever present danger in Italy.
The region is seismically very active, and some events bear comparison with the most catastrophic anywhere in the world.
In 1908, thousands died in a 7.2-magnitude quake which reduced the Sicilian city of Messina to rubble. A tidal wave followed causing more devastation.
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.
The US Geological Survey said Monday's quake was a 6.3-magnitude event, striking at 0330 (0130 GMT) close to L'Aquila city, about 95km (60 miles) north-east of Rome.
Italy's National Institute of Geophysics put it at 5.8. Disagreements about the amount of energy involved are not unusual in the immediate aftermath of a quake and later analysis will likely refine numbers further.
The hypocenter - the point below the surface where the strain in the rocks was released - appears to have been some 10km down.
"An earthquake of magnitude six on average we get about one every three days somewhere in the world," explained Dr Roger Musson, head of seismic hazards at the British Geological Survey.
"But of course a lot of these are in places out at sea where they don't cause any damage and so people don't realise they're so frequent. It's when you get an earthquake in a populated area like central Italy that they become a serious matter."
Major fault system
On the big scale, Mediterranean seismicity 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 Monday's event, 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.
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 considerable debate about this. Recent GPS data suggests this region, too, is shifting to the northeast.
What is not in doubt from all this complexity is the major fault system that runs the length of the Apennines and the series of smaller faults that fan off to the sides.
Aftershocks are likely to be common. There was a M4.8 event just a few hours after the main quake on Monday, and others of similar scale could follow, warn seismologists.
The 1997 M6.4 event, which caused serious damage to the Basilica of St Francis in Assisi, ruining priceless Mediaeval frescoes, occurred about 50km to the north.
It was marked by a series of aftershocks on following days. One of these measured 5.1 and caused considerable further damage.
"Routine earthquake prediction is not possible anywhere in the world," said Dr Musson.
"Given the chaotic nature of earthquake occurrence, it may never be possible. In the case of the L'Aquila earthquake, some warning might have been taken by the series of foreshocks that preceded it - though there is no way to discriminate between foreshocks and normal small-magnitude seismicity, other than with the benefit of hindsight. Aftershock activity has been intense so far."