The earthquake which shook Athens on Tuesday came "out of the blue", say seismologists, as no known fault existed there.

However, Greece is no stranger to the shuddering of our restless planet - it suffers more earthquakes than any other European country.

Aftershocks concern rescuers in Athens but are not unusual

The nation is captured right in the centre of the crushing grip of three continents which are grinding into one another. The African tectonic plate is moving north, Europe is pushing south and Arabia is moving east.

Greece bears the brunt, as the Aegean Sea and the Greek islands are driven towards an arc of destruction to the south of Crete. Tuesday's earthquake was magnitude 5.8 and occurred only 10 km down. But the summer of 1995 saw two magnitude 6.6 earthquakes.

Unpleasant surprise

Despite a long history of seismic activity, the latest tremor caught geologists unawares.

Maureen Ritchie, a seismologist at the British Geological Survey in Edinburgh, told BBC News Online: "There was no surface feature for a fault in this area. Greek seismologists have been very surprised by it."

Greece is littered with sharp fault slopes resulting from millennia of seismic activity and ancient temples now submerged below the sea show graphically that the land is on the move.

Larger earthquakes occurred as recently as 1995

BGS seismologist Glen Ford rules out any connection between the Athens quake and the Turkish catastrophe last month. "There is no connection we know of at the moment. If there was, why didn't the Athens earthquake happen sooner?"

The earthquakes occurred 620 km apart and although the Turkish fault does extend westwards into Greece, it tracks well to the north of Athens. Furthermore, the Turkish earthquake occurred as rocks slid past each other, so-called "strike-slip" movement but the Greek earthquake was due to rocks being pulled away from each other, a "normal" movement.

Stress release

Many other earthquakes have happened in the general region in the last few weeks but did not damage buildings or lead to loss of life and were not reported. Over 800 earthquakes of magnitude 6.5 or greater are expected every year around the globe.

Being in a tectonic crunch zone, Greece actually benefits from having a large number of earthquakes.

This means the stress in the rocks is released in more, smaller tremors, rather than in a few large ones. Records show that quakes do not occur much above magnitude 6.5.