A strong earthquake shook Portugal's mid-Atlantic Azores archipelago early today, according to the US Geological Survey's Earthquake Information Center.

The quake measured 6.3 on the open-ended moment magnitude scale, now often used by seismologists, which measures the area of the fault that ruptured and the total energy released.

Portugal's Lusa's news agency said there were no immediate reports of damage, although emergency services had received a number of calls.

The quake hit at 1:56am (0356 GMT), and was centred at 93km east of Ponta Delgada, the Azores capital, the USGS said. The epicenter was at a depth of 10km.

The archipelago, located some 1,200km west of mainland Portugal, is full of extinct volcanos, and earthquakes and tremors are a frequent occurrence.
The nine-island chain has a population of about 237,000.