ATHENS, Greece (CNN) -- A strong earthquake and an almost equally strong aftershock struck southern Greece just after midday Thursday, U.S. and Greek experts said.
The U.S. Geological Survey (USGS) reported on its Web site that the initial 6.7 magnitude quake was centered about 140 miles (230 km) southwest of the capital, Athens, and was located about 12 miles offshore.
The Athens Geodynamic Institute said an aftershock measuring 6.4 on the Richter scale -- which is no longer used by the USGS -- rumbled through the same region soon afterward.
The USGS also measured the aftershock at 6.4 magnitude and said it took place about two hours after the initial quake. The aftershock was centered 45 miles southwest of Kalamata and 150 miles southwest of Athens, the USGS reported.
Experts and civil authorities had warned residents to get out of buildings and remain outside until the forecasted aftershock hit.
There were no immediate reports of injuries or damage from either the earthquake or the aftershock, according to Greek media, which said the quake could be felt as far south as the island of Crete.
The earthquake hit the country's Methoni region -- the site of heightened seismic activity in recent months.
Due to its fault lines, Greece is one of the most quake-prone countries in the European Union. The last major earthquake there was in 1999 and left about 100 people dead. E-mail to a friend
Journalist Anthee Carassava in Athens contributed to this report.