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The earthquake struck at 2:29 am (18:29 GMT) around 105 kilometres (65 miles) southwest of Olongapo on the Philippines' Luzon island and 125 kms southwest of the capital Manila, the US Geological Survey said.
It hit at a relatively shallow depth of 24 kms (15 miles), the USGS said.
People in the Philippines have been nervous about a potential killer quake following the devastating ones to have hit Japan and New Zealand recently.
Like those countries, the Philippines sits on the Pacific Ring of Fire a belt around the Pacific Ocean dotted by active volcanoes and tectonic trenches.
One fault line runs directly under Manila, and government seismologists have warned the city is unprepared for a major quake.