A strong undersea earthquake has hit parts of western Indonesia, but no tsunami warning was issued and there have been no immediate reports of damage or casualties.The U.S. Geological Survey says Friday's 6.2-magnitude quake struck at a depth of 12 miles.Indonesia's Meteorology and Geophysics Agency says it was centered about 100 miles southeast of Mentawai island, off western Sumatra.The Indonesian agency said the earthquake was felt in the cities of Padang and Bengkulu and along the southern coast of Sumatra.Indonesia is prone to seismic upheaval due to its location on the Pacific "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin.