A strong earthquake rocked parts of Indonesia's North and West Sumatra provinces Wednesday. There were no immediate reports of a tsunami. The 6.1-magnitude earthquake occurred at 6:16 p.m. (7:16 a.m. EDT) and was centered under the Indian Ocean about 60 miles northwest off Nias island, said Lukito, an official at the Meteorology and Geophysics Agency.

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He said the quake was felt in Gunung Sitoli, the main town on Nias island, in the North Sumatra towns of Porsea and Sibolga, and in Padang, the capital of West Sumatra.

"It strongly jolted Gunung Sitoli, but so far, we have not yet received any report of damage or casualties," said Lukito, who uses one name.

A tsunami caused by an undersea earthquake killed some 600 Indonesians last week.

Indonesia, the world's largest archipelago, is prone to seismic upheaval due to its location on the so-called Pacific "Ring of Fire," an arc of volcanos and fault lines encircling the Pacific Basin.