Earthquake of 6.1-magnitude strikes south of Sibolga, Indonesia: USGS

JAKARTA (Reuters, AFP) - An earthquake measuring 6.1 magnitude struck 107km south of Sibolga, Sumatra, Indonesia on Sunday, the United States Geological Survey said.

The quake was 75.1 km deep and struck 11 kilometres south of the city of Sibolga, in North Sumatra province, the US Geological Survey said.

No tsunami warning was issued and there were no immediate reports of damage or casualties.

Mochammad Riyadi, a senior official from Indonesia's meteorology, climatology and geophysics agency, said the quake was felt strongly for 15 seconds in Mandailing Natal district, which is around 85 kilometres from the quake's epicentre.

"Moderate tremors" were also felt in Padang, the capital of West Sumatra, he said.

"We have no reports of damage or casualty so far. No tsunami alert was issued," he said.

An AFP correspondent in Padang said "the earth swayed back and forth for around 10 seconds", but the majority of residents remained calm.

Indonesia sits on the Pacific "Ring of Fire" where tectonic plates collide, causing frequent seismic and volcanic activity.