JAKARTA, Sept 17 (Reuters) - A massive earthquake that shook Indonesia's Sumatra island last week triggered more activity in three volcanoes in the area, but all have since calmed down, the head of the country's volcanological survey said on Monday.

The 8.4 magnitude quake that struck off the coast of western Sumatra on Wednesday and a series of aftershocks spurred fresh magma movement in the volcanoes, which lie close to the quake's epicentre, Saut Simatupang told Reuters.

"It is true that the shocks have spurred an increased number of tremors in surrounding volcanoes. The number of tremors indicate the movement of magma," Simatupang said.

"If you ask me whether any of the volcanoes will erupt because of the shocks, my definite answer is no. There is nothing to worry about."

Mount Talang -- which lies just 30 km (19 miles) from Padang, the city nearest to the epicentre of one of the quakes -- recorded almost 40 times the usual number of volcanic tremors a day after the quake, Simatupang said.

But activity on the 2,597-metre (8,520-foot) volcano dropped dramatically the day after, and the number of tremors is now close to the normal six a day.

Two other volcanoes, Mount Dempo and Kaba in the worst-hit province of Bengkulu, have also calmed down after the aftershocks began to ease.

A series of tremors, ranging in intensity from 4.9 to 7.8 since Wednesday's quake, have repeatedly set off tsunami warnings in Indian Ocean countries.

More than 20 people have died and over 100 have been injured in the severe earthquake, but with no reports of the sort of tsunami that caused widespread devastation around the Indian Ocean rim on Dec. 26, 2004, following a 9.15 magnitude quake.

Indonesia has the highest number of active volcanoes of any country, sitting on a belt of intense seismic activity known as the "Pacific Ring of Fire".

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