The quake struck at a depth of about 290 kilometres, too deep to unleash a tsunami, geologists said. But it was felt as far afield as North Sumatra to the west and in Bali about 880 kilometres to the east.

"We are closely monitoring Mount Ceremai and Mount Slamet," Surono, head of the energy ministry's Volcanology Centre told Elshinta radio, referring to two volcanoes on densely populated Java.

"If the pressure at the volcanoes is quite high, it may trigger volcanic activity," he said.

The Meteorology and Geophysics Agency said the probability of such an event however was low. Prih Harjadi, head of the geophysics department at the agency, said that "considering the depth of the quake, the probability that the quake will trigger volcanic activity is quite low, although not impossible."

"The nature of deep quakes such as this is that they are widely felt but usually not destructive," he told AFP.

The earthquake frightened many Indonesians, who have endured repeated major and deadly tremors in recent years, including a powerful December 2004 quake that unleashed a tsunami across the Indian Ocean, killing 168,000 people in the province of Aceh alone.

"I was asleep... but I awoke and felt the strong shake. I was on the third floor and when I looked out there were many people outside the building," Ricky, a resident on the island of Bali, told the Detikcom online news portal.

The state-run energy firm Pertamina shut down a refinery in the west Java town of Indramayu following the quake due to a power failure, it said.

"We are still assessing the condition of the Balongan refinery," Pertamina president Ari Soemarno told reporters, noting that while initial reports indicated no damage, the plant would be shut as a precaution.

Operations were expected to resume in three days and the shutdown was not expected to have a significant impact on fuel stocks. Indonesia, an archipelago nation, sits on the Pacific "Ring of Fire" where continental plates meet, causing frequent seismic and volcanic activity.

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