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By Olivia Rondonuwu

JAKARTA, Oct 3 (Reuters) - Indonesia's earthquake-ravaged city of Padang remains at risk of being wiped out during the next decade by a more powerful quake that could match the 2004 Indian Ocean tsunami for destructiveness, an expert said on Saturday.

Geologist Danny Hilman Natawidjaja, of the Indonesian Science Institute, said pressure had been building in the fault line on which the city sits during a series of tremors in recent years, and that Wednesday's 7.6 magnitude quake, was not the "big one".[ID:nSP462922]

"What is important now is this quake is not the quake we often said will happen, the megacrush of more than 8 on the Richter Scale that causes a tsunami," said Natawidjaja, who has been studying the Mentawai fault, off Sumatra, for 12 years.

Natawidjaja, and his partner Professor Kerry Sieh at Singapore's Nanyang Technology University, believe that when the accumulated pressure is released it could result in a quake of 8.8 magnitude, which he said would be 30 times larger than Wednesday's tremor.

"Scientifically, I can say it might happen tomorrow, or next month or next year. But I'd be quite surprised if it doesn't happen within 10 years," he said.

"I'm personally worried it could happen within months or years, but as a scientist, we are often surprised by nature, so it's hard to tell."

In 2004 a 9.15 magnitude quake, with its epicentre roughly 600 km (375 miles) north of Padang, triggered a tsunami that killed 232,000 people around the Indian Ocean, with the highest death toll in Indonesia's Aceh province.

Natawidjaja said an 8.8 magnitude quake could send tsunami waves 4-10 metres (16-33 ft) high slamming into Padang, a city three times the size of Aceh's main population centre, Banda Aceh, at the time of the 2004 disaster.

The geologist also said that, despite living on top of the world's most active fault line, Padang's authorities had failed to prepare the city for Wednesday's quake, which toppled scores of buildings and left thousands feared killed or trapped.

"I think Padang is totally unprepared. Generally, the existing structures are not designed to be quake-proof and that's why the devastation is so great," he said.

The latest quake left buildings from hotels to schools crumpled and with little left but a pile of masonry.

"Since 2004 there were efforts to anticipate tsunamis. So there were several tsunami evacuation drills. But since 2004 there is almost no effort to anticipate the danger of earthquakes."

(Writing by Alex Richardson; Editing by Sanjeev Miglani)

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