China aftershocks set for weeks, months even years

SYDNEY (Reuters) - China's devastated Sichuan region can expect to be rocked by aftershocks for weeks and months, possibly years, but the power of the aftershocks, one destroyed some 420,000 houses, will gradually diminish, say seismologists.

Since the May 12, 7.9 quake which killed more than 68,000 people, a series of strong aftershocks have occured along the quake's 250 kms (155 mile) fault line, running southwest to northeast outside of Chengdu.

Seismologists have recorded around 150 aftershocks above a magnitude of 4, with the highest a 6, and warn that a major aftershock of around 6.9 was still possible.

"I think we have been incredibly lucky that a 7.9 quake has not had aftershocks bigger than 6. There is still a chance, but as time goes by it is reducing," said Gary Gibson, chairman of the International Seismology Centre in London.

"The one that I dread, with people trying to fix these dams, is a large aftershock. Even after weeks you are likely to get the odd rogue one," Gibson told Reuters on Thursday.

China has evacuated more than 150,000 people living below the swollen Tangjiashan lake, one of 35 lakes formed by the quake, amid fears it could burst and trigger massive flooding.

Seismologists said the duration and severity of aftershocks depended on the size of the main quake and its depth. The larger and shallower the main quake, the more powerful the aftershocks and the longer their duration.

"There is no way of predicting the duration of aftershocks, but aftershocks can go on for years and years and years," said Adam Pascale from the Seismology Research Centre in Australia.

"I would imagine they will have quite reasonably strong aftershocks for a couple of months and then earthquakes in that sequence will continue for years," Pascale said.

The May 12 quake at 7.9 was approaching the most powerful for a continental quake, one inside a continent and not on a tectonic plate boundary, and at 10 km (six miles) deep was very shallow.

SHALLOW

Shallow, powerful earthquakes cause widespread damage.

"The most important thing about aftershock duration and severity is the size of the main shock. Usually the biggest aftershock is one unit less than main shock, so you'd expect a 6.9, but we haven't had them fortunately," said Gibson.

"Shallow earthquakes tend to have a lot more aftershocks."

Earthquakes on the Pacific's Ring of Fire, such as the 2004 Indonesian quake which caused an Indian Ocean tsunami, are the result of two tectonic plate boundaries pushing against each other.

The Sichuan quake was a continental quake on the Longmenshan fault, resulting from tectonic pressure caused by the convergence of the Tibetan Plateau in the west butting up against the Sichuan Basin and southeastern China, said the U.S. Geological Survey.

"On a continental scale, the seismicity of central and eastern Asia is a result of northward convergence of the India plate against the Eurasia plate with a velocity of about 50 mm per year," said the USGS.

Seismologists said the weakness of rock along the fault line in Sichuan will also determine the frequency of aftershocks.

"The majority of the (tectonic) stress is relieved from the main shock...but then you have fractures all the way along the fault line that continue the aftershocks," said Pascale.

"The aftershocks will happen on that fault, anywhere along the fault. But the time frame can be years. The aftershocks continue until it settles into a new form of stability."

(Editing by Sanjeev Miglani)