China aftershocks set for weeks, months even years

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.

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