Region may now be seismically hyperactive

Geologists worried, since Yaan earthquake occurred on same fault as major quake in 2008

PUBLISHED : Tuesday, 23 April, 2013, 12:00am
UPDATED : Tuesday, 23 April, 2013, 4:07am

The 7.0-magnitude earthquake that hit Sichuan on Saturday occurred on the same Longmenshan fault as the 8.0-magnitude earthquake in 2008.

That has deepened geologists' concerns that seismic activity in the region has entered a hyperactive period, which could endanger the millions of people living along the fault.

Xiong Xiong , a researcher with the Institute of Geodesy and Geophysics at the Chinese Academy of Sciences in Wuhan , Hubei , said the earthquake occurred on the southwestern part of the fault.

The biggest displacement of earth reached 1.5 metres at about 13 kilometres deep on one of the fault's three major subsidiary fault lines, according to the institute's analysis of data from seismic monitoring stations across the world.

The earthquake lasted about 20 seconds, with most of the energy released in the first 10 seconds. It tore apart surface areas within 10 kilometres of the epicentre in Lushan county, Yaan .

In a 2009 paper, Xiong's team warned that the 2008 earthquake, which occurred on the northeastern part of the Longmenshan fault, had significantly increased the geophysical stress on the area that was hit by Saturday's earthquake.

"This event could have been triggered by the last one," he said. "We are racing against the clock to calculate whether this earthquake has increased the stress in another area."

A total of 2,283 aftershocks had been recorded by 2pm yesterday, the strongest with a magnitude of 5.4, according to the seismological authorities.

After the 2008 earthquake, a growing number of geologists worried that the Longmenshan fault was awakening from a long period of dormancy, and the latest strike has deepened those fears.

According to the American Geophysical Union, the elevation in the Longmenshan fault zone rises from about 600 metres in the southern Sichuan Basin to a peak of higher than 6.5 kilometres less than 50 kilometres away.

This type of long, steep slope could have been created only by volatile geological movements, but geologic records showed that the region had not been hit by earthquakes greater than magnitude 6.5 for about 3,000 years. This has led some researchers to believe that the Longmenshan fault had entered an long period of relative inactivity.

Professor Ji Shaocheng , a geologist with Polytechnique Montreal in Canada, wrote on Saturday in his blog on China's biggest scientific online community, that Sichuan authorities should be worried that two major earthquakes hit the region in less than five years.

"Did the houses that were rebuilt after the 2008 earthquake withstand this second hit? I am very worried," he said.

Professor Long Enshen , deputy director of the Institute for Disaster Management and Reconstruction, jointly formed by Sichuan University and Hong Kong Polytechnic University, said he had visited the hard-hit county of Baoxing in Yaan prior to Saturday's quake, and he was now deeply concerned by the latest devastation.

Beijing officially included the region in reconstruction plans after the 2008 quake, but due to the relatively mild damage sustained there, only a few homes were repaired, Long said.

"This area is plagued by a large number of fault lines with high population density," Long said.

"Even after the earthquake, it would be extremely hard to convince them to give up the land where they have lived for generations," he said.


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