Earthquake One Of The Biggest In The Past Century

The Age

Wednesday May 14, 2008

Carolyn Webb and Alok Jha, London, with Guardian, AFP, New York Times

AT A magnitude of 7.9, the violent earthquake that shook China's Sichuan province was as powerful as the most destructive quake recorded last century: in Tangshan, China, in 1976, when 240,000 were killed in an event that reached magnitude 7.8.

The epicentre of Monday's event lies on the border of the Quing-Tibet plateau and the Sichuan basin. The Longmenshan fault runs along this boundary, with the plateau being pushed up relative to the basin.

India, once a giant island before crashing into the underside of Asia about 40 million to 50 million years ago, continues to slide north at a geologically quick pace of five centimetres a year. The tectonic stresses push up the Himalaya mountains and generate scores of earthquakes from Afghanistan to China.

On Monday afternoon, an upward-thrust fault broke, generating an earthquake with a magnitude of 7.9. An aftershock with a magnitude of 6.0 followed 15 minutes later, and smaller aftershocks continue in the area.

The epicentre lay in the Longmenshan, mountains that rise steeply to the west of Chengdu, the capital of Sichuan.

Melbourne seismologist Gary Gibson said this tectonic collision went on continuously, occasionally erupting into an earthquake. "Energy builds up until eventually it exceeds the strength of the rocks and they break," he said.

Unlike in the Pacific rim, where one plate tends to dive deeply - up to several hundred kilometres - under another, this week's earthquake was due to part of the India-Australia plate slipping under the Asian plate at shallow depth, causing a rupture extending down to only about 35 kilometres beneath the surface.

And so the fault ruptured horizontally, for about 250 kilometres to the north-east from its origin about 90 kilometres north-west of Chengdu.

Mr Gibson said the US Geological Survey's world data centre in Boulder, Colorado, had upgraded the Sichuan earthquake from 7.8 to 7.9 on the moment magnitude scale, which replaced the Richter scale. "And an earthquake of magnitude 7.9 being shallow is a lot worse in terms of amount of damage caused than most 7.9s on plate boundaries."

Mr Gibson said the quake's shallowness meant the level of shaking people would have felt was "exceptional in the epicentral area ... it would seriously affect even well-designed buildings".

People in Sichuan would have felt a distinct rocking motion lasting about a minute or more. "It would be very frightening indeed," Mr Gibson said. "It is an extreme earthquake."

Mr Gibson said there would be aftershocks, but it was unlikely that such a shallow earthquake would recur soon. It was earthquakes that occurred on the edges of plates that sometimes tended to happen "in twos and threes".

Earthquakes are frequent and deadly along the fringes of the Tibetan Plateau.

The quake struck close to densely populated areas of Sichuan province and was felt across South-East Asia.

In addition to creating the Himalayas, India's northward push generates an eastward spread away from the Tibetan plateau.

Brian Baptie, a seismologist at the British Geological Survey, said the plateau was "one of the most tectonically active places on the planet. We know historically and from the deformations involved that there is potential for these large earthquakes".

The region last suffered a major earthquake in August 1933. The epicentre then was near the town of Diexi, about 90 kilometres north-east of this week's quake. The magnitude 7.5 earthquake destroyed the town and surrounding villages and killed about 9000 people.

The 2004 Indian Ocean earthquake that caused the Boxing Day tsunami and killed more than 300,000 people measured 9.1. -- With GUARDIAN, AFP, NEW YORK TIMES

THE WORST HIT

SICHUAN PROVINCE

Site of the earthquake. Population 87 million. The province sits in the Sichuan basin and is surrounded by the Himalayas to the west. The epicentre of the quake was on the border of the Quing-Tibet plateau and the Sichuan basin.The Longmenshan fault runs along this boundary. The quake was caused by the Tibetan plateau colliding with the basin.

WENCHUAN COUNTY

Epicentre of the disaster. Rescuers have yet to reach many towns, with roads blocked by landslides. More than 100,000 people live in the mountainous county. Main town: Weizhou, population 31,000.

The county is also home to the Wolong Nature Reserve, the largest panda reserve in China.

MIANZHU

Population 510,000. State media reported that 10,000 people remain buried in the city. A steam turbine factory was destroyed, killing at least 60 workers.

DUJIANGYAN

City southeast of the epicentre. Population 600,000. At least six schools were destroyed; 900 pupils were buried under rubble when the three-storey Jiangyan Middle School collapsed. At least 50 are confirmed dead. At another school, fewer than 100 students out of 420 are believed to have survived.

BEICHUAN COUNTY

50 kilometres from the epicentre. Population 160,000. One of the worst-hit areas with several thousand dead. A further 10,000 people were injured.

About 80% of the buildings are reported to have collapsed. Local radio reported 15,000 people were still trapped.

DEYANG

A city of around 4 million people near the epicentre. At least five schools collapsed, causing at least 100 deaths.

SHIFANG

Two chemical plants in Shifang city were reportedly flattened, burying hundreds, possibly thousands, of workers and spilling more than 80 tonnes of toxic liquid ammonia.

CHENGDU

Sichuan's provincial capital is a city of 11 million people. It is 80 kilometres north of the epicentre. Tens of thousands of people were evacuated from buildings but few structures collapsed, a sign of the city's modernity.

GANSU PROVINCE

A 40-car freight train carrying bulk petrol was derailed and caught fire. The 13 tank cars are still on fire.

RESCUE China has deployed 50,000 troops to help with relief efforts.

16,000 have reached the area. -- NEW YORK TIMES, WASHINGTON POST, BBC

© 2008 The Age

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