'No link' between Chinese quakes

By Paul Rincon
Science reporter, BBC News

Hundreds have been killed in the Qinghai quake

The southern Qinghai area of China has been hit by very few large earthquakes since 1900.

And there is no direct relationship between this earthquake and the devastating event in Sichuan, China, in May 2008.

"It's not the same fault, it's a consequence of the same bit of global tectonics, which is the collision of India with Asia. That's the only link I'd make," said Dr David Rothery, from the Open University in Milton Keynes, UK.

"It's not stress release that you sometimes hear reported where one part of a fault goes and then, for example, 100km along the fault another bit goes. That's not what we're seeing here."

This earthquake was caused by a different mechanism to the event in May 2008. While the Sichuan earthquake was caused by a "thrust" movement, this latest event was a "strike-slip" event, which involves sideways movement along a fault.

"India bumped into Asia (millions of years ago) and threw up the Tibetan plateau. That high ground is now being squeezed out towards the east and down towards the south-east," said Dr Rothery.

"There is thrusting at the edge of the mountain belt where the Sichuan quake was but this earthquake is further north and west."

Dr Rothery said the earthquake did not occur at a boundary between Earth's tectonic plates: "The plate boundaries get very messy as you go into the middle of a continent. There's not a single line you can point to and say that's a plate boundary.

"You can more or less do that on the San Andreas fault in California.

"But here you have one continent that has collided with another. You have lots of faults and the continent is changing shape gradually because of these faults."

While earthquakes are common in this region, there has been no seismic event as big as this within 200km of the current epicentre since at least 1900.

The Qinghai quake happened at a depth of 10 km, which is slightly more shallow than the recent Haiti quake.

When quakes occur at shallower depths, the ground tends to shake more.

Kevin McCue, director of the Australian Seismological Centre, said: "It doesn't qualify as a major earthquake even though the result may be a major disaster."

He added: "Population density is the difference in the effects of these earthquakes."

Dr Rothery commented: "As usual in earthquakes, people have died because of the collapse of cheaply constructed buildings, in a poor region where it seems little regard has been paid to building codes that could have offered better protection to the people inside."

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