Earthquake damage in Yushu, China. Credit: NEWSCOM

The deadly earthquake that hit western China earlier this month may have put more stress on a nearby fault, increasing the risk of another quake in the devastated region, according to researchers who have been analysing the event.

At 7:49 local time on 14 April, an earthquake recorded as magnitude 6.9 by the US Geological Survey struck the Yushu Tibetan Autonomous Prefecture in Qinghai. As of 20 April, the death toll had reached 2,046, with 12,135 people injured and 193 missing, according to official figures.

Geologists familiar with the Chinese region say it is possible that April's quake may increase the risk of a future earthquake on another fault. "If there is a major seismic hazard in the foreseeable future, the people in the region need to know," says Jian Lin, a geophysicist at the Woods Hole Oceanographic Institution in Cape Cod, Massachusetts.

The earthquake resulted from a type of fault movement known as strike-slip, in which the two sides of the fault move horizontally past each other. At about 4,000 metres above sea level, Yushu is on the eastern edge of the Tibetan plateau, which has risen as a result of the Indian subcontinent colliding with Asia.

Criss-crossed with faults, the region is no stranger to quakes, but the 270-kilometre-long Zhuanglang River fault that caused April's event, "is not one that had been characterized as heightened anticipation", says Walter Mooney, an expert on Chinese tectonics at the US Geological Survey in Menlo Park, California.

Future stress

As China is still trying to recover from the devastating 7.9-magnitude Wenchuan earthquake in 2008, some speculated that the two incidents were related. Mooney says that "the faults are simply too far away to directly affect one another".

The area directly affected by an earthquake is normally within a region that has a radius equal to the length of the fault. Therefore, the Longmenshan fault that caused the Wenchuan event is unlikely to have triggered an earthquake in Yushu, more than 700 kilometres away.

However, the Yushu earthquake may have put further stress on the Xianshui River fault to the west of the Longmenshan fault, says Lin. Studies by Lin and his colleagues show that the Wenchuan earthquake has pushed this fault from the east1.

"Now it got another kick from the west," says Lin. "This has really brought the spotlight back to the Xianshui River fault."

Some sections of the stressed fault haven't moved for more than a century. Lin and his team are now trying to find out how much more stress is in the system and whether this really has increased the risk of a future quake.

Buildings to blame

Immediately after the disaster, a research aircraft equipped with high-resolution optical cameras set off to survey the quake-hit area. Within 48 hours, researchers were able to get a detailed map of the region, with a resolution of 30 centimetres, says Guo Huadong, an expert in remote sensing at the Beijing-based Centre for Earth Observation and Digital Earth at the Chinese Academy of Sciences, who led the project. Using a computer program capable of distinguishing collapsed houses from intact ones — which was developed during the rescue efforts of the Wenchuan earthquake and was subsequently used after the recent Haiti quake — the team found that 62% of the buildings in Yushu had collapsed. Most residential houses in the region are made of mud, stones and bricks, with little reinforcement. These buildings were the hardest hit as 83% of them collapsed, whereas only 20% of reinforced-concrete structures were damaged.

"Both the Haiti and Yushu earthquakes remind us yet again of the poor building standards in some of the world's most quake-prone regions," says Lin. "We can't predict when or where the next earthquake will strike, but we can save some lives by reinforcing buildings and raising public awareness of earthquakes."