A series of strong earthquakes struck a far western Tibetan area of China on Wednesday, killing at least 400 people and injuring thousands as houses made of mud and wood collapsed, trapping many more, officials said.Paramilitary police were forced to use shovels to dig through the rubble in a Qinghai province township where most of the homes had been flattened, footage on state television showed. Officials said excavators were not available and with most of the roads leading to the nearest airport damaged, equipment and rescuers would have a hard time reaching the area.Downed phone lines, strong winds and frequent aftershocks also hindered rescue efforts, said Wu Yong, a local military chief.Workers were racing to release water from a reservoir in the disaster area where a crack had formed after the quake to prevent a flood, according to the China Earthquake Administration.The magnitude-6.9 temblor struck Wednesday morning in an area in the south of Qinghai province, near Tibet, the U.S. Geological Survey said. The quake was centered on Yushu county, with a population of about 100,000, mostly herders and farmers.The USGS recorded six temblors in less than three hours, all but one registered 5.0 or higher. The China Earthquake Networks Center measured the largest quake's magnitude at 7.1.The main quake sent residents fleeing as it toppled houses made of mud and wood, said Karsum Nyima, the Yushu county television station's deputy head of news, speaking by phone with broadcaster CCTV."In a flash, the houses went down. It was a terrible earthquake," he said. "In a small park, there is a Buddhist pagoda and the top of the pagoda fell off. ... Everybody is out on the streets, standing in front of their houses, trying to find their family members."The death toll rose to about 400 by afternoon, according to CCTV. Emergency official Pubucairen was quoted as saying that the number of injured has risen to more than 10,000.The earthquake comes a little less than two years after a magnitude-7.9 quake in neighboring Sichuan province left almost 90,000 people dead or missing.That quake flattened several schools, killing thousands of students. Poor design, shoddy construction and the lax enforcement of building codes were found to be rampant.In Jiegu, a township 20 miles (30 kilometers) from the epicenter that appeared to be one of the worst hit, the local fire brigade was trying to rescue 20 students stuck inside a school, Kang Zifu, head of the rescue team, told state television. They were also working to pull out 40 to 50 people trapped in a toppled four-story building, according to CCTV. It did not say what type of school it was.More than 85 percent of houses had collapsed in Jiegu, while large cracks have appeared on buildings still standing, the official Xinhua News Agency cited Zhuohuaxia, a local publicity official, as saying."The streets in Jiegu are thronged with panic and full of injured people, with many of them bleeding from their injuries," he said.The provincial emergency office told Xinhua that 700 soldiers were trying to clear the rubble and rescue buried people and that 1,000 more troops would be dispatched.A local military official, Shi Huajie, told CCTV rescuers were working with limited equipment."The difficulty we face is that we don't have any excavators. Many ofthe people have been buried and our soldiers are trying to pull themout with human labor," Shi said. "It is very difficult to save peoplewith our bare hands."Five thousand tents and 100,000 thick,cotton coats and heavy blankets were being sent to help survivors copewith strong winds and temperatures of around 43 degrees Fahrenheit (6degrees Celsius), the Qinghai provincial government said in a statement.Theepicenter of the first quake was 235 miles (380 kilometers)south-southeast of Golmud, a large city in Qinghai, at a depth of sixmiles (10 kilometers), the USGS said.Xinhua cited officials atthe China Earthquake Networks Center as saying at least 18 aftershockshave been reported and that more temblors exceeding magnitude 6 werelikely to occur in the coming days.