At least one person has been killed and 400 hurt when a 5.8 magnitude tremor hit south-west China.The quake occurred nearly a fortnight after a big quake killed tens of thousands in the same area.More than 70,000 houses toppled during the aftershock in Sichuan province.It happened as hundreds of troops carrying explosives are attempting to reach a "quake lake" that threatens a secondary disaster.Concerned by a steep rise in the water level of a giant lake at Tangjiashan, authorities want to blast a hole in the barrier before it bursts and causes a flash flood. Thousands have been evacuated below the lake as a precaution.Meanwhile, another miraculous story of survival has emerged with reports an 80-year-old partially paralysed man had been pulled alive from rubble, 266 hours after the 7.9 magnitude quake hit.Xiao Zhihu was rescued in Mianzhu city, where he had been trapped under a collapsed pillar of his house. He had survived after being fed by his wife.Premier Wen Jiabao, who believes the overall death toll from the May 12 earthquake could exceed 80,000, has said the main concerns are now secondary disasters like flooding and landslides, epidemics and providing shelter for nearly 5 million displaced.The biggest concern is the lake at Tangjiashan which rose 1.93 metres on Saturday to 723 metres.State media reported that bad weather had prevented the airlift of personnel and equipment into the area, and that 1,800 soldiers were sent to the lake by foot.Each soldier was carrying 22 pounds of explosives.The lake is just 2 miles upstream from Beichuan, a town so badly hit that it will be rebuilt in a new location and the ruins of collapsed buildings left as a quake memorial.Workers also plan to dig tunnels to drain water from dozens of other unstable lakes that built up after landslides blocked rivers in the Sichuan province.The government fears that as water levels rise they could burst, drowning survivors and rescue workers downstream.