Twin earthquakes and a spate of aftershocks have struck southwestern China, toppling thousands of houses and sending boulders cascading across roads. At least 80 people were killed and hundreds injured in the remote mountainous area, and more than 100,000 residents were evacuated.

Damage was preventing rescuers from reaching outlying towns, and communications were disrupted after the midday quakes hit along the borders of Guizhou and Yunnan provinces, a region of small farms and mines where some of China's poorest people live.

The first magnitude-5.6 quake struck just before 11:30 a.m. and was followed by an equally strong quake shortly after noon, joined by dozens of aftershocks. Though of moderate strength, the quakes were shallow, which often causes more damage.

Hardest hit was Yiliang County, where all but one of the deaths occurred, according to the Yunnan provincial government's official website. Another 730 people in the area were injured, the state-run Xinhua News Agency said. Yiliang's high population density, flimsy building construction and landslide-prone hillsides were blamed for the relatively high death toll.

China Central Television showed roads littered with rocks and boulders and pillars of dust rising over hilltops from the landslides. One image taken just as one quake struck showed people running out of a supermarket as the ground shook.

Other footage showed several hundred people crowding into a school athletic field in Yiliang's county seat, a sizable city spread along a river in a valley, as well as soldiers carrying injured people and rescue materials.

Foreign Affairs Minister John Baird said Ottawa is in close contact with the Canadian Embassy in Beijing and its consulate in Chongqing. He reminded Canadians in the region that diplomatic offices are ready to offer assistance.

"Our thoughts and deepest sympathies are with those affected by the earthquakes," Baird said in a statement. "China has significant experience with natural disasters and proven capacity in responding to them. However, Canada stands ready to provide assistance if requested."

Though quakes in the area occur frequently, buildings in rural areas and China's fast-growing smaller cities and towns are often constructed poorly. In 2008, a magnitude-7.9 quake that hit Sichuan province, just north of Yunnan, killed nearly 90,000 people, with many of the deaths blamed on poorly built structures, including schools.

Friday's quakes destroyed 6,650 homes across several counties and townships, Xinhua said. The Yunnan seismology bureau said more than 100,000 people were evacuated from their homes. All told, 700,000 people had their lives disrupted by the quake, Xinhua said.

Falling rocks

In Luozehe, a town in Yiliang near a zinc mine, residents and state media said boulders hurtled off hillsides and houses collapsed.

"It is scary. My brother was killed by falling rocks. The aftershocks struck again and again. We are so afraid," Xinhua quoted miner Peng Zhuwen as saying.

Wu Xuhong, a goat herder in Luozhe, said only tiles fell from his relatively solid cement and brick sheds.

"But I heard that a lot of buildings built of clay and wood collapsed and we temporarily lost power and mobile phone signal," Wu said.

Red Cross spokesman for East Asia, Francis Markus, said 2,000 quilts, 2,000 jackets and 500 tents were being rushed to the area, one of China's poorest, which is largely inhabited by members of the Yi ethnic minority.

He said the use of light construction materials would likely create far more injuries than deaths.

A government official in Jiaokui town said a large number of houses had collapsed.

"The casualty number is still being compiled. I don't know what it was like for the other towns, but my town got hit badly," he said. Like many Chinese officials, he refused to give his name.

Mobile phone services were down and regular phone lines disrupted. Phones were cut off to clinics in four villages in Qiaoshan, another town in Yiliang, which has about half a million people.

Authorities sent thousands of tents, blankets and coats to the area, Xinhua said.

It said that so far no casualties had been reported in neighboring Guizhou, but that homes had been damaged or destroyed there.

Friday's quakes were relatively shallow, about 10 kilometres deep, creating an intense shaking even at a lower magnitude.

By comparison, the 7.6-magnitude quake that struck Costa Rica this week was 41 kilometres below the surface, and combined with strict building codes, kept damage and deaths to a minimum.