As of 3 a.m. Tuesday, the Qinghai provincial seismic network had monitored more than 550 aftershocks, the biggest of which measured 4.4 on the Richter scale.

The earthquake administration has installed two mobile tremor detection devices in the epicenter to ensure 24-hour surveillance.

The quake and aftershocks have caused no fatalities so far, but three miners were injured when they were evacuated from a coal mine 15 km from town. They were immediately sent to a hospital in Haixi prefecture's capital Delinha.

"The quake has not caused too much havoc mainly because the area is sparsely populated," said Lu Ning, an official with the Qinghai earthquake administration.

Da Qaidam town, more than 700 kilometers northwest of the provincial capital Xining, has about 16,000 people, including Mongolians, Tibetans, Han and Muslim Hui. Its population density is about one person on every square km.

Besides Da Qaidam, the quake was clearly felt in the neighboring cities of Delinha, Golmud and even Xining.

"Some residents said it was the biggest tremor they felt since the 8.1-magnitude quake jolted Hol Xil in 2001", said Luo Zhenggang, an official in Golmud.

But even in the hardest-hit areas, people said the tremor didn't last long. A taxi driver in Da Qaidam said he felt his house rocking for about 20 seconds. "It had ended when I realized what was going on."

Da Qaidam, which literally means "a big salt lake" in the Mongolian dialect, is rich in mineral resources, with one of the largest lead-zinc mines in western China and several coal mines.

The quake has left cracks in many buildings in the mining area, including the boiler rooms, dormitories, staff canteens and warehouses, a spokesman with Haixi government said.

"Power and water supplies were disrupted for a few hours but were restored by Monday night," he said.

The local government ordered all the 74 factories and mining companies in Haixi to suspend production and all miners were evacuated from underground.

Traffic remained unaffected along the Qinghai-Tibet Railway, the world's highest plateau railroad. But the railway company said Tuesday that when the quake hit, four trains, including a passenger train, were halted for 20 minutes at Yinmaxia station, about 33 km from the epicenter.

"We have tightened safety surveillance along the route, and equipped every train with additional mechanical engineers," said a company spokesman.

Starting from 10 a.m. Monday, the company imposed a speed limit on all trains passing the quake-battered areas. The upper limit is 60 km per hour between Delinha and Hairag in the west of Qinghai, and 45 km between Delinha and Golmud, about 160 km from the epicenter. The average speed along the railway is 100 to 120 km per hour.

(Xinhua News Agency November 12, 2008)