DA QAIDAM, Qinghai, Nov. 10 (Xinhua) -- An earthquake measuring6.3 on the Richter scale hit northwest China's Qinghai Province at9:22 a.m. on Monday, but as yet no casualties have been reported.
The epicenter, at 37.6 degrees north and 95.9 degrees east, was in the Mongolian-Tibetan Autonomous Prefecture of Haixi, about 716kilometers west of the provincial capital Xining, according to the National Seismic Network.
By noon, the Qinghai Seismic Network had monitored at least 25 aftershocks in the area, with the strongest measuring 5 on the Richter scale. But seismologists expected no further quakes above 6.3 magnitude imminently.
The earthquake toppled several huts in Da Qaidam district near the epicenter, and mud houses had cracks in their walls, said Gu Xiaodong, a deputy official of the administrative committee of the district.
"No casualties have been reported as yet," Gu said.
Schools in Da Qaidam had been closed for safety reasons and classes would be suspended for at least one day, he said.
Da Qaidam is sparsely populated with about 16,000 people, including Mongolians, Tibetans, Muslim Hui and Han, according to the county government's website.
Da Qaidam, which literally means "a big salt lake" in Mongolian, is rich in mineral resources, with one of the largest lead-zinc mines in western China and several coal mines.
Gu said the epicenter was close to two coal mines, Kaiyuan and Dameigou, "but no serious damage has been incurred".
"Altogether 119 workers underground and 38 above ground were evacuated to safe areas by 10:35 a.m.. Only three people were slightly injured, and they have been sent to hospital," said Duan Mingdao, deputy manager of Yihai Energey Company that runs the Dameigou Coal Mine, about 15 kilometers from the epicenter.
"The quake cut the mine's communication network and power supply, which resumed at midday and 5 p.m., respectively," he told Xinhua.
Big cracks could be seen on most buildings at the mine, and workers' dorms were in a dangerous condition. The company bought 20 tents to accommodate about its 700-strong workforce.
"The mine has suspended operation," Duan said.
At least seven mines in Haixi prefecture, near the epicenter, reported they felt the tremor, but there were no reports of casualties.
The tremor was noticeably felt in Haixi's industrial city Golmud -- the starting point of the Qingha-Tibet Railway, the world's highest railway -- and Haixi prefecture's capital Delinha. Schools were also suspended in the two cities.
When the quake struck, many people ran out of buildings. "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.
Hol Xil, a major nature reserve and habitat for the critically-endangered Tibetan antelope, is in the Kunlun Mountain Range, near Golmud.
The quake was also felt in Xining, where Xinhua reporter Ma Yong said, "We all jumped up in the middle of our morning editorial meeting." The office is on the third floor of a four-storey building downtown.
The Qinghai-Tibet railway seemed largely unaffected by the earthquake. "Railway operation remains normal," said a company spokesman. "But we have tightened safety surveillance along the route, and equipped every train with additional mechanical engineers."
An earthquake measuring 6.6 on the Richter scale hit the same region in April 2003, but did not cause casualties, and only damaged old buildings.
The China Earthquake Administration suggested Monday's earthquake would have caused limited losses.
The quake occurred almost six months to the day after the 8-magnitude May 12 earthquake hit southwest China's Sichuan Province, leaving more than 80,000 dead or missing. It was the deadliest and strongest tremor to hit China since the 1976 Tangshan earthquake.
Map highlighting Qinghai province. 
According to the United States Geological Survey (USGS), an earthquake with magnitude of 6.5 has struck China. ''Xinhua'', one of China's main news agencies, reports the quake at a magnitude of 6.3.

According to the USGS, the quake was centered 55 kilometers (35 miles) East, Southeast of Da Qaidam, Qinghai province and was reported at a depth of 10km (6.2 miles). It struck at approximately 9:22 a.m local time on Monday morning. 

Although there have been no immediate reports of damage or injuries, the USGS says damage in the area could be "very heavy". The shaking was felt as far away as Golmud, and Xining, where witnesses say buildings shook, but there was no damage.  This is adjacent to Tibet Autonomous Region and just at the foothills of Tibetan Plateau.

The Da Qaidam region is an area of China where there are several launch sites for DF-4 nuclear missiles. At least two nuclear missiles are reported to be stored in the Da Qaidam Basin and were deployed there in 1998. Da Qaidam is one of five locations throughout China where launch sites exist. There is no word if the sites suffered any damage from the quake.