March 21 (Bloomberg) -- A magnitude 7.2 earthquake struck northwestern China at 6:33 a.m. local time today, the U.S. Geological Survey said. There were no immediate reports of casualties in China's most powerful quake since 2002.

The quake struck in the border region between Xinjiang Uyghur Autonomous Region and Tibet, 225 kilometers (140 miles) southeast of the city of Hotan, the USGS said by e-mail. The earthquake's depth was 37 kilometers, the agency said.

The city of 280,000 people, about 3,160 kilometers west of Beijing, lies on the edge of the Taklamakan desert and the Tarim Basin, where a third of China's oil reserves are. PetroChina Co. and China Petroleum & Chemical Corp. said the Hotan quake's epicenter is far from their oilfields and don't expect adverse effect from the tremors.

Xinjiang province, which produced a third of China's 2005 cotton crop and 22 percent of the country's wool, has been blanketed by China's worst snowstorms in half a century. Today's earthquake comes as the weather bureau is forecasting more snow and for average temperature to drop by 5 degrees Celsius.

The earthquake occurred in the sparsely populated mountains of Xinjiang, inhabited mainly by the region's Uyghur ethnic minority group, China's state-run Xinhua News Agency reported, citing a Hotan official it didn't name.

The towns of Aqqan and Bostan, with a combined population of 13,400, were affected, the Chinese news agency said without giving details. Zhang Jinbiao, the top Communist Party official of the region, had departed for the earthquake zone with a team of medical workers and public security staff, Xinhua said.

Biggest Since 2002

The last major earthquake to shake Xinjiang took place in February 2003, when a 6.4 magnitude tremor struck 105 kilometers east of Kashgar in the Tarim Basin, leaving at least 261 people dead, according to the USGS.

Today's earthquake was the most powerful to hit China since June 2002, when a magnitude 7.3 temblor struck the country's northeast, according to the USGS. Today's Hotan quake measures 7.3 on the scale used by the China Seismological Bureau.

The earthquake struck about 1,000 kilometers southwest of Urumqi, the capital of Xinjiang. The area was hit by two aftershocks of magnitude 5.3 and 5.5 in the 45 minutes that followed, according to the agency.

PetroChina, the country's largest oil company, said the company hasn't received any damage reports at its oilfields, which are far from the epicenter of the Hotan earthquake spokesman Mao Zefeng said today by phone from Hong Kong.

China Petroleum, China's biggest oil refiner, doesn't expect any ``major impact'' from the quake on its Xinjiang oilfields, which are more than 1,000 kilometers from the epicenter, spokesman Huang Wensheng said today in Beijing.

To contact the reporter on this story: Ed Johnson in Sydney at ejohnson28@bloomberg.net

Last Updated: March 21, 2008 00:01 EDT