The second temblor measuring magnitude 5.1 hit about 15 minutes later, some 60 miles west of the Tibetan capital, it said.
Earlier reports from China's official Xinhua News Agency said at least 30 people died, but the agency revised the death toll to nine on Tuesday, saying the previous figure was inaccurate as it came from "unauthoritative sources" and required verification.
Hundreds of houses collapsed in Gedar township near the epicenter in Dangxiong County, and traffic and telecommunications were cut. Nineteen people were injured, Xinhua said, citing Hao Peng, deputy chairman of the Tibetan regional government.
An unknown number of people were trapped, and soldiers and rescue workers were dispatched to the site, Xinhua said.
Deaths also were reported in a neighboring county, Xinhua said, but no figures were available. The Lhasa airport and the Qinghai-Tibet railway - which stretches from western Qinghai province to Tibet - continued to operate, the agency said.
China says Tibet has been part of its territory for centuries, although many Tibetans say their homeland was essentially independent for most of that time. On March 14, monk-led protests against Chinese rule turned violent in the Tibetan capital of Lhasa, and ethnic Chinese residents were attacked.
China's State Seismological Bureau said the initial temblor was centered in Dangxiong county, which has a population of about 42,000 people, mostly herdsmen.
"I felt the building shaking a little bit and saw a bench overturn," said Ge San, an employee at the Baima Hotel in Dangxiong, who was sitting in a room with about five other employees.
"The shaking was not heavy. We stayed in the room and were not frightened," she said, adding that all the hotel's guests remained on the premises.
In Lhasa, employees at the Civil Affairs Bureau rushed out of their building when the tremors began but returned soon after, said an official who refused to give her name.
"I was in my office on the third floor," she said. "The shaking lasted for about half a minute."
Xinhua said that so far, none of the city's landmarks, such as the Potala Palace, appeared to be damaged.
One of the agency's reporters in Lhasa said shops remained open and there was no panic on the streets.
Authorities said seismologists and officials were sent to the area and were assessing the situation.
China's far west is fairly earthquake prone. On Sunday, a magnitude-5.7 earthquake shook the Xinjiang region, which borders Tibet, Tajikistan and Kyrgyzstan, which also suffered a 6.6-magnitude quake hours later. At least 60 people were killed when a village collapsed.
Tibet, a sparsely populated region, has been hit by several moderate earthquakes in recent weeks.
Last month, a magnitude 6 quake struck near its border with Nepal but there were no reports of damage or casualties.
In late August, the USGS reported that an earthquake with a magnitude of 6.7 hit the region. Chinese state media said schools, a hydropower station and 622 homes were damaged and about 2,000 people forced to seek temporary shelter.
A 7.9 magnitude earthquake on May 12 devastated parts of Sichuan province, just east of Tibet, killing 70,000 people and leaving 5 million homeless.