The quakes with magnitudes of 6.6 and 5.6 struck Minxian and Zhangxian counties in Gansu Province.
Over 400 aftershocks had been recorded in the quake-hit region, with the strongest measuring 5.6 in magnitude, Chang Zhengguo, spokesman for the provincial government, said.
Official Xinhua news agency reported that the death toll has climbed to 75.
According to latest figures, 73 people were killed in Dingxi City and two in its neighboring Longnan City, said the Dingxi municipal government.
Aftershocks and minor landslides with falling rocks were seen in the mountainous region, posing dangers for rescuers arriving at quake-hit villages.
In the villages in the seriously-hit Meichuan Township, of Minxian County, many rural buildings had been reduced to ruins and others had cracks in the walls, Xinhua reported.
A total of 31 people have been confirmed dead in Meichuan, according to rescuers.
Villagers said the victims were mainly elderly and children, who failed to escape from collapsing houses.
President Xi Jinping has urged all-out rescue effort and put "saving life" as the top priority.
