A 5-magnitude aftershock rattled windows in Sichuan’s provincial capital of Chengdu in the early hours of Tuesday, scaring residents who feared another earthquake after the devastating May 12 main quake.

According to China’s Xinhua news agency, the number killed in that province had risen to 39,577.

A number of fresh aftershocks and forecasts of heavy rain are threatening to hinder efforts by China's military, government and private workers to ensure food and housing for millions made homeless by last week’s 7.9 magnitude.

“You expect to see aftershocks following a major earthquake,” said Susan Potter, a geophysicist with the U.S. Geological Survey in Golden, Colorado, “but they become less frequent and smaller as time goes on,” she added, quoted by CNN.

“Any ‘prediction’ of an earthquake at a certain time and in a certain place is certainly a rumor. Because currently in the world the level of earthquake prediction is impossible to achieve this kind of accuracy,” the notice from the Seismological Bureau said.

Aftershocks mostly occur on the same fault line as the main shock, and are believed to be triggered by the main shock. It seems that sometimes the stress caused by the main shock is great enough to trigger aftershocks on nearby faultlines, Reuters reports. Aftershocks also follow patterns, decreasing in number and magnitude over time. Scientists claim the second day has about half the number of aftershocks of the first day. The 10th day has about 1/10 the number of the first day.