Tremors have been detected near North Korea's nuclear site. The movement is likely the result of aftershocks following country’s massive nuclear test in early September. “They’re probably relaxation events from the sixth nuclear test,” said an official from the U.S. Geological Survey. “When you have a large nuclear test, it moves the earth’s crust around the area, and it takes a while for it to fully subside." Pyongyang said the September test was of an H-bomb.