WASHINGTON - Two minor tremors were detected on Saturday from near North Korea's nuclear test site and were probably aftershocks from the country's massive nuclear test in early September, a US Geological Survey official said.The aftershocks, of magnitude 2.9 and 2.4, were detected at 0613 and 0640 GMT (1:13 a.m. and 1:40 a.m. EST) respectively, said the USGS and Lassina Zerbo, executive secretary of the Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization.A tweet from Zerbo said analysts had confirmed that the activity was "tectonic" in origin.The USGS official said the tremors had been in the vicinity of the Punggye-ri nuclear test site, where North Korea conducted its sixth and largest underground nuclear test on September 3."They're probably relaxation events from the sixth nuclear test," the official said. "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. We've had a few of them since the sixth nuclear test."Pyongyang said the September test was of an H-bomb, and experts have estimated it was 10 times more powerful than the US atomic bomb dropped on Hiroshima in 1945.