(CNN) -- The U.S. Geological Survey reported a seismic disturbance in northern North Korea on Tuesday, centered near the site of the secretive regime's two previous nuclear tests.
The area around the reported epicenter of the 4.9-magnitude disturbance has little or no history of earthquakes or natural seismic hazards, according to U.S. Geological Survey maps. The disturbance took place at a depth of about 1 kilometer, the USGS said.
There were no initial reports concerning the activity on the North's state-run Korean Central News Agency on Tuesday.
The reclusive, Stalinist state announced last month that it planned a new nuclear test and more long-range rocket launches, all of which it said were part of a new phase of confrontation with the United States.
U.S. analysts say North Korea's first bomb test, in October 2006, produced an explosive yield at less than 1 kiloton (1,000 tons) of TNT. A second test in May 2009 is believed to have been about two kilotons, National Intelligence Director James Clapper told a Senate committee in 2012.
By comparison, the bomb the United States dropped on Hiroshima in 1945 was a 15-kiloton device.
In May 2012, North Korea said it had amended its constitution to formally proclaim itself a "nuclear state."