 On February 11, 2013, the U.S. Geological Survey detected a magnitude 5.1 seismic disturbance, reported to be a third underground nuclear test.North Korea has officially reported it as a successful nuclear test with a lighter warhead that delivers more force than before, but has not revealed the exact yield.Multiple South Korean sources estimate the yield at 6–9 kilotons, while the German Federal Institute for Geosciences and Natural Resources estimates the yield at 40 kilotons.