On 12 February 2013, North Korean state media announced it had conducted an underground nuclear test, its third in seven years. A tremor that exhibited a nuclear bomb signature with an initial magnitude 4.9 (later revised to 5.1) was detected by the China Earthquake Networks Center, Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization and the United States Geological Survey. In response, Japan summoned an emergency United Nations meeting for 12 February and South Korea raised its military alert status. It is not known whether the explosion was nuclear or a conventional explosion designed to mimic a nuclear blast; as of two days after the blast, Chinese, Japanese, and South Korean investigators had failed to detect any radiation.