Ryoo Yong-gyu, Earthquake and Volcano Monitoring Division Director, points at where seismic waves observed in South Korea came from, during a media briefing at Korea Meteorological Administration in Seoul, South Korea. The blast, on the 68th anniversary of North Korea's founding, was more powerful than the bomb dropped on Hiroshima, according to some estimates. It was condemned by the United States, which said it would work with partners to impose new sanctions on North Korea, and by China, Pyongyang's main ally. REUTERS/Kim Hong-Ji 

A sales assistant watches TV sets broadcasting a news report on North Korea's fifth nuclear test, in Seoul, South Korea. REUTERS/Kim Hong-Ji 

Ryoo Yong-gyu, Earthquake and Volcano Monitoring Division Director, points at a chart showing seismic waves observed in South Korea, during a media briefing at Korea Meteorological Administration in Seoul, South Korea. REUTERS/Kim Hong-Ji 

An employee looks at where seismic waves observed in South Korea came from, during a media briefing at Korea Meteorological Administration in Seoul, South Korea. REUTERS/Kim Hong-Ji 

Japan Meteorological Agency's earthquake and tsunami observations division director Gen Aoki speaks next to screens showing the seismic event was indicated on North Korea and observed in Japan during a news conference at the Japan Meteorological Agency in Tokyo, Japan. REUTERS/Kim Kyung-Hoon