The resulting explosion could have been roughly as strong as four megatons of tnt -- 250 times more powerful than the atom bomb dropped on hiroshima .
最后产生的爆炸大概相当于4百万吨的tnt的能量--比投向广岛的原子弹的威力还要大上250倍。
South korea 's defence ministry says the seismic impact of the blast indicated a power equivalent to 6-7 kilotons of tnt , compared with 2-6 kilotons in 2009 .