SEOUL, South Korea — North Korea fired another missile over the northern Japanese island of Hokkaido on Friday morning, just a day after Pyongyang threatened that the four main Japanese islands “should be sunken into the sea” by its nuclear bomb. This was the second time in less than three weeks that North Korea had sent a missile over Japan, and immediately sparked angry reactions in Tokyo and Seoul. The missile was launched from the Sunan airfield just north of Pyongyang around 6:30 a.m.