A magnitude 5.9 earthquake off North Korea early Thursday has jolted watchers of the country's weapons development but experts say it was not caused by a nuclear test.

Analysts say North Korea needs to conduct another atomic test explosion to perfect a nuclear-tipped missile capable of reaching the continental U.S. On July 4, Pyongyang test-launched its first intercontinental ballistic missile. North Korea's five previous nuclear tests caused signs of artificial quakes.

According to the U.S. Geological Survey, the quake struck 187 kilometers (116 miles) southeast of the northern port city of Chongjin. The epicenter was 559 kilometers (347 miles) below the seabed.

Cho Ik-hyun at South Korea's state weather agency says the depth shows it was a natural event, too deep for a possible nuclear blast.


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