A magnitude 5.9 earthquake off North Korea early Thursday 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. 4. North Korea’s five previous nuclear tests caused signs of artificial quakes.

Here’s what experts say about the quake:

First off, the quake was centered far offshore and very deep while North Korea’s past nuclear tests were conducted on land.

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 said the depth shows it was a natural event, too deep for a possible nuclear blast.

Natural earthquakes create different seismic patterns from ones caused by humans. South Korea’s Defense Ministry said there was no indication that North Korea had carried out a nuclear test.

Cho said any earthquake deeper than 70 kilometers (43 miles) normally causes little damage on the surface. Even if a ship was sailing over the epicenter at the time of the quake, it wouldn’t have noticed anything, Cho said.

Earthquakes are rare on the Korean Peninsula, unlike in neighboring Japan. Two quakes measuring 5.1 and 5.8 jolted southeastern South Korea on Sept. 12, causing no casualties.