BEIJING/SEOUL, (Reuters) – A small earthquake near North Korea’s nuclear test site on Saturday was probably not manmade, the nuclear proliferation watchdog and a South Korean official said, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.
Chinese earthquake officials said the magnitude 3.4 quake detected at 0829 GMT was a “suspected explosion” but both the CTBTO, which monitors nuclear tests, and a South Korean meteorological agency official said they believed it was a natural quake.
“A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake,” said the South Korean official, who asked for anonymity. “In this case we saw none. So as of now, we are categorising this as a natural earthquake.”
The earthquake, which South Korea put at magnitude 3.0, was detected in Kilju county in North Hamgyong Province, where North Korea’s known Punggyeri nuclear site is located, the official said.
All of North Korea’s previous six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test on Sept 3 registered as a 6.3 magnitude quake.
A secondary tremor detected after that test could have been caused by the collapse of a tunnel at the mountainous site, experts said at the time. Satellite photos of the area after the Sept 3 quake showed numerous landslides apparently caused by the massive blast, which North Korea said was an advanced hydrogen bomb.
The head of the nuclear test monitoring agency CTBTO said on Saturday that analysts were “looking at unusual seismic activity of a much smaller magnitude” than the Sept 3 test in North Korea.
“Two #Seismic Events! 0829UTC & much smaller @ 0443UTC unlikely Man-made! Similar to “collapse” event 8.5 mins after DPRK6! Analysis ongoing,” CTBTO Executive Secretary Lassina Zerbo said in a Twitter post, referring to Sept 3 test.
Russia’s emergency ministry says background radiation in nearby Vladivostok was within the natural range.
The U.S. Geological Survey said it could not conclusively confirm whether the quake, which it measured at magnitude 3.5, was manmade or natural.
“The depth is poorly constrained and has been held to 5 km by the seismologist,” USGS said. “The Air Force Technical Applications Center (AFTAC) is the sole organization in the U.S. federal government whose mission is to detect and report technical data from foreign nuclear explosions.”
There was no immediate reaction from China’s Foreign Ministry, but the news was widely reported by Chinese state media outlets and on social media.