South Korea's weather agency said a magnitude 3.2 earthquake was detected in North Korea on Saturday close to where the country recently conducted a nuclear test, but it assessed the quake as natural.
The quake, which South Korea's Meteorological Agency put at magnitude 3.0, was detected 49 km from Kilju in North Hamgyong Province, where North Korea's known Punggye-ri nuclear site is located, the official said.
The area isn't where natural earthquakes normally occur.
"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 man-made natural disaster", said the South Korean official, who asked for anonymity.
A US government intelligence analyst said the events could have been a "mine-type" collapse of tunnels damaged by North Korea's previous nuclear test, but was more likely a small quake.
An natural disaster with a magnitude of 3.4, was recorded on Saturday in North Korea, according to the Service seismic chinese CENC.
A small quake near North Korea's nuclear test site on Saturday was probably not man-made, the nuclear proliferation watchdog and a South Korean official said, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one. She spoke on condition of anonymity, citing office rules.
An official of South Korea's Meteorological Agency said acoustic waves should be detected in the event of a man-made quake.
Nuclear proliferation watchdog CTBTO said on Saturday it had detected two seismic events in North Korea on Saturday but they were probably not deliberate explosions in the isolated country."Two #Seismic Events!"
So-called atmospheric tests of nuclear weapons were banned by the USA, the then-Soviet Union and numerous world's nations in a 1963 treaty, though China conducted the last such test known worldwide in 1980. The latest test was followed by a second magnitude 4.1 quake that experts said could have been caused by landslides or a tunnel collapsing after the explosion.
That is about 20km (12m) south-east of where the North conducted its sixth and most powerful nuclear test on September 3, an official from the Korea Meteorological Administration in Seoul said.
North Korea said its recent nuclear test was a detonation of a thermonuclear weapon built for its developmental intercontinental ballistic missiles.
Atmospheric tests of nuclear-armed missiles, as NPR's Geoff Brumfiel reported, are risky for a host for reasons, including the potential for sending nuclear fallout in unpredictable patterns and, if a test were to be administered by North Korea, in the direction of the U.S.