An official of South Korea's Meteorological Agency said acoustic waves should be detected in the event of a man-made quake.
The United States Geological Survey (USGS) said the quake struck around 20 kilometres (12 miles) away from the North's nuclear test site, where earlier this month Pyongyang detonated its sixth and largest device, which it claimed was a hydrogen bomb capable of being launched on a missile. On Thursday, the US unveiled its own set of economic sanctions targeting North Korea.
"This event occurred in the area of the previous North Korean Nuclear tests".
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" in North Korea, and that it would have more details to come.
Ri did not respond when asked by reporters whether North Korea had conducted a new nuclear test. Similar to "collapse" event 8.5 mins after DPRK6! "So as of now we are categorising this as a natural quake". 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. "I see no reason to doubt that it was an quake".
Earlier on Saturday, China said it will limit exports of refined petroleum products from October 1 and ban exports of condensates and liquefied natural gas immediately to comply with the latest United Nations sanctions.
Until now, the North has not tested its missiles and nuclear weapons together, opting instead to fire them separately - the missiles above ground and the nuclear weapons below.
There was no immediate reaction from China's Foreign Ministry.
The quake also caps a week in which President Trump and North Korea leader Kim Jong Un exchanged increasingly bellicose insults and follows a threat on Thursday by the North to carry out a significantly more unsafe nuclear test.
The president assured those in attendance, without details, that he would defend the US against any offensives launched by North Korea: "I can tell you one thing, you are protected, OK?"
A day earlier, Kim reacted to Trump's provocative speech at the U.N.by equating the USA president to "a frightened dog" and a "mentally deranged US dotard", using an obscure insult for someone declining into senility.
North Korea has been maintaining a torrid pace in nuclear and weapons tests as it accelerates its pursuit of nuclear weapons that could viably target the United States and its allies in Asia.