North Korea was likely to have conducted its fifth and largest nuclear test on Friday, South Korean officials and analysts said after meteorological agencies reported a surface-level seismic tremor near the North’s known test site.
U.S., European and Chinese agencies reported that the tremor was recorded at 0030 GMT. The U.S. Geological Survey and European agencies measured the tremor at a magnitude of 5.3.
A South Korea government official and the meteorological agency said it was a suspected nuclear test, and Seoul called an emergency meeting of its National Security Council.
“Today at around 9:30 a.m., we detected an artificial earthquake wave at magnitude 5.0 in the area of North Korea’s Punggye-ri and we are analysing whether it is a nuclear test,” South Korea’s Joint Chiefs of Staff said in a statement, referring to the North’s nuclear test site.
The China Earthquake Networks Center also said the tremor was a suspected explosion. China’s environment ministry began emergency radiation monitoring along its borders with North Korea in northeast China, state television reported.
Japanese Prime Minister Shinzo Abe said such a nuclear test could not be tolerated. Japan’s foreign minister lodged a protest and Tokyo also sent two military jets to begin measuring for radiation.
Jeffrey Lewis of the California-based Middlebury Institute of International Studies said the highest estimates of seismic magnitude suggested this was the most powerful nuclear test conducted by North Korea so far.
He said the seismic magnitude and surface level indicated a blast with a 20- to 30-kilotonne yield. Such a yield would make this test, if confirmed, larger than the nuclear bomb dropped on the Japanese city of Hiroshima in World War Two and potentially bigger than that dropped on Nagasaki soon after.
“That’s the largest DPRK test to date, 20-30kt, at least. Not a happy day,” Lewis told Reuters, using the North’s official title of the Democratic People’s Republic of Korea.
“The important thing is that five tests in, they now have a lot of nuclear test experience. They aren’t a backwards state any more,” he said.
Initial analysis of North Korea’s January test estimated a yield of just 6 kilotonnes.
South Korea’s Yonhap news agency reported that the South’s military believed the seismic event indicated a blast with a 10-kilotonne yield, still the largest ever conducted by the secretive North.
Kune Y. Suh, a nuclear engineering professor at Seoul National University, said the latest test, if confirmed, appeared to be two to three times larger than earlier tests, with a potential yield of more than 10 kilotonnes.
“So North Korea can now show off their nuclear capability proudly,” Suh said.
Lassina Zerbo, the head of world monitoring agency the Comprehensive Nuclear-Test-Ban Treaty Organisation, said the seismic activity registered on Friday was larger than that triggered by the North’s test in January.
That test was recorded with a magnitude of 5.1, according to the U.S. Geological Survey.
The White House’s National Security Council said it was aware of seismic activity on the Korean Peninsula. “We are monitoring and continuing to assess the situation in close coordination with our regional partners,” an NSC statement said.
U.S.-based 38 North, a North Korea monitoring project, said in a note late on Thursday that satellite images showed fresh activity at North Korea’s Punggye-ri test site.
Isolated North Korea’s fourth nuclear test in January resulted in tough new U.N. sanctions. It has conducted a series of missile tests since then, including of a submarine-launched missile, in defiance of those resolutions.
North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006.