An earthquake of magnitude 2.1 occurred Wednesday in Punggye-ri in North Korea's North Hamgyong Province, where the regime had conducted six nuclear tests, the Korea Meteorological Administration here said Thursday.It was the sixth earthquake near the nuclear test site since early this year and the 44th that hit the area since the North's last nuclear test in September 2017. "Ground subsidence in Punggye-ri is clearly serious," a geologist said. "A fresh nuclear test could cause a nuclear disaster." Until the sixth nuclear test, no natural earthquakes occurred in the area. The test itself caused an artificial quake of magnitude 5.7 due to an estimated 150 kt yield from what the regime claimed was a "hydrogen bomb." The yield of the atomic bomb that dropped on Hiroshima in 1945 was 15 kt. The North built the underground test site in an area of hard granite rock which it believed could withstand the powerful shocks and vibrations of nuclear blasts and prevent leakage of radioactive materials.
Only artificial quakes were reported from the first nuclear test in 2006 to the fifth test in 2016. But since the sixth nuclear test the natural quakes have been frequent and getting stronger. Just as a big natural earthquake is usually followed by a series of smaller tremors, the artificial quake seems to have triggered a series of aftershocks.Six quakes of magnitude 2 to 2.9 have occurred in the first half of this year alone within a 50 km radius of the test site, progressively destabilizing the ground. Things could get even worse if the North conducts another nuclear test there, and experts fear the ground could collapse entirely and leakage of radioactive materials could contaminate all of Northeast Asia.A KMA official said, "A geological survey of the Punggye-ri area is necessary, but it seems that the ground is already dangerously weak." Radioactive materials could spread on westerly winds and contaminate soil and water. A North Korean defector from the region said, "Kilju is a bowl-shaped area where water gathers after flowing from Mt. Mantap in Punggye-ri, where the nuclear test site sits." Underground water veins in the area are probably already contaminated.Five of 10 defectors who arrived here from Kilju after the sixth nuclear test showed a radiation dose exceeding 250 mSv, the chromosomal anomaly detection threshold, in exposure testing. One 48-year-old woman even had a dose of 1,386 mSv, the level for a high risk of cancer.The annual permissible radiation for engineers at nuclear power plants is about 50 mSv.