On September 23, 2017, China's earthquake administration at 0830 GMTdetected possible earthquake tremors of 3.4 magnitude sparking suspicion that North Korea has conducted another "explosion". An official at South Korea’s meteorological agency said they were analyzing the tremor, which they put at magnitude 3.0. A South Korean official said “We use several methods to tell whether earthquakes are natural or manmade.” The earthquake was detected in Kilju county in North Hamgyong Province, where North Korea’s known Punggyeri nuclear site is located. 

(Comment: 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.)