"Our map data shows the locations of mines across North Korea, and we also detected soundwaves, proving that this quake was caused by an explosion,'' he said. He didn't want to be named, citing office rules.
 
   The area is distant from the northeastern region where the North tested six nuclear devices until 2017. The North last year unilaterally demolished tunnels at its nuclear testing ground as it initiated diplomacy with Washington and Seoul.
 
   North Korea's weakest nuclear test, its first one, conducted in 2006, generated a magnitude 4.3 quake. The U.S. Geological Survey measured the North's sixth and most powerful nuclear test in September 2017 at magnitude 6.3. The North then claimed it detonated a thermonuclear weapon built for its long-range missiles targeting the U.S. mainland.