Authorities in China on Saturday said a “suspected explosion” was thought to be behind a 3.4-magnitude earthquake in North Korea, raising concerns of yet another nuclear test by Pyongyang. The quake was detected in North Hamgyong Province, near North Korea’s Punggyeri nuclear site. South Korean authorities, however, said they believed the tremors had been caused by a natural earthquake. “We use several methods to tell whether earthquakes are natural or man-made,” an official was cited as saying by Reuters. “A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake. In this case we saw none. So as of now we are categorizing this as a natural earthquake.” The U.S. Geological Survey said it detected a 3.5-magnitude quake near the site of previous nuclear tests but could not confirm whether the event was natural or man-made.