Seoul/Beijing, Sept 12, SPA -- South Korea said Sunday it was investigating reports of a huge explosion in North Korea's northern province of Yanggang near the Chinese border, but ruled out a nuclear blast. "The incident seems to be not related to a nuclear test," a government spokesman quoted Unification Minister Chung Dong Young as saying. A report by Yonhap news agency, which had cited unnamed sources in Seoul and Beijing, had sparked speculation about detonations of possible nuclear test weapons. The blast was reported to have occurred on Thursday as North Korea marked the 56th anniversary of its founding. Yonhap cited diplomats in Seoul who said the explosion produced a mushroom-like cloud with a diameter of 3.5 to four kilometers. It was detected by satellites. Rocket silos are stored in the mountainous region. The strength of the reported detonation was said to be greater than the huge train explosion on April 22 in North Korea's Ryongchon railway station which left 170 dead and 1,300 injured. Official North Korean media reported the incident two days later. While there were no reports in China of a massive explosion on Thursday in the region bordering North Korea, there were reports of a quake in the same area on Wednesday evening. Authorities in Jelin province told Deutsche Presse-Agentur, dpa by telephone that an earthquake measuring 3.7 on the Richter scale was detected at 22.24 local time (15.24 GMT). Quakes in the Tianchi mountain region are not unusual, they said. Local authorities and a hotel in Tianchi confirmed the quakes on Wednesday evening. "That happens occasionally," said a member of hotel staff near the mountain lake owned partially by North Korea. Authorities, trading companies with contacts to North Korea and residents of Helong, Tianchi, the Changbai district and in Jian along North Korea's Yanggang province were unable to confirm the reports of a massive explosion and a huge mushroom-like cloud. No one had heard of it, they said. --SPA 1326 Local Time 1026 GMT www.spa.gov.sa/200674