The "artificial quake" in North Korea Sunday, thought to be its sixth nuclear test, was five to six times more powerful than the tremor from Pyongyang's fifth test, the South's weather agency said.
US monitors measured a 6.3-magnitude "explosion" near the North's main testing site site at Punggye-ri.
"The scale of the energy was five to six times more powerful than the fifth nuclear test," Lee Mi-Sun, head of the Korea Meteorological Administration's earthquake and volcano centre, told a televised briefing.
The South's Yonhap news agency, citing the state weather agency, had previously reported that the quake was 9.8 times more powerful than the tremor from Pyongyang's fifth test.
The North carried out its first nuclear test in 2006, and successive blasts are believed to have been aimed at refining designs and reliability as well as increasing yield.
Its fifth detonation, in September last year, caused a 5.3 magnitude quake and according to Seoul had a 10-kiloton yield -- still less than the 15-kiloton US device which destroyed Hiroshima in 1945.
Sunday's explosion came just hours after the North claimed to have developed a hydrogen bomb that could be loaded onto the country's new intercontinental ballistic missile.