Controversy is brewing over the size of North Korea’s nuclear capability as varying readings put the yield of its nuclear explosion last Tuesday at between 6 kilotons to 40 kilotons.

South Korea’s Korea Meteorological Administration and Institute of Geo-science and Mineral Resources detected artificial seismic activity with a magnitude of 4.9 after the detonation. It said the explosive power was between 6 and 7 kilotons based on a model of the U.N.-affiliated Comprehensive Nuclear Test Ban Treaty Organization.

A kiloton is equivalent to 1,000 tons of TNT.

The figures mean that the blast was more powerful than Pyongyang’s two previous atomic tests in 2006 and 2009, which generated 1 kiloton and 2-6 kilotons, respectively.

But foreign experts offered starkly different assessments.

On Wednesday, Germany’s state-run geological research institute BER estimated the magnitude at 5.2 and the explosive yield at nearly 40 kilotons.

Russia’s defense ministry said the output should be at least 7 kilotons, while the U.S. Geological Survey reported a 5.1-magnitude tremor, which indicates a 10-kiloton explosion.

North Korea called its third test “high standard,” claiming that it used a miniaturized, lightened weapon.

“Unlike previous times, a smaller and lighter nuclear bomb with more explosive power was used to conduct a high-standard nuclear test safely,” the official Korean Central News Agency said after the detonation.

Some scholars and foreign media questioned “political motivations” behind Seoul’s constantly lower gauges than those elsewhere in the all three tests in 2006, 2009 and 2013.

The CTBTO said the first two tests produced 4.1 and 4.52 magnitude seismic events, whereas the USGS announced 4.3 and 4.7, respectively. BER estimated the size of the first two blasts at 2 and 13 kilotons, respectively.

In particular, the South Korean government downgraded its magnitude estimates from the initial 5.1 within the space of an hour, dismissing speculation that North Korea may have used a boosted fission weapon.

Seoul officials claim to have a more accurate number given the geographical advantage and a denser observation network.

“There are disparities in methods for assessing the scale of the nuclear test,” Defense Ministry spokesperson Kim Min-seok said last week. “The government uses KIGAM’s system.”

Others pointed to different calculation methods that can culminate in a wide range of numbers.

Based on an International Monitoring System measure, the same 5.2 magnitude can be translated into outputs as great as 15.9 kilotons, equivalent to the U.S. bomb dropped in Hiroshima in 1945, experts noted.

The KMI stressed the methodological difference such as between body-wave magnitude derived from the amplitude of P-waves and local magnitude commonly known as the Richter scale.

“The IMS formula is sort of a world average,” Chi Heon-cheol, head of Daejeon-based KIGAM’s earthquake research center, was quoted as saying by Yonhap News.

“The actual number should be rated lower because of North Korea’s solid bedrock landscape and its small damping effects.”

By Shin Hyon-hee (heeshin@heraldcorp.com)