At a meeting with President Vladimir Putin, Russian Defense Minister Sergei Ivanov stated that "the power of the tests carried out was 5 to 15 kilotons", though this early estimate is much higher than any other international estimate. An early report by the Institute of Geoscience and Mineral Resources of South Korea said the blast was equivalent to an earthquake registering 3.58 on the Richter scale, which corresponds to the explosion of 100 tons of TNT. This was later revised to at least 800 tons, corresponding to a blast wave of 4.2. The U.S. Geological Survey also estimates the blast wave at 4.2. (Note that 4.2 is considerably more powerful than 3.58 because the Richter scale is a logarithmic scale.)