SEOUL — North Korea's powerful nuclear test this month may have been even stronger than first reported, equivalent to roughly 17 times the strength of the bomb that destroyed Hiroshima, according to a new analysis by a U.S. monitoring think tank.
North Korea's Sept. 3 nuclear test, its sixth and biggest, showed how much progress the country has made on its nuclear program.
Estimates of the bomb's yield, or the amount of energy released by the blast, have ranged from South Korea's 50 kilotons to Japan's 160 kilotons, although some analysts have said the 6.3 magnitude of the earthquake caused by the detonation could put it into the "hundreds of kilotons." This would put it into the realm of thermonuclear weapons, supporting North Korea's claim that it had tested a hydrogen bomb.
In comparison, the bomb detonated over Hiroshima in 1945 released about 15 kilotons of energy.
The new analysis by 38 North, run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies, found North Korea's test may have been much stronger.
Updated seismic data showed the magnitude of the resulting earthquake was greater than initial estimates — between 6.1 and 6.3. That means the yield of the latest test was roughly 250 kilotons, reported 38 North's Frank V. Pabian, Joseph S. Bermudez Jr. and Jack Liu.
In other words, the North Korean test may have been almost 17 times stronger than the bomb detonated over Hiroshima. This is close to what 38 North previously calculated as the maximum yield that could be contained at the underground Punggye-ri test site.
This new estimate by 38 North is much higher than initial estimates from U.S. intelligence sources and allies. The United States intelligence assessment put the blast at 140 kilotons, Japan at 160 kilotons and South Korea at 50 kilotons.
Experts at the Center for Nonproliferation Studies in Monterey, Calif., said that the size of the earthquake triggered by the explosion also suggested that the bomb could have had a force in the hundreds of kilotons.
The Pentagon declined to comment, calling it an intelligence matter.
A U.S. intelligence official said the 38 North analysis is consistent with the range of estimates by the intelligence community. The Air Force Technical Applications Center's early estimate was a range of 70 to 280 kilotons, based on the possible magnitude, the official said.
Satellite imagery showed the test resulted in many more landslides than after any of the previous five tests, according to the 38 North analysis.
North Korea described the device it had detonated as a hydrogen bomb designed to be carried by a long-range missile capable of reaching the U.S. mainland. The international community widely condemned the test and within 10 days, the U.N. Security Council unanimously approved its toughest sanctions on the country to date.
In the wake of the North Korean test, both the United States and South Korea are highlighting their own military readiness.
Defense Secretary Jim Mattis was traveling Wednesday to Minot Air Force Base in North Dakota, the center of the American nuclear arsenal, with more than 100 land-based nuclear missiles and aircraft.
Meanwhile, the South Korean Air Force on Wednesday conducted its first live-fire drill to test its preemptive strike capability, according to the South Korean Defense Ministry.
Anna Fifield in Tokyo and Ellen Nakashima in Washington contributed to this report.