North Korea atom test resembled blast, quake: treaty body

VIENNA (Reuters) - North Korea's nuclear test this week was bigger than the one in 2006, but not on the scale first reported, the atomic test ban monitoring agency said Friday.

The Vienna-based Comprehensive Test Ban Treaty Organization (CTBTO) said absolute proof that Monday's blast was Pyongyang's second nuclear test awaited findings of any radioactive particles and noble gases, expected next week at the earliest.

But it said data so far showed it had resembled both an explosion and earthquake.

Russia said shortly after Monday's test by the mercurial Stalinist state that the blast was 20 times larger than North Korea's one-kiloton test three years ago, or about equal to the U.S. atom bomb dropped on Nagasaki in Japan in World War Two.

However, the CTBTO and the United States assessed the strength of North Korea's underground experiment to be smaller.

"It was in the low one-digit kiloton (range) in magnitude," CTBTO Executive Secretary Tibor Toth told a news conference on Friday. A senior U.S. official said Monday the explosive yield was roughly a few kilotons TNT.

"An explosion-like signal is very clearly seen ... in the initial data ... There is no doubt," he said.

"There was an epicenter of earthquake-like and explosion-like signals from the same source," said Lassina Zerbo, head of the CTBTO center that collects data from seismic monitoring stations worldwide.

The 2006 test touched off only blast waves.

Toth said the earliest expected readings of noble gases and radio-nuclei would come from a station in Japan.

He said the North Korean test bore a magnitude measuring 4.52 on the Richter scale, while in 2006 it was 4.1.

The CTBTO, the independent world body observing possible breaches of the test ban, has obtained readings from 61 of its 130 seismic stations on Monday's test, up from 22 in 2006, underlining the network's wider reach and improved technology.

The 1996 CTBT prohibits nuclear test detonations globally, but cannot take force before being ratified by all 44 countries that took part in the negotiations and also have nuclear power or research reactors.

Of the five nuclear weapons states, the United States and China have yet to ratify the treaty.

North Korea quit the nuclear Non-Proliferation Treaty in 2003 but embarked on a denuclearization-for-aid process with world powers in 2007, only to bolt from it last month, calling it worthless, and begin reactivating its atomic bomb program.

In New York, the United States and Japan have circulated a draft U.N. Security Council resolution, condemning Pyongyang's second nuclear test and demanding strict enforcement of sanctions adopted after its first test in October 2006.