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North Korea's nuclear test was low-powered and is believed to have been conducted in a horizontal tunnel dug deep inside a mountain on its northeast coast.
The first scientists knew of it was when they detected seismic waves caused by an artificial explosion, but there was no immediate report of radioactivity.
The activity measured 3.6 on the Richter scale, which could be caused by the explosion of the equivalent of 800 tonnes of dynamite, said Chi Heon-Cheol, head of the Korea Earthquake Research Centre.
Another unidentified expert quoted by Yonhap news agency said the blast was equivalent to about 550 tonnes of TNT judging by the seismic tremor.
The US atomic bomb which destroyed Hiroshima during World War II was comparable to 12,500 tons of TNT.
Intelligence officials told South Korea's parliament the test appeared to have been carried out in a 360-meter-high (1,200 feet) mountain northwest of the Musudan missile base in the Hwadaeri region, according to lawmaker Chung Hyong-Keun.
He quoted an intelligence official as saying: “In consideration of the height of the mountain, the test appeared to have been done in a horizontal tunnel.”
The North's official media said it had successfully conducted an underground nuclear test under secure conditions, with no radiation leak.
“It has been confirmed that there was no such danger from radioactive emission in the course of the nuclear test, as it was carried out under scientific consideration and careful calculation,” the Korean Central News Agency said.
“The nuclear test was conducted with indigenous wisdom and technology, 100 percent,” it added.
Seoul officials said the test was detected through seismic waves coming from the Hwadaeri region near the town of Kilju in North Hamgyong Province at 10:36 am (0136 GMT).
Chi Heon-Cheol, head of the Korea Earthquake Research Centre, said the seismic activity took place 15.4 kilometers (10 miles) northwest of Hwadaeri.
“The peculiarity of the seismic waves indicated there was an artificial explosion, not a natural earthquake,” Chi told journalists.
No excessive radioactivity was immediately detected in South Korea, experts said.
“No radioactivity has not yet been detected from the alleged nuclear test,” said Han Seung-Jae, director of the state-run Nuclear Emergency Preparedness Department (NEPD), before confirmation of the blast.
“It might not be detected at all if the alleged nuclear testing was conducted in a tightly sealed atmosphere such as a deep tunnel, and all radioactive rays and fallout are contained,” he said.
The NEPD operates 37 observation posts to detect radioactivity, including one on the southeastern island of Ullung. Prevailing winds might have carried any radioactivity in that direction.
In an October 3 statement announcing the planned test, the North pledged never to use nuclear weapons first and strictly to ban the transfer of nuclear weapons and technology.
“The … nuclear weapons will serve as reliable war deterrent for protecting the supreme interests of the state and the security of the Korean nation from the US threat of aggression and averting a new war…” it said at the time.
North Korea is believed to have produced enough weapons-grade plutonium to make several crude nuclear bombs, according to US and South Korean experts.
It also has an advanced missile programme, although it is not known whether it has the technological skill to arm one with a nuclear warhead.
On July 5 North Korea test-fired seven missiles into the sea — including a Taepodong-2 believed to be technically capable of hitting the United States.
