Nuclear watchdog looking at 'unusual seismic activity' in North Korea

BEIJING/SEOUL, Sept 23 (Reuters) - A small earthquake near North Korea's nuclear test site on Saturday was probably not manmade, the nuclear proliferation watchdog and a South Korean official said, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.

Chinese earthquake officials said the magnitude 3.4 quake detected at 0829 GMT was a "suspected explosion" but both the CTBTO, which monitors nuclear tests, and a South Korean meteorological agency official said they believed it was a natural quake.

"A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake," said the South Korean official, who asked for anonymity. "In this case we saw none. So as of now, we are categorizing this as a natural earthquake."

RELATED: Kim Jong Un observes North Korea's latest missile launch

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Kim Jong Un observes North Korea's mid-September missile launch
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Kim Jong Un observes North Korea's mid-September missile launch
North Korean leader Kim Jong Un watches the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT. TPX IMAGES OF THE DAY
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un (not pictured) guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT. TPX IMAGES OF THE DAY
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
Officials react after North Korean leader Kim Jong Un (not pictured) guided the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
North Korean leader Kim Jong Un (not pictured) guides the launch of a Hwasong-12 missile in this undated combination photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THIS IMAGE. NO THIRD PARTY SALES. SOUTH KOREA OUT.
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The earthquake, which South Korea put at magnitude 3.0, was detected in Kilju county in North Hamgyong Province, where North Korea's known Punggyeri nuclear site is located, the official said.

All of North Korea's previous six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test on Sept 3 registered as a 6.3 magnitude quake.

A secondary tremor detected after that test could have been caused by the collapse of a tunnel at the mountainous site, experts said at the time. Satellite photos of the area after the Sept 3 quake showed numerous landslides apparently caused by the massive blast, which North Korea said was an advanced hydrogen bomb.

The head of the nuclear test monitoring agency CTBTO said on Saturday that analysts were "looking at unusual seismic activity of a much smaller magnitude" than the Sept 3 test in North Korea.

"Two #Seismic Events! 0829UTC & much smaller @ 0443UTC unlikely Man-made! Similar to "collapse" event 8.5 mins after DPRK6! Analysis ongoing," CTBTO Executive Secretary Lassina Zerbo said in a Twitter post, referring to Sept 3 test.

Russia’s emergency ministry says background radiation in nearby Vladivostok was within the natural range.

TENSIONS HIGH

The U.S. Geological Survey said it could not conclusively confirm whether the quake, which it measured at magnitude 3.5, was manmade or natural.

"The depth is poorly constrained and has been held to 5 km by the seismologist," USGS said. "The Air Force Technical Applications Center (AFTAC) is the sole organization in the U.S. federal government whose mission is to detect and report technical data from foreign nuclear explosions."

There was no immediate reaction from China's Foreign Ministry, but the news was widely reported by Chinese state media outlets and on social media.

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Tensions have continued to rise around the Korean peninsula since Pyongyang carried out its sixth nuclear test, prompting a new round of U.N. sanctions.

North Korea's Foreign Minister Ri Yong Ho, currently in New York for a United Nations meeting, warned on Thursday that Kim could consider a hydrogen bomb test of an unprecedented scale over the Pacific.

Ri is due to address the United Nations later on Saturday.

U.S. President Donald Trump called the North Korean leader a "madman" on Friday, a day after Kim dubbed him a "mentally deranged U.S. dotard" who would face the "highest level of hard-line countermeasure in history."

North Korea's nuclear tests to date have all been underground, and experts say an atmospheric test, which would be the first since one by China in 1980, would be proof of the success of its weapons program.

North Korea has launched dozens of missiles this year, several of them flying over Japan, as it accelerates a weapons program aimed at enabling it to target the United States with a nuclear-tipped missile.

While China has been angered by North Korea's repeated nuclear and missile tests and has signed up for the increasingly tough U.N. sanctions, it has also stressed the need to resume dialog and for all sides to take steps to reduce tensions.

In a series of meetings this week at the United Nations, Chinese Foreign Minister Wang Yi has reiterated to various foreign counterparts that apart from sanctions, the resolutions also call for dialog to resume and that this needs to happen.

Earlier on Saturday, China said it will limit exports of refined petroleum products from Oct. 1 and ban exports of condensates and liquefied natural gas immediately to comply with the latest U.N. sanctions. It will also ban imports of textiles from North Korea.

RELATED: Images show North Korea's nuclear test site

26 PHOTOS
Satellite images of the Punggye-ri nuclear test site in North Korea
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Satellite images of the Punggye-ri nuclear test site in North Korea
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 1. Activity continues at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 2. Possible new dumping observed at the North Portal spoil pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 3. Probable personnel in formation or equipment in rows at the Main Administrative Area. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 1. No vehicles or trailers remain around the North Portal but well-worn paths are observed. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 2. No new dumping of material on the North Portal spoil pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 3. Small collection of crates or trailers seen in previous imagery has been removed. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 3B. Formations seen in the Main Administrative Area, similar to what was seen in lead up to 2013 nuclear test. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 2. Material dumped at the North Portal tailings pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 4, 2013. Figure 3A. Formations seen in the Main Administrative Area in lead up to 2013 nuclear test. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 1. Continued activity at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 25, 2017. Figure 1. Probable cabling and water drainage seen at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 19th, 2016: Figure 6: Excavation continued underground in the North Portal area suggesting more tests to come in the same tunnel complex directly under Mt. Mantap. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 5th, 2017: Figure 7: The North Portal spoil pile continued to expand into 2017, becoming increasingly broader. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 22nd, 2017: Figure 8: Late January 2017 imagery showing new spoil on top of recent snow. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 19th, 2016: Figure 9. A close-up of the North Portal spoil pile as it appeared in late October 2016. The unstable spoil can sometimes lead to accidents, as in this case of toppled rail cars downslope. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - FEBRUARY 12th, 2017: Figure 10. A close-up of the North Portal spoil pile from February 2017 shows that accumulations had begun move westward with a broadening of the top and bottom west side of the pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 1. DigitalGlobe imagery showing large shipping container or crate seen at the North Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 2. DigitalGlobe imagery showing no changes to pattern and texture of tailings (spoil) pile at the North Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 3. DigitalGlobe imagery showing a small vehicle present at the West Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 4. DigitalGlobe imagery showing a truck present in the southern courtyard of the Main Administrative Area. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 5. DigitalGlobe imagery showing a truck present at the sites Command Center. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 6. DigitalGlobe imagery showing snow cleared at guard barrack and security checkpoint. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 24, 2016: Figure 2. No activity seen at the Sohae launch pad. Date: October 24, 2016. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 24, 2016: Figure 3. Environmental shed remains adjacent to the vertical engine test stand. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 29, 2016: Figure 1C. Increased activity around the North Portal throughout October. Date: October 29, 2016. (Photo DigitalGlobe/38 North via Getty Images)
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The United States and South Korea are technically still at war with North Korea because the 1950-53 Korean conflict ended with a truce and not a peace treaty.

The North accuses the United States, which has 28,500 troops in South Korea, of planning to invade and regularly threatens to destroy it and its Asian allies.

(Reporting by Andrew Galbraith in Shanghai, Ben Blanchard in Beijing, Christine Kim and Heekyong Yang in Seoul, Michael Shields in Zurich, Denis Pinchuk in Moscow; Editing by Lincoln Feast)

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