* China detects quake near N.Korea nuclear test site
* N.Korea nuclear tests register as earthquakes
* S.Korea says signs point to natural tremor, not man-made
* Tensions high after N.Korea's sixth nuclear test on Sept 3
A small earthquake near North Korea's nuclear test site on Saturday was probably not man-made, the nuclear proliferation watchdog and a South Korean official said, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.
China's Earthquake Administration said the quake was not a nuclear explosion and had the characteristics of a natural tremor. The administration had said earlier the magnitude 3.4 quake detected at 0829 GMT was a "suspected explosion".
The CTBTO, or Comprehensive Test-Ban Treaty Organization, which monitors nuclear tests, and officials of the South Korean meteorological agency said they believed it was a natural quake.
The Pentagon and the US State Department did not immediately respond to requests for comment
A US intelligence official and US-based non-governmental experts said their initial assessment was that the quake was either natural or connected to North Korea's latest and largest nuclear test on Sept.3, and not caused by a new nuclear test.
"It seems likely that these small tremors are related to the shifts in the ground due to the recent large test," said David Wright of the Union of Concerned Scientists in the United States.
The seismic activity came just hours before North Korea's Foreign Minister Ri Yong Ho, who warned on Thursday that North Korea could consider a hydrogen bomb test of an unprecedented scale over the Pacific, was due to address the United Nations General Assembly in New York.
Ri did not respond when asked by reporters whether North Korea had conducted a new nuclear test.
A US government intelligence analyst said the events could have been a "mine-type" collapse of tunnels damaged by North Korea's previous nuclear test, but was more likely a small earthquake.
An official of South Korea's Meteorological Agency said acoustic waves should be detected in the event of a man-made earthquake.
"In this case we saw none. So as of now, we are categorising this as a natural earthquake."
The earthquake, which South Korea's Meteorological Agency put at magnitude 3.0, was detected 49 km from Kilju in North Hamgyong Province, where North Korea's known Punggye-ri nuclear site is located, the official said.
All of North Korea's six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test 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 international 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 the Sept. 3 test.
Russia’s emergency ministry says background radiation in nearby Vladivostok was within the natural range.
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