Photo above from http://www.scmp.com/news/asia/article/1137842/activity-seen-north-korea-atomic-test-site with caption "This January 4  satellite image shows North Korea's Punggye-ri test facility where roads have been kept clear of snow over the past month. Photo: AP. [article further states] ...The North’s two previous detonations in 2006 and 2009 followed a similar pattern of a missile launch, followed by UN measures, followed by a nuclear test".
Map indicates Phunggye-ri [re] North Korea's 2006 nuclear test site and site of the test (probably) in the next few hours or days. Nearby Kilju was the 2009 test site.
Separately Agency France Press via The Age, February 12, 2013, reports http://www.theage.com.au/world/n-korea-withdraws-from-nuclear-test-site-20130212-2e9gz.html
"N Korea withdraws from nuclear test site
Seoul: North Korea has pulled manpower and equipment out of its nuclear test site, says a report, which cites the move as a possible sign of an imminent blast.
South Korea's Yonhap news agency says Seoul has been keeping close watch on the underground Punggye-ri nuclear test site, as no movement of manpower and equipment has been observed there since Friday.
The agency quoted an unnamed source as saying the apparent withdrawal could point to a nuclear test being carried out soon.
...Seoul has predicted Pyongyang may carry out its third nuclear test between the end of the holiday and the late leader Kim Jong-il's birthday on February 16, Yonhap said..." WHOLE ARTICLE