A 38 North exclusive with analysis by Nick Hansen and Jack Liu.

New commercial satellite imagery from April 25, 2014 confirms a further increase of activity at North Korea’s Punggye-ri Nuclear Test Site likely related to preparations for an underground nuclear test.[1] However, it is unclear from the imagery whether any test tunnels have been sealed, a sign that a test may be imminent, as claimed by South Korean sources in press briefings on April 25.

Imagery from April 25 indicates that the pace of movement of vehicles and equipment near tunnel entrances in the South Portal area has increased consistent with what would be expected during pre-test preparations.[2] The number of crates and boxes near the entrances that may contain equipment related to a test has increased. Moreover, it appears the equipment is being moved into the tunnels since the numbers of crates and boxes as well as their positions have changed in the six-day period from April 19 to April 25. Finally, a panel truck in the area appears to be in the process of unloading boxes and crates since they are visible around the vehicle.[3]

Of particular interest has been the rapid increase of materials near the entrance of the western most tunnel in the South Portal area. No crates or boxes were present on April 19 but by April 25, up to nine containers can be seen near its entrance (figure 1). Whether the rapid buildup means this tunnel will be the location of the test is unclear, but it is worth noting as a possibility.

In addition to activity in the area of the South Portal tunnel entrances, imagery from April 25 also indicates the presence of a vehicle parked in exactly the same spot in the Main Support Area as during the days prior to the 2013 test (figure 2). While it is not possible to positively identify the vehicle in the most recent imagery, the one present in this location in 2013, covered with camouflage netting and surrounded by personnel, was believed to be associated with test preparations.

While 38 North’s earlier conclusion that a test during President Obama’s visit to Seoul “appeared unlikely” has proven to be accurate, contradicting briefings given by the South Korean Ministry of Defense, predicting when the fourth test will take place remains extremely difficult.

One key indicator that a detonation is imminent is the sealing of a test tunnel entrance prior to a detonation. Press reports on April 25, once again based on briefings by senior South Korean officials, claimed a tunnel had been sealed and a test could be expected in 7 to 14 days.

However, based on a close examination of imagery from April 25, it is unclear whether any of the test tunnels in the South Portal area have been sealed. One possible indicator that they had not yet been sealed is the activity near the entrances, particularly increases in numbers and movement of crates and boxes. Moreover, based on observations of the entrance areas, maneuvering earth-moving equipment to seal the tunnels around those crates and boxes would be difficult.

Another important indicator will be an end to activity at the test site. Just before the 2013 detonation, personnel, equipment and vehicles were cleared from the area, only to return a few days later (figure 3).

Figure 1. Significant activity at both South Portal tunnel entrances.

Figure 2. Activity at Main Support Area consistent with test preparations.

Figure 3. Activity at Punggye-ri before and after the 2013 nuclear test.