The North Koreans claim to have tested a nuclear bomb at about 10:30 this morning (11:30am eastern Australia time). Can geology tell us anything about what actually happened? Let us see.
First stop, the comprehensive test ban treaty organization. In theory, they have a monitoring network designed to detect nuclear tests. Unfortunately, as a UN organization, their webpage is full of schemes and classifications and plans and protocols, and as of 6 this evening, there is no data, and no press announcement or other hint that anything happened.
The Geoscience Australia earthquake page is great for small quakes near Australia, but does not contain any information on any events today.
What about the evil, super-secret world-dominating Americans?
The USGS earthquake list shows a little orange square in NE north Korea. Clicking either it or the “M2.5 to 4+ earthquake list” gives the following quake listing:
|4.2||2006/10/09 01:35:28||41.294||129.134||0.0||NORTH KOREA|
Clicking the appropriate entry gives the summary page for that earthquake
So, what can we see? First, the depth of the earthquake is listed as 0 km. Most of the earthquakes in this region are very deep (see “historical seismicity” under the “maps” tag and look at the color scale). These deep quakes happen in the downgoing slab, as the old pacific plate subducts beneath Japan. Thus they are quite different than the zero depth M 4.2 event from this morning.
What else can we determine? In theory, an explosion should have an all-positive first motion tensor. Unfortunately, first motion tensors only seem to be calculated for quakes larger than M 5.0, so that test will need to wait until the North Koreans detonate a larger bomb. It might be possible to put a DIY tensor together by looking at the raw data, but I don’t know where that is archived.
Similarly, I seem to recall from my deep, dark past that explosions have a higher P/S wave ratio than normal quakes. But once again, this will require having the S wave data, and I don’t know where to find that, either. Hopefully some of the real seismologists who occasionally visit the lounge can give us some hints.
The only conclusion that I can draw is that there was a M4.2 event on land in North Korea with a zero depth, which was dissimilar to other seismicity in the area.