What are the distinguishing features of an "artificial seismic wave" (e.g. a large underground explosion) IN the NPR news item North Korea Possibly Conducts Sixth Nuclear Test, South Korea Says:

The U.S. Geological Survey says it detected a 6.3 magnitude &quot;possible explosion&quot; near Sungjibaegam, North Korea Sunday afternoon, &quot;located near the site where North Korea has detonated nuclear explosions in the past.&quot;
South Korean Joint Chiefs of Staff say they detected an &quot;artificial seismic wave&quot; at 12:29 p.m. local time and are &quot;currently looking into the possibility of it being a nuclear test.&quot;
The incident was near North Korea's Punggye-ri nuclear test site, which is in the country's northeast and the site of the five previous tests.

The magnitude and location of the event are strongly suggestive of the cause, but if the location were very different, it sounds like the nature of the seismic waves would already indicate that the source was artificial rather than a natural geological seismic event.
Question: What is it about a seismic wave measurements that would allow one to distinguish between natural geological and artificial causes?

below: &quot;North Korea has conducted six nuclear tests. As this chart of seismic activity shows, the latest test on Sept. 3 was roughly an order of magnitude larger than earlier ones.&quot; From the NPR new item Here's How Big North Korea's Latest Nuclear Test Actually Was, Original: CTBTO
Several things can indicate that a seismic event is an explosion and not an earthquake.

Firstly, the depth of the source. The shallowest earthquakes tend to have a hypocentre that is at least a couple of km below the ground surface and some earthquakes in certain parts of the world can go down to about 700 km. Artificial explosions on the other hand, will be very shallow, with a hypocentre that is essentially at the surface.

Secondly, and more importantly, the P-wave radiation pattern (and S-wave as well) for an explosion is fundamentally different to that of an earthquake. This can be investigated by plotting the polarities of the first arriving waves of a seismic event on a stereonet, with the position on the net being determined by the azimuth of the source from the receiving station and the outgoing angle of the wave from the source.

Earthquakes radiate P-waves in four lobes, with alternating polarities. This is called a double couple source and it produces a fault plane solution with two positive and two negative quadrants. 

An explosion, radiates P-waves equally in all directions and so produces an isotropic pattern.

You may have seen reports of a second seismic event after the nuclear test which has been interpreted as a tunnel collapse. This also has a distinctive focal mechanism, called a compensated linear vector dipole (CLVD).



The photo shows directions of first motion at the source (a) (outwards = positive, inwards = negative), the P-wave radiation pattern (b), the steroplot of each focal mechanism (c).

These links give some further introduction to how stereonets are used to analyse earthquakes:

https://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-510-introduction-to-seismology-spring-2010/lecture-notes/lec18.pdf

http://www.ucl.ac.uk/EarthSci/people/lidunka/GEOL2014/Geophysics4%20-%20Seismic%20waves/Seismology/Earthquake%20Seismology2.htm

http://rses.anu.edu.au/~nick/teachdoc/lecture11.pdf