What are the distinguishing features of an "artificial seismic wave" (e.g. a large underground explosion)
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