Impulsive
There is also a clear difference between the radiation pattern of an explosion, which is a positive pressure pulse in all directions, and real earthquakes which have sectors of positive and negative polarity corresponding to the direction of fault rupture.
You can, in principle, disguise a bomb signature - notably by detonating off-centre in the cavity produced by a previous bomb, but it's not easy.
The solution is to go for small devices, in a known seismic area, and never repeat.