I honestly do not know if it is a visual bug or intended, I hope it is a bug though. Whenever you use Earthquake on monsters (I have not tried with aftershock unspecced or converted to spell) giant black lines appear across the screen, sometimes covering as much as 50% of the screen. Only happens on this skill and never when I am hitting the ground without mobs presents.
I have played the skill in Offline and Online, different characters, tried plenty of different gear etc. Seems to be very much tied to the skill and not other factors.
Video of it happening: https://youtu.be/6F4gneD0TuI
Cheers.