Either aftershock is bugged, or the tooltip is completely wrong.

Just lost to a Riven running aftershock. I was SHOCKED (HAHAHAHA) to see my Vlad empowered Q deal only 59 damage to her far after her aftershock exploded. I went back in to the replay to see why. I timed her bonus stats at lasting 3.5 whole seconds. (14.11s was my best effort at recording it at .25x speed.) Let me copy+paste the aftershock tooltip for y'all.

PASSIVE: Stun icon Immobilizing an enemy champion grants 70 − 120 (based on level) bonus armor and bonus magic resistance for 2.5 seconds. After the duration, you release a shockwave that deals 10 − 120 (based on level) (+ 3% of your maximum health) (+ 15% bonus AD) (+ 10% AP) magic damage to nearby enemy champions and monsters.