Upper trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
Hexagram 40 of 64
☳☵ Deliverance
The tension breaks; relief arrives.
Lower trigram
☵
Water · Kan
Danger and depth.
FamilyMiddle son
ElementWater
The image
Thunder above, Water below.
Thunder over Water: arousal and shock meeting danger and depth.
That combination is the whole meaning of hexagram 40. Nothing about it is arbitrary — change either trigram and you get a different hexagram with a different reading, which is why the 64 form a system rather than a list.