Upper trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
Hexagram 16 of 64
☳☷ Enthusiasm
Movement that carries others with it.
Lower trigram
☷
Earth · Kun
Receptive yielding.
FamilyMother
ElementEarth
The image
Thunder above, Earth below.
Thunder over Earth: arousal and shock meeting receptive yielding.
That combination is the whole meaning of hexagram 16. 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.