Upper trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
Hexagram 34 of 64
☳☰ Great Power
Strength that must not be misused.
Lower trigram
☰
Heaven · Qian
Creative force.
FamilyFather
ElementMetal
The image
Thunder above, Heaven below.
Thunder over Heaven: arousal and shock meeting creative force.
That combination is the whole meaning of hexagram 34. 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.