Zhen, Thunder refers most of all to an earthquake, but also to thunder or a lightning strike. Other translations: shake, emotional, invigorate and rejuvenate, power, majesty, respectful, frightened. The trigram brings new ideas or beginnings, impulses and intuition, a breakthrough, sudden changes, a wake-up call.
When Thunder is above or outside, the lower trigram expresses itself in accord with the properties of Thunder. Or your surroundings demand it. Actions can be rash and unrestrained. Things happen suddenly and can have radical consequences. Or things happen wich look frightening but don’t do any damage.
When Thunder outside is out of balance, actions are rash and destroying, ideas have no structure but depend on wild guesses, revolutions are destroying instead of opening up new possibilities.