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.
Thunder below or inside has to do with sudden and intense emotions and impulses. Ideas come spontaneous, intuitively, sudden and often very original. They can be a breakthrough, or just transient despite their fierceness.
When inner Thunder is out of balance, it can cause blindly lashing out, a crisis, a chance lost through shortsightedness or rashness.