Zhen, a moment in time, time-marker. Attribute: Shock (which is a better translation than Thunder).
Zhen is about sudden change and the start of things. Zhen's attribute 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, but it has no direction on its own, except to start (or startle).
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.