In the MaWangDui YiJing: Chen, which is the bottom part of zhen and the old way of writing zhen. It is the 3d stem, corresponding with the 3d month, when thunder wakes everything up and the work in the fields begins. It is the 5th branch, from 7 to 9 AM, when the day begins.

Zhen: initiative, shock

"God comes forth in the sign of Zhen.
the myriad things come forth in the sign of Zhen."

  Life comes up and goes down, it appears suddenly and disappears when growth declines. Thunder renews it over and over, which makes life cyclical, after the decline new growth starts, every end brings the space for a new beginning. The seasons follow each other in endless succession, but every season is new in itself.

"Thunder acts in the dragon."

  Thunder is beginnings. Every change starts with Thunder, every action or command. Everything big grows from a small seed. When the inherent electric and magnetic energies of earth receive the power of heaven, this power finds a direction through Thunder and charges and discharges, creates and destroys. Thunder is the flying dragon, the voice of Heaven which resonates on earth.

"All living things come forth in the sign of Thunder."

  It is the manifestation on earth of the power of heaven, which makes all things emerge, from a simple act or thought up to the creation of a species. Thunder is Heaven's fertilizing force. Thunder wakes up the seeds in spring and motivation in the mind. It opens up the time for renewal and change.

"Finally, it is the strong, that which grows luxuriantly."

  Thunder is usable energy. Heaven sends its laws down to the earth, and with the power of Thunder we can make use of them. Nowadays as electricity, but in all times physically as energy and mentally as clear thinking, serendipity, insight, creativity. Absorb this strength, when you pass this on to others as inspiration or motivation you yourself make use of Thunder.

  "Thunder brings about movement."

  If you have been trying without results, Thunder signals relief. This can come in many ways, from solving the problem to overthrowing the entire situation or finding rest in resigning. Praying is an act of Thunder, connecting with Heaven. Thanks to Thunder there is always an opening to a new beginning where everything seems at its end.

"Thunder is the oldest son of Heaven and Earth. He presides over the altars of grain and soil."

  The clouds and rains follow the dragon and make the seasons dry or rainy, warm or cold, all according to their time. In the fields of earth the dragon's voice can be heard as Thunder. In spring it tells all that the growing season will start, in autumn it clears the sky for the winter time.

Inner and outer trigram
Thunder "through anxiousness and fear sets in order and examines".

  When Thunder is INNER (lower) trigram, it gives the outer trigram the energy to become reality.

  In 25 the powerful order of Heaven (The ancient kings made use of lush growth in accordance with the seasons to nurture the myriad beings).
  In 24 the generous space of Earth (The first kings closed the frontiers at sun solstice, peddlers did not travel, the Hou did not inspect the region).
  In 51 the energy of Thunder (The noble one through anxiousness and fear sets in order and examines)
  In 42 the influence of Wind. (When the noble one sees good, his rule is to improve, when there are transgressions, his rule is to reform).
  In 3 the reshaping power of Water (The noble one unravels the warp).
  In 21 the clarity of Fire (The ancient kings made the penalties clear to enforce the law).
  In 27 the individuality of Mountain (The noble one uses cautious language and limited food and drink).
  In 17 the reflection of Lake (The noble one, towards nightfall, goes inside to eat and relax).

 When Thunder is OUTER (upper) trigram it brings the powers of the inner trigram into action.

  In 34 the powerful order of Heaven (The noble one will not tread without ritual).
  In 16 the generous space of Earth (The ancient kings composed music, honored virtue, with dancers worshipped the Lord on High. They associated with the deceased ancestors).
  In 32 the influence of Wind (The noble one stands and does not change his bearings).
  In 51 the energy of Thunder (The noble one through anxiousness and fear sets in order and examines)
  In 40 the reshaping power of Water (The noble one pardons transgressions and is lenient towards crimes).
  In 55 the clarity of Fire (The noble one decides lawsuits and carries out punishments - in 21 ‘use’ punishments, in 55 carry them out)
  In 62 the individuality of Mountain (The noble one exceeds in respect in actions, exceeds in sorrow at funerals, exceeds in thrift at spending)
  In 54 the reflection of Lake (The noble one through the eternity of the end understands the transitory).. It is a situation in which social rules connect very closely with their big inherent meaning. The maiden has an important task, even though her position is low, certainly in the beginning. She has to look after the altars of the ancestors, but as the new wife she harbors the potential for making the lineage continue into the future.

Shuo Gua
discussion of the trigrams, ch.3. (Transl. Wilhelm-Baynes)

 3. Thunder and wind arouse each other.
 4. Thunder brings about movement.
 5. God comes forth in the sign of the Arousing.  All living things come forth in the sign of the Arousing. The Arousing stands in the east.
 6. Of all the forces that move things, there is none swifter than thunder. Of all the forces that bend things, there is none swifter than wind. Therefore: thunder and wind do not interfere with each other.
 7. The Arousing means movement.
 8. The Arousing acts in the dragon.
 9. the Arousing manifests itself in the dragon.
 10. In the trigram of the Arousing she seeks for the first time the power of the male and receives a son. Therefore the Arousing is called the eldest son.
 11. The Arousing is thunder, the dragon. It is dark yellow, it is a spreading out, a great road, the eldest son. It is decisive and vehement; it is bamboo that is green and young, it is reed and rush. Among horses it signifies those which can neigh well, those with white hind legs, those which gallop, those with a star on the forehead. Among useful plants it is the pod-bearing ones. Finally, it is the strong, that which grows luxuriantly.