Q: What did Poseidon use to cause rough seas and earthquakes?
A: Poseidon used trident to cause rough seas and earthquakes. Poseidon used his trident into the seas or into the ground to cause rough seas and earthquakes.
Poseidon used his trident to strike a rock upon the hill of the Acropolis to produce a well of seawater.
The trident is the weapon of Poseidon, or the God of the Sea in classical mythology.