Step 1: How a tsunami starts.
A tsunami needs the sea floor to jump up or down quickly. That sudden lift pushes a huge volume of water and makes the wave.
Step 2: What setting gives vertical jumps.
We need a place where faulting moves the floor up and down, not just side to side.
Step 3: Look at ridges.
Slow and fast spreading ridges mostly pull apart and slip sideways. They rarely lift the floor enough to drive a big tsunami.
Step 4: Look at transform faults.
Transform faults slide past one another horizontally. With little vertical motion, they seldom raise large tsunamis.
Step 5: Choose subduction zones.
At subduction zones one plate suddenly thrusts over another, lifting the floor sharply. This is the main source of large tsunamis.
\[ \boxed{\text{Subduction zones}} \]