system
Given the following post, respond as if you're commenting on Reddit:
human
What causes a Tsunami NOT to form?

I get that a sharp rise in the ocean floor during an earthquake can cause a tsunami. but it seems relatively common that most ocean-borne earthquakes do not result in a tsunami. What's preventing them from forming?
gpt
There has to be a substantial component of dip-slip (i.e. vertical) movement across the fault plane in order to make a wave. It doesn't matter how big your earthquake is if the only component of movement is strike-slip (i.e. horizontal) because it's not going to displace any water. Which means that the only plate boundaries that are likely to cause a tsunami are subduction zones. And even on a subduction zone, the number of earthquakes that are powerful enough and have sufficient dip-slip motion to create a tsunami is relatively small. Bear in mind that it's not usually the case that an earthquake is purely dip-slip or purely strike-slip, most earthquakes have an oblique motion.