Question

What happens when two oceanic plates collide? What negative impacts for human populations can result from this type of action?

Answer 1

When 2 oceanic plate collide, an earthquake is usually made, sometimes causing a tsunami, which is a very large wave of water that speeds towards land, getting larger the closer it gets. 
A tsunami can have very adverse effects on human populations that are living on the land that is hit by the tsunami, such as flash flooding, power outages, the destruction of homes, among other things.

Answer 2

Good Morning!

Well, first of all, when a oceanic plate hits another, an earthquake is usually about to pass. The problem though, is that after, the earthquake, a tsunami is expected and tsunamis are know to destroy cities and flood them with lots of water. Note, that also in this same manner, islands are sometimes formed, or even underwater volcanoes! Cool, right? 
Well happy studying and tag me if you need something! :)

Thank you,
Darian D.