2. The under water earthquakes and subduction zones are the most common cause.

How a Tsunami Is Caused

When an oceanic plate hits a continental plate the plates press together and pressure builds.
Eventually the heavier oceanic plate slips under the lighter continental plate and causes an earthquake.
The earthquake lifts part of the ocean up and drops other parts down.
What happens on the ocean floor is mirrored on the surface of the water above.
The gravity acts fast to even out the water’s surface. The seismic energy created that big wave and it doesn’t just disappear. Waves start moving up to 600 miles per hour but you can’t see them.
The Tsunami extends thousands of feet deep into the ocean.
Tsunamis carry lots of water and energy so they can travel very far.

1. They can destroy lives/drowned loved ones.