Usually underwater earthquake cause tidal waves, but on shore earthquakes with at least a magnitude of 6.5 can also cause them. What happens is very similar to dropping a pebble in a still pond, the water ripples outwards of the epicenter. These waves carry a lot of force, but mainly stay underwater. As these waves come closer to shore, the water depth gets shallower and shallower, forcing these waves up out of the ground.

The waves that travel through the water can move at speeds up to 370 mph, and can be detected by scientests, allowing people to be warned about an incoming tsunami in advance of it happening. This has saved many people's lives.

More information about what a tidal wave is can be found [here]

Information about major past tidal waves can be found [here]

Tidal wave links can be found [here]

More tidal wave pictures can be found [here]