Tidal waves have nothing to do with the tides, which are caused by the gravity of the moon and the sun. A better term for a tidal wave is a seismic sea wave, or a tsunami, a word which comes from Japan, where tidal waves are common.
Tsunamis are caused by earthquakes under or near the ocean. These quakes shift the ocean bed and send sharp vibrations through the water. Underwater volcanoes or avalanches may also cause a tsunami.
A tsunami is not a single wave, but a series of waves. At sea, these waves may be just a few feet high, traveling along at more than 400 miles per hour.
As the waves near land, they move more slowly and their height increases. By the time they reach land, they may be over 100 feet high, moving at 100 miles an hour! One tsunami that hit Alaska after a 1964 earthquake was close to 220 feet high!
A tsunami can cross an entire ocean, and these massive waves often cause great damage when they strike land. In 1883, a tsunami killed 36,000 people in the East Indies, and an 1896 tsunami killed 27,000 people in Japan.
A single earthquake in 1960 sent tsunamis to places as far apart as Chile and Japan!