How they form
A tsunami is a sea wave generated by an undersea earthquake or possibly an undersea landslide or volcanic eruption. When the ocean floor is tilted or offset during an earthquake, a set of waves is created. Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mi) long, that encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the Hawaiian Islands.
A tsunami can have wavelengths, or widths, of 100 to 200 km (60 to 120 mi), and may travel hundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 km/hr (about 450 to 500 mph). Upon entering shallow coastal waters, the wave, which may have been only about half a meter (a foot or two) high out at sea, suddenly grows rapidly. When the wave reaches the shore, it may be 15 m (50 ft) high or more. Tsunamis have tremendous energy because of the great volume of water affected. They are capable of destroying coastal settlements.