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Tsunami Waves in Open Seas
In the open sea, and at speeds that make tsunamis capable of traversing across the Pacific Ocean in about 24 hours, the waves themselves are often rarely more than a couple of feet tall. That makes the waves undetectable from aircraft and typically unnoticed by ships at sea. In deep parts of the ocean, the bulk of the waves̵7; energy is locked below the surface. The waves rise and fall nearly to the ocean floor, so the crest is little more than a ripple on the surface as the tsunami races across the ocean.
Tsunami Waves Approaching Land
As a tsunami closes in on the shore, its speed begins to diminish rapidly. Friction produced by the water as it moves against the shallow floor of a bay, beach or inlet forces the leading edge of the wave to move more slowly than the back of the wave. The trailing portion can be miles behind the leading edge, still traveling at great speed. When the back end of the wave catches up to the leading edge, the water and the energy behind it can only go up. Sometimes, tsunamis consist of more than one wave. Those additional waves may also meet up with the first one̵7;s leading edge and pile onto it, creating an even larger wave. Depending on wave speed and how they meet the narrowing ocean floor toward a shoreline, those secondary waves create a surge that crashes into shore several times after the first wave hits.
Attacking the Shoreline
In some cases, water near the beach begins to rise as the tsunami reaches the shoreline. But if the wave trough is still in front of the tsunami wave, the water recedes back toward the ocean. Entire bays may empty, leaving the muddy, mucky bottom exposed. Beaches can suddenly become wider as the tsunami approaches. Typically, the receding waters only last about five minutes before the tsunami strikes with crushing force. People may be tempted to venture onto the new beach, as they did in Hawaii in 1946 and Portugal in 1775. Both cases resulted in large loss of life as the tsunamis charged into shore after an eerie calm.
Resulting Damage
By the time a tsunami reaches the shore, it can unleash upwards of 100,000 tons of water per 5 feet of shoreline. The surge of water travels inland rapidly, and it can travel 1,000 feet or more depending on the shoreline structure and the contours of the shallow sea bottom. Cars, buildings, homes and other structures are no match for the force of tsunami waves. The crushing force of all that water smashes objects and carries them further inland. The worst part of the destruction occurs inside the inundation zone -- the land that lies closest to the coastline -- but the force of some tsunamis has been known to push miles inland, and even partway up inland hills.