Detecting tsunamis is a very difficult thing to do. When a wave begins in the deep ocean waters, it may only have a height of about twelve to twenty-three inches and look like nothing more than the gentle rise and fall of the sea surface. An example of how easy tsunamis are to overlook, is the Sanriku tsunami, which struck Honshu, Japan, on June 15, 1896. Fishermen twenty miles out to sea didn't notice the wave pass under their boats because it only had a height at the time of about fifteen inches. They were totally unprepared for the devastation that awaited them when they returned to the port of Sanriku. Twenty-eight thousand people were killed and 170 miles of coastline were destroyed by the wave that had passed underneath them.
Tsunamis are characterized as shallow-water waves. Shallow-water waves are different from the wind-generated waves many of us have observed from the beach. Wind-generated waves usually have a period (the time between two successive waves) of five to twenty seconds and a wavelength (the distance between two successive waves) of about 330 to 660 feet (100 to 200 meters).
Tsunamis in deep water can have a wavelength greater than 300 miles (482 kilometers) and a period of about an hour. This is very different from the normal California tube, which generally has a wavelength of about 330 feet (100 meters) and a period of about ten seconds.
As mentioned above, tsunamis are shallow-water waves, which means that the ratio between water depth and wavelength is very small. These shallow-water waves move at a speed equal to the square root of the product of the acceleration of gravity (9.8m/s/s) and the water depth. The deeper the water, the faster and shorter the wave is. For example, when the ocean is 20,000 feet deep, a tsunami travels at 550 miles per hour. At this speed, the wave can compete with a jet airplane, traveling across the ocean in less than a day. Because a wave loses energy at a rate inversely related to its wavelength, tsunamis can travel at high speeds for a long period of time and lose very little energy in the process.
When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, or a series of breaking waves. Reefs, bays, entrances to rivers, undersea features, and the slope of the beach all help to modify the tsunami as it approaches the shore. Tsunamis rarely become great, towering, breaking waves. Sometimes the tsunami may break far offshore. On occasion, a tsunami may form a bore, a step-like wave with a steep breaking front, which can happen if the tsunami moves from deep water into a shallow bay or river.
The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity, while in another area, destructive waves can be large and violent. The flooding of an area can extend inland by 1,000 feet (305 meters) or more, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat.
The vertical height reached by a tsunami onshore above sea level is called a run-up height. In extreme cases, the water level can rise to more than 50 feet (15 meters) above sea level for tsunamis of distant origin, and over 100 feet (30 meters) for tsunamis generated nearby. A notable exception to this is the 1958 landslide-generated tsunami in Lituya Bay, Alaska which produced a 1,722 foot (525 meter) wave.
Tsunami