Everyone has heard about a tsunami, whether that be the one that hit Japan not too long ago or some other instance. Regardless of how you've heard of these water monsters, I was interested to find out more about the physics behind these.
Tsunamis are basically a massive scale version of the waves that we've studied throughout our physics experiences. Rather than wavelengths in centimeters and periods measured in seconds, the waves of tsunamis are measured in kilometers and their periods are measured in hours. Their wavelengths have been measured to be as large as 500 km. Interestingly enough, the speed that these waves travel at is dependent only upon the water depth and the force of gravity. In the ocean, water depth can be 5000m, and utilizing the equation that the speed of the wave = sq rt(g·H), that means that waves would travel 221 MPH at a depth of 5000m.
Tsunamis caused by earthquakes, however, have wavelengths and periods that are determined by the size of the underwater disturbances caused by the earthquake.
As tsunamis approach land, the water depth decreases, thus causing the speed that the waves are traveling at to decrease. The tsunamis energy flux, which depends on speed and height of the waves, remains almost constant. As the speed decreases and the energy remains constant, this causes the heights of the tsunami waves to become much greater as they approach land. Because of this effect, known as "shoaling", tsunamis can go completely unseen at water but grow rather tall as they approach land. This is why tsunamis are often characterized by their massive waves.