This is a picture of the P wave shadow zone. Waves bend as they hit the liquid outer core. There’s a shadow zone at the surface, into which seismic waves from A cannot travel – it’s actually a ring of points.

There’s a huge S waves shadow zone too, because S waves can’t pass through liquid.

From the bending of different types of seismic waves, we get a good idea of the Earth’s internal structure.

Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".subduction zoneplate tectonic

As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.

Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered.

The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline.

Tsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours.tsunami produced by an earthquake along the coast of Chile in 1960

Wave Motion As the depth below the free surface increases, the radius of the circular motion decreases The motion ceases at a depth of half the wavelength

Ocean waves travel at speeds limited by the density of water and the pull of gravity. At sea a tsunami is hardly noticeable even though it travels so fast, typically rising less than a meter over a period of seconds The speed of a tsunami wave is limited by the depth of the sea, to about 450 knots (830 km/hr) in the Pacific Ocean. May 1906 tsunami traveled ~900 km/h. As the waves approach the coast, their speed decreases and their amplitude increases. Unusual wave heights have been known to be over 100 feet high. However, waves that are 10 to 20 feet high can be very destructive and cause many deaths or injuries.

Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.