Most tsunami are caused by earthquakes with epicenters near or on the ocean floor. Not all earthquakes generate tsunami and some tsunami can be small and cause little or no damage. Because of this, people and surfers sometimes go to the beach when there is a tsunami warning. This is not a good idea because it is difficult to know when tsunami will be small and when they will be deadly. It usually takes an earthquake greater than 7.5 on the Richter scale to produce dangerous tsunami. Click here for a tsunami animation (you will need MPEG viewing software).
Sometimes people use the words tidal wave and tsunami to mean the same thing. However, the two are not related. While tsunami refers to dangerous waves caused by underwater disturbances, tidal waves are simply the crest of tides as they travel around the Earth. Tsunami have nothing to do with tides.