A、Tsunamis are caused by underwater earthquakes. B、Earthquakes of about magnitude 6.75 on the Richter scale will cause tsunamis.

A tsunami is a series of huge waves that can cause great devastation and loss of life when they strike a coast. The word tsunami comes from the Japanese word meaning "harbor wave". Tsunamis are sometimes incorrectly called "tidal waves".  Actually tsunamis are not caused by the tides for the gravitational force of the moon on the sea. Regular waves are caused by the wind.
     (30) Tsunamis are caused by an underwater earthquake, a volcanic eruption, a sub-marine rockslide, or, more rarely, by an asteroid or meteoroid crashing into in the water from space. (31) Most tsunamis are caused by underwater earthquakes, but not all underwater earthquakes cause tsunamis.  An earthquake has to be over about magnitude 6.75 on the Richter scale for it to cause a tsunami. About 90 percent of all tsunamis occur in the Pacific Ocean.
     Many tsunamis could be detected before they hit land, and the loss of life could be minimized, with the use of modern technology. If you see the water recede quickly and unexpectedly from a beach (this is called drawback), run toward higher ground or inland, there may be a tsunami coming. Also, if you are on the coast and there is an earthquake, it may have caused a tsunami, so run toward higher ground or inland. Some beaches have tsunami warning sirens, do not ignore them. The first wave in a tsunami is often not the largest; if you experience one abnormally-huge wave, go inland quickly, even bigger waves could be coming soon.
     (32)After the huge volume of water has moved, the resulting wave is very long but not very tall (roughly 3 feet tall). The wave spreads across the sea in all directions; it can travel great distances from the source at tremendous speeds.