When a seism occurs under the sea, the abrupt lowering of the bottom can start a tsunami.
The catastrophe which touched Asia on December 25th, 2004 has been caused by more the violent one seism recorded in the world for 40 years.
The Tsunamis were felt until on the coasts of East Africa to more than 6.000 kilometers.
It records seismic waves which are propagated much more quickly than the hydraulic waves. Thus, the data of the seismic networks make it possible to locate the seism, to estimate its magnitude and thus to envisage if it will cause a tsunami.
It also records the heights of water with the marigraphs installed on the coasts of the Pacific and the islands. These recordings have the disadvantage sometimes of measuring disturbances due to the coasts and not generated by tsunamis.
In order to stage with this disadvantage, of the pressure pick-ups, insensitive with the swell, and installed on the ocean floor off the coasts Japanese woman and of the Aleutian Islands, record more reliable data; they can measure tsunamis lower than the centimetre.