formation of tsunami waves starts when the seismic energy from an offshore earthquake forces out a mass of sea water. the waves may start off as less than a meter in height, with wave lengths between 100-150km, and travelling at speeds of 800km/h, and may pass off as undetected. upon reaching shallower waters, the increased friction with the sediments on the seabed forces the tsunami waves to slow down and increase in height. at the point of impact with the coast, tsunami waves may be travelling at speeds of 30-50km/h and reach heights of 15m. sometimes, the sea recedes before advancing on shore, but this happens only a few minutes before the tsunami reaches the shore, this is because the water rushes to fill the void and is forced out shortly after causing a tsunami