Drawback – Alerting Tsunamion May 21, 2010 at 7:16 am
If the first part of a tsunami to reach land is a trough known as drawback, rather than a wave crest. The water along the shoreline recedes dramatically, exposing usually submerged areas.
A drawback generally occurs because the tectonic plate on one side of the fault sinks suddenly during the earthquake, causing the overlaying water to propagate outwards with the trough of the wave at its front. This is why there would not be any drawback when the tsunami traveling on the other side arrives ashore, as the tectonic plate is “raised” on that side of the fault line.
Drawback starts to begin before the wave arrives at an interval equal to half of the wave’s period. If the slope of the coastal seabed is small, drawback can exceed about hundreds of meters. People who are unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed. During the Indian Ocean tsunami, the sea withdrew and a number of people went onto the exposed sea bed to investigate. Photos show people walking on the normally submerged areas with the advancing wave in their background.