Tsunami characteristics

A tsunami is a series of rapidly ‘travelling’ super waves generated from a (deep) sea disturbance of the water column in the ocean water. Most harmful tsunamis are generated by an earth quake in the ocean floor with a magnitude as from 7.5 on Richter’s scale. (Deep) sea volcanic eruptions and massive subaerial or submarine landslides can also cause tsunamis. The potential energy in the vertical water column on top of the rupture location is then transferred into a horizontal propagation of tsunami waves.

Irrespective of its cause - line or pointed ruptures* - tsunami waves radiate into all directions. Within minutes, the original tsunami splits into a ‘local’ wave heading for the nearest coast and a distant wave that radiates out from the disturbance zone in the deep ocean for thousands of kilometres, loosing little of its energy en route. The wave magnitude and height of the tsunami at its point of origin is related to the inclination of the fault plane and the degree of vertical displacement of the fault.

Depending on the character and magnitude of the rupture, the deep ocean tsunami ‘travels’ with a speed up to 900 kms per hour. The local one slows down to approximately 30 – 40 kms/hour in shallow waters. In deep water the tsunami is hardly noticeable, but once running up on the rising sea bottom in shallow waters (ground effect**), the waves build themselves up in height and increase dramatically. Local tsunamis usually arrive too quickly for any formal warning system to be effective.

The treacherous December 2004 tsunami struck like a giant breaking surf wave with a peak preceded by a trough or ‘valley’ (negative tsunami)in the eastern part of the Indian Ocean. A positive tsunami type, as occurred in the centre and western part of the Indian Ocean, rushes in like a strong and fast moving tide current with a rapidly rising sea level. In both cases, part of the tsunami’s energy is reflected back into the open sea after its coastal run-up. It is a comprehensive phenomenon, which is not only characterized by its sequence in secondary and tertiary waves, but also by its intermediate back and forth rolling waves parallel to its shore line. In addition, the first tsunami wave is not always the largest and may be followed by stronger ones. This is a variable scene in repetition with a great destructive power.

* Line and pointed ruptures
Line and pointed tsunamis reflect the character of the rupture. E.g. the 2004 event was a classic example of a sea floor line subduction, where the Australian and Indian plates collided on a stretched course covering some 1200 kilometres. Whereas a pointed rupture just concentrates on a centre scale, also with devastating consequences.  

** Ground effect
The front wave meets resistance from the rising sea bottom and vigorously slows down in speed, whilst the middle and tail end are still moving on full speed. Hence, this water mass is ‘pressed together’ and simultaneously pressed up. Pushed by its rear end, the front end rolling in on the shallow coast is building up in height rapidly. Once approaching the coast, it is rushing in like a wall of water causing vast devastations, particularly in lower situated populated areas (see figures below).     

                        The wave slows down and builds itself up fed by the coastal water
                        (in the case of a negative tsunami).