When an underwater earthquake occurs, the sea floor splits in two and the parts shift vertically in relation to each other, creating a fault. The fault can be up to 5-10 metres high, and occurs during just a few tenths of a second.
All the water that is on top of the sea floor is forced up with the movement of the fault, which therefore propagates through the water. If the depth is 5 000 metres and the fault is 10 metres high, then a 5 000 m water column is lifted 10 metres in just a few tenths of a second. A large amount of energy is transferred in a very short time from the earthquake in the earth’s crust to the sea water.
The displacement at the sea surface is less than on the sea floor. It might only rise by a few decimetres up to a metre. A wave forms on the surface that is tens or hundreds of kilometres wide, but only up to a metre high. The horizontal extent of the wave is the wavelength.
How a tsunami travels across the ocean
As soon as the wave has been formed it starts to move away from the epicentre. If you could watch if from space it would radiate out like rings on the water after throwing in a stone. Since a tsunami wave is so long and low, with maybe just a metre rise over an area of 10-100 km, it won’t be seen or felt by anyone out on the ocean in a ship.
The tsunami moves away from the epicentre. Average ocean depths are about 4000 metres, so the wave moves at speeds of around 700 km an hour. If the tsunami is 100 km long then it would take about 10 minutes for the entire wave to pass under a ship, which is why it goes unnoticed.
A tsunami is a special sort of wave, behaving like a soliton it interacts only very weakly with other waves and so almost entirely retains its power (energy) as it crosses the deep ocean. It may pass across the whole of the Pacific, nearly half of the planet, without any noticeable damping.
When a tsunami reaches land
Before the tsunami reaches land the water becomes shallower and two things happen: the wave speed reduces (as does its wavelength) and the height of the tsunami increases dramatically.
When the depth is as shallow as 10 m the speed will have reduced to around 50 km/h and the wave height increased to 5 m. The front of the tsunami steepens, giving the appearance of a wall of water rushing towards the coast.
When it finally reaches a beach it rushes far up onto land, where the huge volume of water causes devastation, drowning people and animals and washing them out to sea.
Warning signs
A tsunami is often preceded by a drawdown of the water. Before anyone has time to suspect that something is about to happen, the sea level drops by several metres, showing the sea floor and probably exposing interesting objects that are usually covered by the sea.
Unsuspecting people walk out to look at what has been uncovered and maybe even salvage some of the things. But then the tsunami wave rolls in faster than a person can run.
The advice is therefore if a tsunami approaches, even if the water level appears to drop, leave the beach and the sea, get to higher ground but keep out of buildings.
The wave energy
The tsunami mainly spreads radially and therefore the wave height decreases by 1/fourthroot(r). However there are only very limited energy losses unless the wave meets land or very shallow water.
In the ocean, the energy is focused or diffracted due to bending (refraction) of the wave. Shallow areas such as oceanic ridges or island shelves function as wave lenses. A focal point forms behind these where the tsunami can become magnified several times.
Could a tsunami reach Sweden?
A tsunami can cross a deep ocean without losing much energy but when it meets land, the energy is dissipated through the violent turbulence of the wave breaking. Tsunamis are most common in the Pacific and Indian Ocean. As the tsunami’s path to Sweden is blocked by land, only a tiny fraction of the energy could reach Swedish shores, making them very difficult to notice.
A tsunami is a wave movement that can be compared with light. Light travels in straight lines and cannot go round corners. Tsunamis behave in almost the same way; although they can go round peninsulas and islands to a certain extent, for example around Sri Lanka or the southern tip of India. But a tsunami can not travel around Africa and spread through the Atlantic around the UK and then in to the Skagerrak and the West coast.