By Rachel Hunt
‘Tsunami’ is a Japanese word that translates to ‘harbour wave’1. Sounds pleasant enough, but unfortunately this name doesn’t emphasise the destructive potential of these hazards. When most people think of tsunamis, they picture a wall of water crashing into a coastline. So why is it that in some cases the water is seen to recede to an extensive distance before surging back onto the shore? This phenomenon is known as the Initial Withdrawal of the Ocean (IWO)2. Confusingly, this spectacle doesn’t take place during all tsunami events and can even occur in certain areas but not others during the same event!
But let’s back up for a second and take a closer look at how tsunamis work.
Water, water, everywhere
Tsunamis can be broken down into four successive stages; generation, propagation, runup, and inundation.
Tsunamis are mostly commonly generated by earthquakes under the ocean that displace the seafloor. These events are most likely to occur at the boundary between oceanic and continental tectonic plates. These boundaries are known as subduction zones, where the denser oceanic plate is pulled down beneath the lighter continental plate1. In areas like these, shallow earthquakes cause the seafloor to rapidly rise or fall on either side of the boundary, this transfers energy to the water above and generates a tsunami3.
Tsunamis often travel in wave trains that are typically made up of between five and ten waves3. In the deep ocean, tsunamis are characterised by long wavelengths and small wave heights so they would appear as no more than a ripple at the surface. But these deep-water tsunamis can travel at velocities of up to 1,000 kilometres per hour, now that’s speedy!
As a tsunami approaches the coast, the wavelength shortens whilst the wave height drastically increases1, resulting in towering waves up to 30 metres tall. This process, known as shoaling, is due to the wave energy being forced into a much smaller volume of water as the ocean gets shallower towards the shore3.
The tsunami hits the land! Flat coastal terrain allows tsunamis to travel several kilometres inland, whereas steep terrain can help to shelter shorelines from tsunami inundation3. The powerful waves can transport debris both inland and back out to sea, these objects might include; trees, cars, houses, and even people!
But enough about the ocean coming in, let’s talk about why the water goes out.
Tsunami and the IWO: Where the heck did the ocean go?
We now know that tsunamis are generated by the sudden uplift or subsidence of the seafloor and we learned at the start of this blog that the IWO doesn’t occur during every tsunami event.
Have you guessed it? These two factors are linked!
If the generating earthquake causes the seafloor to rise on the landward side of the plate boundary, the first wave in the tsunami wave train will be classed as a crest or elevation wave, causing the water level to increase before the wave inundates the shore4. This is because the water is trying to return to equilibrium and so must travel towards the shore.
On the other hand, if the earthquake causes the seafloor to fall on the landward side of the plate boundary, the leading tsunami wave will be classed as a trough or depression wave, meaning that the water level will decrease just before the inundation occurs4. Here, in order to reach equilibrium, the water must flow away from the shore, producing the IWO phenomenon.
Staying alive, staying alive
Tsunamis can devastate and destroy buildings, infrastructure, and livelihoods. Here’s two recent examples; 230,000 people lost their lives in the 2004 Indian Ocean tsunami, with a further 16,000 people being claimed by the sea in the 2011 Tohoku tsunami.
But how could the catastrophic impacts of future tsunamis be reduced? The IWO could be the answer we’ve been looking for as it has the potential to act as a natural warning for approaching tsunami hazards2. Moments before the 2004 tsunami hit Phuket in Thailand, ten-year-old Tilly Smith who was on holiday with her family, noticed the warning signs of an impending tsunami after learning about them in her geography class at school. Tilly’s family and 100 other tourists were evacuated from Mai Khao Beach, they all survived the tsunami waves thanks to Tilly’s knowledge!
However, there have been many cases where members of the public have gone down to the shore to investigate the strange occurrence of the sea withdrawing, only to be caught in the tsunami as it reaches the coastline moments later. This highlights the considerable need for tsunami preparedness programmes in order to educate local people to heed the warning of the IWO4.
So, remember, if you’re near the coast and feel a strong earthquake or you see the ocean quickly receding from the shoreline, this could mean that a tsunami is on the way! Make sure you get to high ground or travel further inland as quickly and as safely as possible.
- Bryant, E. (2010). Tsunami: The Underrated Hazard. (2nd ). Berlin, Germany: Springer.
- Nirupama, N., Murty, T. S., Rao, A. D., & Nistor, I. (2007). A Partial Explanation of the Initial Withdrawal of the Ocean during a Tsunami. In T. S. Murty, U. Aswathanarayana, & N. Nirupama (Eds.), The Indian Ocean Tsunami (pp. 73-80). London, United Kingdom: Taylor and Francis.
- Nott, J. (2006). Extreme Events: A Physical Reconstruction and Risk Assessment. Cambridge, United Kingdom: Cambridge University Press.
- Tadepalli, S., & Synolakis, C. E. (1996). Model for the Leading Waves of Tsunamis. Physical Review Letters, 77(10), 2141-2144. doi: 10.1103/PhysRevLett.77.2141
- Geoscience News and Information. (2017). Tsunami Geology – What Causes a Tsunami?