Tsunamis are violent phenomena happening across the globe as a bi-result of external stressors. In this blogpost we will dive into some key aspects of tsunamis.

It’s important to understand the nature of tsunamis as these effect the lives of millions of people across the globe. They are violent and causes huge loss of life, economic and social damage in the millions.

Tsunamis the what

Tsunamis are natural phenomena occurring as a result of huge displacements of water. Equally they are caused by natural external pressures such as landslides, earthquakes and volcanic eruptions.

To describe them imagine a huge wave of water rushing in, that keeps coming as time passes. As a matter of fact this causes massive flooding as a result with devastating forces impacting structures, flood gates, towns, roads, trees and houses alike.

The Great Sendai Earthquake Tsunami

To understand how devastating the events of tsunamis can be, lets try and examine one recent catastrophic event: the Tsunami of 11’th of March 2011, caused by the Great Sendai Earthquake.

In the video below, you can see the entire video of the 11’th of march 2011 tsunami hitting and overtopping the flood barriers. It is clearly seen how the water keeps rushing in over the flood barriers even after the first hit. This event was catastrophic and resulted in the loss of lives amounting to 20.000 lives lost.

Video 1: Tsunami hitting the town of Miyako of Japan.

Numerical wave models WAM – Wave watch III – NOAA

To model a Tsunamis impact, one can use a numerical weather prediction tool which is a wave-modelling tool. Multiple wave-modelling suites exists which all have differences in treatment of source-sink terms. The wave-watch III model is a numerical wave model which have been utilized for creating a wave map.

A wave amplitude map showcases differences in wave attenuation across the entirety of the pacific ocean. Here a valid source term describes that the tsunami source arises from the earthquake epicenter. NOAA have released a map showcasing numerical results of wave attenuation across the entire pacific, see Figure 1.

Figure 1: Map prepared by the U.S. National Oceanic and Atmospheric Administration depicting the tsunami wave height model for the Pacific Ocean following the March 11, 2011, earthquake off Sendai, Japan.

Other theoretical frameworks exists for describing and prediction of wave models. The main ones are MIKE – models (State-of-the-art), DELFT and WAM.

Theoretical wave theories

Important to realize is that there exist multiple theoretical frameworks to try and understand the propagation of tsunamis. To highlight a few of the most wide-spread theoretical wave methodologies are as follows: Cnoidal, Linear (Airy wave theory), 2’nd and higher order (Stokes waves), Stream function wave theory and Boussinesq wave theory. Consider this blogpost which goes into the differences and similarities of the different wave theories.

Wave measurements of the Tsunami

Theoretical considerations for describing the propagation of wave energy is important. At the same time they can’t stand alone and as a result we need measurements of the surface displacements as these are more valuable when trying to understand Tsunamis. Luckily, there exists measurements freely available for users to download.

The NDBC system, allows users to download real-time and historic data. By looking at historic quality controlled measurements we can understand what the tsunami amplitude at sea is. This allows us to answer questions such as what are tsunami amplitudes at sea? Where do they arise from? How fast are tsunamis moving?

Tsunamis the when

Tsunamis are caused by underwater eruptions, landslide displacements and volcanic eruptions to mention a few of the important ones.

History shows that tsunamis happens as a result of undersea earthquakes displacing large masses of water developing a wave train with large amplitudes. Legends have it that the Krakatoa Eruption caused mayor destructions and a worldwide tsunami travelling across the world causing havoc.

Tsunamis the where

When trying to understand Tsunamis and where they might be potential dangers there are certain rules of thump to follow.

Firstly, in order to be a potential hazard area, a considerable amount of water needs to be present.

Secondly, when water are present some of the potential sources of tsunamis need to be present. This can include subduction zones caused by the continental plate tectonics, earth hotspots with volcanic activities and large terrain variations with potential rock-slides or icebergs melting and cracking causing tsunami events.

Related read: How do we measure Tsunamis?

Thirdly, people need to reside in shores connecting these waters and especially nearby the epicenters of earthquakes are a significant potential hazard area.

To conclude, there are multiple factors which need to be present before a significant risk of tsunamis however in certain places, this causes quite large burdens for societies at large.

Concluding remarks

Tsunamis are unpredictable in nature and violent catastrophes causing large problems for the nearby societies. They are difficult to effectively mitigate and require long term investments in due time whenever peace is sufficient. They can be described as a huge volume of water which keeps rushing inland and the combination of high amplitude and long period makes the wave increasingly dangerous.

The unpredictability arises from the underlying natural phenomena driving these catastrophic events. Methodologies exist with geophysical sounders listening to the rumbling of the earth allowing early warning of incoming earthquakes which may cause tsunamis. Another example of early warning signs is the receding coastline immediately before the wave arises on shore.

The locations of tsunamis require quite a few preconditions before posing a threat to society. Firstly, there needs to be a significant amount of water present. Secondly, underlying driving mechanisms such as earthquakes, volcanic eruptions and rock slides need to be present. Thirdly, people need to reside and live in the neighborhood. All of these conditions are rarely met, luckily since they are one of natures most devastating events.

References

Tsunami video hitting the Japanese town of Miyako on March 11’th 2011 – https://www.youtube.com/watch?v=86ThCibkHQw&ab_channel=Rachid82UK*
Wave-height map of tsunami from NOAA –https://www.britannica.com/event/Japan-earthquake-and-tsunami-of-2011

NOAA Wave Watch III –https://polar.ncep.noaa.gov/waves/wavewatch/