Wednesday 18 October 2017

Tsunami: Causes, Effects and Mitigation Strategies

Overview

The purpose of this blog is to examine causes, effects, and the solutions to tsunami formation in a way that helps the reader recognize its importance. A look at several of earth’s spheres helps depict a picture of their interaction. And since the earth is dynamic, change in one system of the earth can influence the other. Causes of tsunami are either related to earthquake, volcanoes, and landslides which can be a collaboration between hydrosphere and geosphere. Since tsunamis are known to displace water from its equilibrium position, the displaced water can affect humans in one way or the other. Hydrosphere-geosphere is known to be the major concern in tsunami formation, this blog will elaborate the interaction between different component of spheres and the influence in the other corresponding aspect of earth.

What is a Tsunami and how does it work?



Causes of Tsunamis

Earthquakes - Geosphere & Hydrosphere
Earthquake are the primary source of tsunami. This is due to tectonic activity happening underneath the surface water (Mofield, Symons, Lonsdale, Gonzalez & Titov, 2004). When an earthquake happens beneath the surface water due to the collision of tectonic plates, pressure is created, displacing water from its equilibrium position. Research indicates that when water is displaced from its original position waves are formed under  the influence of gravity (Costa, Leroy, Kershaw, & Dinis, 2005), hence leading to tsunami formation. This is an example of the interaction between geosphere and hydrosphere.



                                                                  Figure 1. Tsunami in action

Volcanoes - Geosphere, Hydrosphere & Atmosphere
Volcanoes are also a major cause of tsunami. This can be generated in a subduction zone through the release of gases from the melting of tectonic plates creating pressure and pushing gases against the crust. The release of gas create tension changing the volume of water. This change in pressure displaces the sea water. Alternatively, an area of a higher temperature within the mantle result in a lower density than the adjacent magma. The excessive strain and high energy magma melts the crust hence pressure is created and volcanoes form. Submerged volcanic eruption displaces the overlaying water causing a tsunami. This is an expression of the interaction between geosphere, hydrosphere and atmosphere resulting to the formation of tsunami.


                                    Figure 2. Volcanoes erupting underneath surface water causing tsunami.

Landslides - Geosphere & Hydrosphere
Landslides that generate tsunamis most often occur where Earth's tectonic plates converge and is compared to the one caused by earthquake where rocks slides over each other (Herbitz, Lovholt, Pedersen & Masson, 2006). The dense plates sink underneath the lighter plates which makes Part of the seafloor snaps upward as the pressure is created. The overlying column of seawater is elevated and displaced toward the surface, creating a surge of seawater. This process involving geosphere and hydrosphere creates waves in the forms of tsunamis as the water reverts to equilibrium state.



Figure 3. Effect of landslide and as the heavy plates sink underneath the lighter plates causing tsunami.


Effects of Tsunamis

Tsunamis have an impact on the earth dynamic system. This effect emerges as a result of interaction between the different component the earth creating a surge movement of water hence leading to destabilize the soil hence making life uncomfortable for human beings. 

Salination - Hydrosphere, Pedosphere & Biosphere
Soil productivity is dependent on water permeability. According to Ondrasek, Rengel and Veres(2011) excessive mineral deposition in the soil threatens water infiltration. Sea water contains a large quantity of minerals in is composition, and when tsunami happens the pedosphere is exposed to this excessive quantity of minerals. Since sea water contains large amount of Na+ it replaces soil coagulators absorbed into the surfaces interfering with the content of other minerals like Mg2+ and Ca2+ which are essential in retaining nutrients (Rashad and Dalts, as cited in Ondrasek et al., 2011). Without proper nutrients in the soil, the habitability of the pedosphere is jeopardized. This effect the biosphere in such an environment hence making hostile to organism living on the surrounding region.


                                          Figure 4. Tsunami in action before it turns the soil unproductive.

Death/Disease - Hydrosphere, Anthrosphere & Pedosphere
Tsunamis affect the astrosphere in two ways, directly and indirectly. The direct impact come because of the force and the velocity at which water mass flows. Waves produced can travel at a speed of 900km/h (Costa et al., 2005). Subsequently this can have an impact on infrastructure and livelihood of those residing in the surrounding area. Illagasekare et al., (2006) claim that the Sri Lanka Tsunami in 2004 caused the loss of approximately 400000 jobs, 85000 homes and claimed a loss of 35000 people. Indirectly tsunamis can contaminate ground water causing a long-term effect for instance diseases. (Ondrasek et al., 2011) suggest that increased salinity in aquifer can enhance the solubility of the water. this can lead to excessive potential contaminants. When tsunami happens water infiltrates into the soil with high contents minerals salinizing the underground water. As a result, with large contents of minerals water becomes unhealthier for consumption. For example, salinity causes mineral deficiency to the people who are relying on the effected water Ondrasek et al (2011). In general hydrosphere can influence astrosphere through different means.  



Tsunami Floods in Miyako, Japan 2011




Mitigation of Tsunamis


Mangroves - Biosphere & Hydrosphere
The danger of tsunami is primarily derived from the momentum which the sea water moves and even though with less recommendation found, mangroves plays significant role in resisting the force induced by the tsunami. Research indicates that presence of mangroves around coastal areas destabilizes the tsunami by amplifying the wave height and consequently reducing velocity at which the mass water flows, hence collapsing the wave (Koh,Teh,Majid and Lau, 2012). Similarly Danielsen and others(2005) believes that mangroves and other coastal forests significantly mitigate the potential damage caused by tsunami. The 2004 Indian tsunami supports this finding, as reports suggest that coastal vegetation like mangroves helped provide protection in nearby communities. This shows a clear evidence between hydrosphere and biosphere interaction in mitigating tsunami.

Figure 5. pre- tsunami tree vegetation cover and post tsunami damages in Cuddalore District, Tamil Nadu, India.

Tsunami Sirens - Anthrosphere & Hydrosphere
Early warning system (EWS) plays acritical role in easing the effect of tsunami across the globe. Larsen and Thomalla (2010) suggest that putting resilience for instance Siren can positively contribute to a better future in terms of minimising death as well as property. This however suggest that people responding to that are more likely saver and incur less problems than those with no resilience measures on the disaster-prone areas. Arguably Smith and Davies (as cited in Larsen & Thomalla 2010) believes that people prone to disaster areas do not always respond to warning system and therefore can respond so that they can be in a better position. Similarly, Hope (2016) believes that there is an urgent need to put up resilience after the Kaikoura earthquake. This signals out the level of risk that can evolve as result of the earthquake and can eventually lead to success in the near future. This clearly states the role of putting EWS to minimise risk for people residing along the coastal parts and prone earthquake areas with the likes of Christchurch.



                                                     Figure 6. man responding to the emergency sirens

Conclusion


Tsunamis are a complex and powerful geological process involving the interaction of many spheres of the earth system, from causation to consequence and human efforts to mitigation their effects. Dynamic Interactions between hydrosphere and geosphere, atmosphere (volcanic eruptions, earthquakes and tectonic plate movements) result in the formation of a tsunami. Tsunami pose various detrimental impacts on the  Earth's dynamic system including the pedosphere and the biosphere. Due to the anthrospheres dependence on these respective spheres, the anthrosphere also exhibits the consequences of tsunami. With rising concern on the emergence of tsunami, solutions to minimise its consequences are implemented relating to the hydrosphere and the biosphere. Overall the interaction that tsunamis exhibit between different spheres of the Earth shows the dynamic nature of the Earth system.




About

This Blog is a personal project made by Mohamud Hassan. I am an undergraduate student studying at the University of Canterbury majoring in Geology. This is part of Geol115 – “The dynamic Earth system” which is focusing on Travis Horton section of the course. I hope you enjoyed reading.
























Reference list



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Costa, P.; Leroy, S.; Kerskaw, S. & Dinis, J. (2005). Tsunamis: Causes, behaviour and sedimentary signature. Studies on the AD 1755 (Portugal). In Martinez Frias, J. Madero, J. (Eds.): Meteoritos y GeologĂ­a Planetaria, (pp. 151-171) Retrieved from file:///C:/Users/mohamud/Downloads/2005_PCosta_et_al_Chapter_SpainBook.pdf


Council to review activation of tsunami warning sirens. (2016, November 15). Retrieved from http://www.stuff.co.nz/national/nz-earthquake/86492060/christchurch-city-council-to-review-activation-of-tsunami-warning-sirens-after-complaints


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Harbitz, C.B., Lovholt, F., Pedersen, G., & Masson, D.G. (2006) Mechanisms of tsunami generation by submarine landslides: A short review. Norwegian Journal of Geology, 86, 255-264.


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Ondrasek, G., Rengel, Z., & Veres, S. (2001). Soil salination and salt stress in crop production. In A. Shanker (Ed.), Abiotic stress in plants: Mechanisms and adoptions (pp. 171-190). Retrieved from https://cdn.intechopen.com/pdfs-wm/18402.pdf

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