The samoa-tonga tsunami

Abstract

     On the 29th September 2009 a magnitude 8.0 earthquake occurring on the outer rise of the Tonga Trench generated a large tsunami. The tsunami struck Samoa, American Samoa and Tonga with devastating consequences. Waves which reached up to 7.4m struck the coastlines of the islands just one minute after the initial underestimated tsunami alert. The tsunami killed at least 182 people. It had a large impact on the populations of all the islands and also had a sizeable impact upon the environment especially coral reefs. Cases of dengue fever and leptospirosis disease have increased and there is expected to be a long term impact upon the region’s economy.

Introduction

     On the 29th of September 2009 a large earthquake of magnitude 8.0 occurred in the vicinity of the northern end of the Tonga Trench. This earthquake triggered a tsunami which had a large scale impact on the islands of Samoa, American Samoa and also in Tonga. It caused many fatalities and devastated many areas. The natural disaster although only directly affecting a small region does have a global significance. How the possibility of such a hazard was treated beforehand and how people responded during and after it occurred can be used as a model to show the impacts of future similar hazards not only in this region, but in others across the earth. This report examines the cause, consequences and impact of the earthquake – generated tsunami. It also assesses the human response and whether the scale of the tsunami impact could have been less.

Cause of the Tsunami

     The tsunami was generated by a magnitude 8.0 earthquake that struck at 17:48:10 UTC at a depth of 18km (United States Geological Survey (USGS) 2009a). It occurred towards the northern of the Tonga Trench, 190 km south of Apia Samoa. Figure 1 show’s its location. The Tonga Trench marks the boundary between the Pacific and Australia plates. This is a convergent plate boundary where the oceanic Pacific plate subducts westward below the continental Australia plate at the Tonga Trench. The rate of this subduction is the fastest to be recorded worldwide with a maximum of 240mm per year at the northern end (Cousteau 2006). This makes this region one of the most seismically active in the world. Close to where the earthquake occurred the velocity has been measured to be 86mm per year (USGS 2009a) however in current worldwide plate tectonic movement this rate is still very fast. The actual earthquake occurred on a thin ridge called the outer rise to the east of the Tonga Trench (Annunziato et al 2009). Earthquakes occur in these regions, on the ocean side of a subduction zone, as normal faults are ruptured when the oceanic plate is forced to flex as it bends under the continental plate. Thus it can be deduced that the earthquake was a normal fault rupture on the outer rise of the Tonga Trench.

The earthquake was felt across the islands in the region and was reported to have shaken the ground for up to 3 minutes (Adetunji and Gabbat 2009). In Ili’ili and Tafuna, American Samoa, the intensity was measured as reaching IV. According to the USGS modified Mercalli Intensity Scale (2009b) this would mean it would have been felt by most with the disturbance of objects such as “windows and doors”, “standing vehicles being noticeably rocked” and a “sensation like [a] heavy truck striking a building”. A higher intensity was felt in Apia, Samoa of V but it was felt strongest in Faleniu, American Samoa with an intensity of VII (USGS 2009a). This could have resulted in “considerable damage in poorly built or badly designed buildings” however there are no reports that any damage was caused by the earthquake before the tsunami hit.

The Tsunami

Tsunamis are generated by a sudden vertical displacement of ocean water. In this instance an earthquake rupture in the sea floor caused water to be pushed upwards. This water collapses producing a tsunami. Through deep water the tsunami moves rapidly with speeds up to 500 km/h but heights of the waves are usually less than 1m. As it nears shore the tsunami slows because of decreasing water depth but is compressed upwards thus growing in height. These waves are then capable of pushing far inshore (Keller and Blodgett 2008).

The Pacific Tsunami Warning Centre first sent out an alert 16 minutes after the earthquake. It had, however, an underestimation of the earthquake magnitude – 7.1 – and so it was only a green alert. It was not until 20 minutes after the earthquake that the alert was scaled up to orange (Annunziato et al 2009). Both these alerts however would have come too late to help those in the regions first in the tsunamis path. Reports that water began to retreat from coastal areas beyond the coral reefs (Mercer 2009) shows the tsunami was approaching Samoa soon after the Earthquake. This report also suggests the tsunami first struck Samoa shortly after 7:00 am local time or 18:00 UTC. The exact time for the first tsunami waves to hit Samoa is thought to be around 17 minutes after the earthquake (Annunziato et al 2009) or 18:05 UTC.

Different sources provide various data for the heights of the tsunami waves. The USGS (2009a) report, lists recorded wave heights as being 314cm in Pago Pago, American Samoa and 140cm in Apia, Samoa. It also details the distance the tsunami travelled with it reaching Wellington, New Zealand where it was 11cm in height. The BBC reported waves of 5m in height (Mercer 2009) whilst the New Zealand Herald informed its readers that the island of Niuatoputapu, Tonga, experienced wave heights of 6m (Tahana 2009). With many of the sensors for measuring tidal height in the region not functioning, Annunziato et al (2009) carried out many calculations so as to try and determine actual wave height. Their results revealed tsunami heights of above 6m in some areas including 7.4m in Alaufu, American Samoa. There are also different reports on the extent to which the tsunami travelled inland causing damage. The Guardian reported the damage extending 100m inshore in Samoa (Adetunji and Gabbatt 2009) where as in eastern Samoa the tsunami caused damage up to 330m inshore (NASA Earth Observatory 2009). In American Samoa there are reports of water surging 1km inland (TVNZ 2009). By utilising all of this data it is evident that this was a large tsunami the impact of which was devastating.

The Tsunami Impact and Consequences

     The tsunami had a large impact upon the islands of Samoa and American Samoa and also proved to be destructive to the island of Niuatoputapu, Tonga. There were at least 149 killed in Samoa, 24 in American Samoa and 9 in Niuatoputapu (USGS 2009). In total it was reported to have affected around 32000 people and left close to 3000 homeless (Atayman 2009). Devastation was widespread with the destruction of tens of villages. Lalomanu, a village come holiday resort in south-eastern Samoa, was flattened by the tsunami (Mclean 2009). It was made up around 100 homes and resort huts, the majority of which were destroyed. The village of Poutasi in south-western Upolu, Samoa was another that suffered large scale destruction (TVNZ 2009). Radio New Zealand (2009) reported that along the south coast of Upolu the majority of resorts and hotels were destroyed. American Samoa’s main village of Pago Pago was “completely devastated” with only the foundations of buildings being left and a large degree of flooding. On a greater scale all of western American Samoa was left with no power. Lieutenant Governor Faoa Sunia of American Samoa reportedly said that water was limited in some areas due to damage to the water system (Samoa news staff 2009). In Niuatoputapu, home to just over 1000 people, 90% of homes had been ruined and the hospital was also badly damaged (Tahana 2009).

     The long term consequences of the tsunami will be substantial. Not only will infrastructure have to be redeveloped but other factors are going to be problematic. The loss of homes means people have been forced to live outdoors. This has resulted in people becoming more at risk of dengue fever. The fever is spread by mosquitoes and so a lack of shelter means people are more likely to get bitten. American Samoa’s only hospital has confirmed that there has been an increase in patients suffering from dengue fever with 62 confirmed cases in October and the first two deaths from the disease in 2009 (Anon 2009). The risk of polluted water being drunk is also greatly enhanced by the poor living conditions. Leptospirosis, a disease spread by the contamination of water by the urine of rats and other species has also seen a rise in confirmed cases in American Samoa, according to the same report.

The economic impact is going to be significant in the longer term. People from the rural areas of Samoa are dependent on agriculture, fisheries and eco-tourism (FAO 2009). Areas of vegetable cultivation were devastated and the tsunami swept away fishing boats and fishing gear. The destruction of holiday resorts around the coasts of the islands and that their owners are fearful of returning means many tourists are likely to look elsewhere (Coopes 2009).

The environmental impacts of the tsunami were also extensive. The Samoa Tsunami Rapid Environmental Impact Assessment Report (Ifopo et al 2009) lists much of the damage caused and what damage was expected. It includes septic tank pollution, solid waste pollution and salinisation pollution which occurred in many areas. Salinisation pollution is going to be a significant impact to the agricultural industry as crops may fail to grow in soil with a high salt concentration. It also details that there was foreshore and beach erosion, damage to wetland areas and in particular mangroves. Damage to ecosystems extended beyond the coast with marine protected areas and no take zones heavily impacted. Coral reefs were also devastated. The Associated Press (2009) reported that some coral reefs had been “obliterated” whilst others may not be able to recover.

Response to the Earthquake and Tsunami

     The first tsunami alert was issued 16 minutes after the earthquake. This does appear to be relatively fast considering the data for the earthquake had to be received and analysed. However the first alert was green – the lowest alert level – due to the earthquake magnitude being underestimated. It wasn’t until 20 minutes after the earthquake when the alert was upgraded to orange by which point the tsunami had already struck the Samoa islands. Many people didn’t realise the earthquake could have generated a tsunami and thus few moved to higher ground (Driscoll 2009). It is therefore apparent that a faster and more accurate response to the earthquake was required so as to have saved at least some of the lives that were lost. As this could save lives in the future further investigations into methods on how to analyse earthquakes more quickly are worthwhile.

     Aid response for the victims of the tsunami was issued within 2 days. The US, New Zealand and Australia provided the initial help with them all sending in planes to provide aid and assess the overall damage, the US also sent in a ship from their naval fleet to assist (Atayman 2009; Australian Government Department of Defence 2009). The American Samoa Power Authority (ASPA) delivered water to those who were affected by water shortages (Samoa news staff 2009) and the Red Cross set up camps for those who had been left homeless (Mercer 2009). The New Zealand Air Force provided medical supplies, medical teams and temporary morgue facilities to assist the hospital in Apia, Samoa (Young 2009). By October 8th all of Samoa was reconnected to the electricity grid and the main water pipes had been fixed as detailed in the 7th United Nations Office for the Coordination of Human Affairs (OCHA) situation report (2009b). The report informed that roads had been cleared in Samoa and Tonga and airports were functional. Financial aid was provided by governments and other organisations worldwide. Using information from OCHA situation reports 6 through to 9 and also 11 (2009a; 2009b; 2009c; 2009d; 2009e;) some of the funding provided can be listed as coming from; the European Union (over $2 million), the Japan International Cooperation Agency ($220 thousand), Australia ($874 thousand for Niuatoputapu), New Zealand (over $1.1 million), the European Commission ($150 thousand for Samoa) and the Asian Development Bank ($1 million). It is evident that the response to the tsunami was fast, well funded and efficient.

Education

     When researching the initial response to the earthquake it was found that local people had previously been uninterested in the dangers of a tsunami (Driscoll 2009). This is likely to have been due to there being no living memory of the last significant tsunami which occurred in 1917 (Mata’afa 2009). If they had knowledge of the consequences of a large earthquake occurring in the ocean they may have responded more quickly by moving to higher ground. In this case in order to have saved lives it would have been essential for the local people of Samoa, American Samoa and Tonga to react quickly as the tsunami alert came too late. By educating people on the affects of earthquakes and how they can generate earthquakes it may be possible to save lives in the future.

Conclusion

     The 29th September Samoa – Tonga tsunami devastated the islands and many of the people living on them. It was caused by a magnitude 8.0 earthquake which occurred on the outer ridge of the Tonga Trench. The Tsunami killed at least 182 people and affected thousands more. Much of the islands infrastructures were destroyed and the tsunami also had a significant environmental impact especially upon coral reefs. Long term consequences have already been seen with an increase in cases of dengue fever and leptospirosis disease. Economically the impacts are also going to be long lasting. The destruction of livelihoods whether it is through fishing, agriculture or tourism will be widespread. It was found that the initial tsunami alert issued was the lowest level and wasn’t upgraded until after the first tsunami waves hit. The response in the days following the tsunami however was quick effective and well funded. By educating people especially in areas where the risk of tsunami is high it is probable that lives will be saved in the future.

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