Risk Of Another Tsunami Disaster: Are We Adequately Prepared? 

By Murali Vallipuranathan –

Sri Lanka was the second worst hit country in the 2004 Indian Ocean tsunami with more than 30,000 deaths[1] [2]. The frequency of the earthquakes has been increasing in Sri Lanka with 9 tremors reported so far in the year 2023 [3]. The aim of this article is to analyze the possibility of another tsunami affecting our island nation and discuss the possible prevention and mitigation strategies. 

Background: earthquake and tsunami risk in Sri Lanka

Earthquakes are caused by the movement of 7 major tectonic plates[4]. Sri Lanka is located on the Indo-Australian plate that stretches from Australia to India. However, following a giant earthquake in 2012 April near Indonesia, scientists have found that this earthquake has occurred in the middle of the Indo-Australian tectonic plate and points to splitting of this plate [5]. The above map depicts the split line between Indian plate and Australian plate which is passing through the Indian Ocean close to the Southern end of Sri Lanka. Consequently the risk of developing earthquakes along this line and the risk of tsunami when an earthquake occurs in the ocean, both are increased. The last tremor affecting Sri Lanka was reported on 24.4.2023 with a magnitude of 4.4 in Richter scale and the epicenter was located 25.8km from Hambantota in the Indian Ocean[3]. 

Earlier Sri Lanka was considered as a country safe from earthquakes. However, in 2022, the island nation has witnessed 07 earthquakes, and in the last three months in 2023, 09 earthquakes have occurred and that projects to a high risk of increasing trend in earthquakes in future[6]. 

The emerging threat of local tsunami

Fortunately the last earthquake was not strong enough to produce a tsunami because tsunamis are generated by earthquakes with magnitude over 7.0 [7]. Tsunamis are classified into local tsunami and distant tsunami depending on the location of the tsunami and where it may strike the land[8]. The danger is greatest for local tsunamis because warning time is limited. A distant tsunami is generated far away from the coast, so there is more time to issue and respond to warnings. Tsunami waves reach very high speeds over 500kmph at ocean depths though it is slower in less deeper oceans [9]. As such, if we get a powerful earthquake in a similar distance as happened near Hambantota we can expect a tsunami to reach the coast in about 5 to 10 minutes before adequate warning can be given to the population at risk. 

Prevention and Mitigation strategies

First of all, the disaster management centers need to recognize the new unforeseen threat posed by the local tsunami and the increasing risk of earthquakes. Though 77 early warning towers were established in the coastal areas of Sri Lanka to protect from distant tsunamis [10], it would be impossible to evacuate and protect the population at risk from a local tsunami which can reach the coast in 5 to 10 minutes. In this context it may be appropriate to revisit the UN tsunami report published in 2005 in the aftermath of the 2004 tsunami. This report urged affected nations to establish a no-build zone up to 200m from the high tide line[11]. Though this recommendation was strictly enforced in Sri Lanka initially, several structures including tourist attractions have encroached into this no-build zone later, believing that the early warning systems developed for the distant tsunamis can give adequate protection and time for evacuation to safer regions. 

In addition the 2005 report stated that the intact coral reefs, vegetated dune systems, deep stands of mangrove forest and other natural barriers appear to have afforded significant protection to the coast beyond them. Therefore these natural barriers need to be developed and established along the coastline. In 2004 it was found that most of Yala and Bundala National Parks were spared because vegetated coastal sand dunes dissipated the tsunami’s energy. The 200 meters no-build buffer zone can be used to develop these natural barriers and therefore both strategies can be conveniently coupled together. The worst tsunami damage in 2004 occurred where mining and damage to coral reefs had been heavy in the past. Dynamite fishing and coral mining should be stopped to protect the coral reefs [12]. Further the tsunami drills and mock evacuations need to be practiced regularly and the public should be kept vigilant to the risk of both local and distant tsunamis and earthquakes. 

*Author Dr. Murali Vallipuranathan is a visiting lecturer at the Universities of Jaffna and Colombo, a Senior Community Medicine Specialist of the Ministry of Health with past experience in the Disaster Preparedness and Response Unit. He claims the opinion expressed in this article with social responsibility aims to improve the public awareness on this issue and is not related to his official positions.

References: 

1. https://www.researchgate.net/publication/236152184_Analysis_of_Plans_for_Tsunami_Related_Evacuations_in_Sri_Lanka
2. https://www.adb.org/sites/default/files/publication/157220/adbi-rp71.pdf
3. https://www.youtube.com/watch?v=p9eIKfXpbCs
4. https://earthhow.com/7-major-tectonic-plates/ 
5. https://www.yalescientific.org/2013/02/tearing-at-the-seams-the-splitting-of-the-indo-australian-tectonic-plate/
6. https://economynext.com/sri-lanka-probes-into-frequent-unexplained-earthquakes-118772/ 
7. https://www.noaa.gov/jetstream/tsunamis/tsunami-generation-earthquakes#
8. https://www.tsunami.gov/?page=tsunamiFAQ
9. http://itic.ioc-unesco.org/index.php?option=com_content&view=article&id=1164&Itemid=2031
10. https://www.adrc.asia/aboutus/vrdata/finalreport/2016B_LKA_fr.pdf
11. https://www.newcivilengineer.com/archive/build-coastal-buffer-zones-says-un-tsunami-report-2-01-02-2005/
12. https://www.fao.org/3/X5627E/x5627e09.htm#

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