Tsunami

by Prem SADDUL

 

Natural disasters will hit us at the time people have forgotten about it Dr Torahika Terada ( Japanese Physics Scientist)

  • Tsunamis in the Indian Ocean : A reminderAlthough not as frequent as in the Pacific Ocean, tsunamis generated in the Indian Ocean pose a great potential threat to all the countries of the region. Especially vulnerable are islands such as the Maldives, Mauritius, Reunion Island, the Seychelles and the low lying small atolls. Although destructive earthquakes and tsunamis have occurred in the Indian Ocean throughout geological history, most of these events have not been adequately documented, leading to the erroneous impression by many that tsunamis do not occur often enough to present a risk that requires the establishment of a Regional Tsunami Warning System in the Indian Ocean.
  •  The road map for the setting up of a Tsunami Early Warning System in the IOSoon after the horrific scenes of the December 2004 natural calamity, international agencies were mobilised to assess and to set up a mechanism for the setting up of the Indian Ocean Tsunami Warning and Mitigation System (IOTWS). Some ten such meetings were held during the first few months of 2005 and such meetings are ongoing. In January 2005, the Intergovernmental Oceanographic Commission (IOC) Secretariat submitted to the United Nations a proposal for an amount of US$ 3.5 million for the establishment, by IOC, of the early-warning component of the IOTWS. The proposal was approved and the activities started in March 2005. An International Coordination Meeting for the Development of a Tsunami Warning and Mitigation System for the Indian Ocean within a Global Framework was held in UNESCO Headquarters in March 2005. The meeting adopted a communiqué that provided guidance to all partners regarding the required actions that would lead towards the establishment of an Indian Ocean Tsunami Warning and Mitigation System. The meeting also recommended the establishment of an InterGovernmental Coordination Group for the Indian Ocean Tsunami Warning and Mitigation System (ICG/IOTWS). This was followed by the Second International Coordination Meeting for the Development of a Tsunami Warning and Mitigation System for the Indian Ocean (Grand Baie, Mauritius, April 2005). Following the meeting, an additional 20 sea-level stations were to be put in place or upgraded by October 2005. This included the installation of seven new gauges and the upgrading of 11 existing gauges. Australia also decided to establish an Australian Tsunami Warning System that will cover the Indian and Pacific Ocean coasts of Australia. Mauritius pledged to develop tsunami inundation maps. Unfortunately, these strategic maps, prepared by the Mauritius Oceanography Institute (MOI), have not been made public yet as, according to the MOI, they constitute highly confidential documents. It is good to point out here that in Hawaii, the authorities have published such maps at the front page of telephone directories for all people to take note. In addition, a study on the generation, propagation, and magnitude of tsunamis would be implemented, covering the whole of the Indian Ocean region. During the Mauritius meeting which was attended by countries of the Indian Ocean region as well as other interested member states, the IOC and observers, donor nations attending the Mauritius meeting, pledged some $5m over and above earlier contributions for specific activities linked to the tsunami warning and mitigation system. Several others, including Australia, France, Italy, Japan, Germany and the United States of America, along with the European Commission, also indicated their continuing support and their willingness to provide more financial aid as the plans for the system became more clearly defined. The Meeting adopted the ?Mauritius declaration? that outlines the way forward towards the development of an effective IOTWS.
  •  The IO Tsunami Warning System up and functioningEighteen months after the tragic tsunami of December 2004, the Indian Ocean Tsunami Warning System is now up and running as scheduled. The entire Indian Ocean region has a warning system capable of receiving and distributing tsunami advisories around the clock. With time, this initial system would be capable of improved and faster detection of strong, tsunamigenic earthquakes, increased precision in the location of the epi- and hypo centres of earthquakes and confirmation of the presence of a tsunami wave in the ocean after a strong earthquake, issuing a tsunami watch, regional watch or a global tsunami ocean warning and calling off tsunami watches and regional tsunami watches. At present, information bulletins are issued from Japan and Hawaii, pending a final decision on the location of regional centres in the Indian Ocean. This will be facilitated when important additional contribution including instruments such as deep-sea pressure sensors and satellites become available in late 2007 and 2008. The new systems need to be tested in real situations and the major challenges addressed. Limitations such as the SMS being vulnerable to saturation when they are most needed and that the siren systems may be heard on one side of a bay but not on the other are also to be addressed. However, two major challenges must still be faced. The first is to reinforce international coordination. The region cannot afford the risk of having a disparate array of national systems with little or no coordination. The open and free exchange of data and the full interoperability of national systems, is absolutely crucial for success. The second challenge is to ensure long-term investment in the system securing the downstream flow of information, from the warning centres to populations and communities at risk. A timely 100 % accurate and precise warning will not provide any protection, if people do not know how to respond to the emergency. Building national preparedness is the most difficult part of establishing Early Warning Systems.

 

  • MauritiusAlthough 80% of tsunamis occur in the Pacific basin, they can threaten coastlines of countries in other regions such as the South West Indian Ocean area. The 27 May and the 17 July 2006 earthquakes have significantly increased the risk of further large earthquakes in the Sumatra and Java region. The consequences this may have upon people living in our region and what can be done to mitigate the effects of a potential tsunamigenic earthquake is the question Mauritian authorities must focus their attention upon. For Mauritius and other islands of the South West Indian Ocean, the danger of a tsunami hitting the coastal areas would be more likely to come from the southern and south eastern segment of the Indonesian subduction arc (Central Sumatra through to the Java Trench) where the curvature faces the group of islands of the South West Indian Ocean including the Chagos Archipelago, the Seychelles, Agalega Islands and St Brandon. If a tsunamigenic earthquake of high magnitude and intensity occurs along that segment, it would generate tsunami waves that will travel southwards and south westwards towards the Mascarene Islands and would also affect Madagascar and the Comoros.

 

  • However, our islands have an advantage over the South Asian and North Australian landmass as they are located some 5 000 km from the tectonically active subduction zone and the time of travel (approx. 5-6 hours) would provide enough time for us to put forward our evacuation plan and in the process considerably reduce loss of life and property. One probability which I would like to put forward in this article is the mitigating role of ocean currents along the Equator and in the southern part of the Indian Ocean and the presence of the Mascarene Plateau which may help to either divert, deflect or-and decrease the impact of the tsunami waves on the coastal areas of the islands of the South West Indian Ocean. This is but a hypothesis which needs to be proven by appropriate modeling. The Mascarene Plateau has isobaths as high as -200 metres. Mauritius is located in the southern extremity of the plateau. The submarine relief of the plateau is a varied one. Although it is broad based in general with a wide level summit, there are fractures and faults producing a rough terrain with horsts and grabens as well as volcanic mounds. These submarine features may further help to mitigate or dissipate the waves energy of south westwards moving tsunami waves

 Other possible causes of giant destructive waves for the Indian Ocean area

We must not write off the possibility of some minor, if not major, tsunamis affecting the south western region of the Indian Ocean from the seismically active mid Indian Ocean ridge which lie some 1 500 km to the east of the Mascarene Islands. This possibility must be taken seriously. The volcanicity of La Fournaise (Réunion Island) and that of Karthala (Grand Comoro Island) is of the Hawaiian type (effusive rather than explosive) and as such do not constitute a risk factor for the generation of tsunamis. However, we must maintain a constant watch in the region north of the Antarctic continent. Owing to the phenomenon of global warming, there is a high risk that huge megablocks of ice may be explosively detached from the continent and drift northwards triggering destructive northward bound tsunamis or tidal waves that may cause havoc to the highly vulnerable coastal environment of the Mascarene Islands and more specifically to the low relief atolls of Agalega and the Chagos Archipelago. Apart from a regional dimension to an effective Early Tsunami Warning and Mitigating System for all small island states, all small islands of the Indian Ocean area including those of the Mascarene basin must immediately come out with a clear tsunami evacuation plan for all coastal communities. This has so far not been released. Such parameters as the coral reefs, the depth and width of the lagoon, configuration of bays and estuaries, sea floor topography, beach profile and the 50, 100 and 150 metre mean sea level contours are important as they determine the impact of the approaching tsunami waves and help determine:

  • the inundation prone areas (maximum horizontal distance of inland penetration of the tsunami waves) and
  • runner up (maximum vertical height above mean sea level that the sea surface may attain during a tsunami).
  • We need to urgently prepare the above along with: (iii) a bathymetric map of the near shore sea floor depth down to 200 metres for wave height predictions and storm surge calculation to facilitate response strategies,
  • the quickest escape and evacuation routes from coastal zones. Evacuation maps to be distributed to all concerned including the coastal hotels.
  • delimitation of safe zones as well as identify regions of higher hazards (bays and headlands etc).
  • Set up beach sirens for sending both alerts and for canceling them on a timely basis. Acknowledgements ■ The Mauritius Meteorological Services. ■ The Mauritius Oceanography Institute. ■

Saddul.P. 2002. ?Mauritius : Geomorphological Analysis?. Moka: MGI Press.

Glossary of terms used in the article ?

 

Tectonic plates: The surface of the Earth is divided into several thin plates ( plaques) carrying our oceans and lands. These plates are always moving at a speed ranging from 4 to 7 cm/ year forming high mountain ranges, volcanoes and earthquakes. ?

 Plate boundary: The line that separates two moving plates. ? Faulting: A fracture on the Earth surface. This happens when two plates move towards each other creating a force that breaks the earth crust causing earthquakes and tsunamis. ? Tsunamigenic earthquake: A deep sea earthquake that can cause a tsunami. ? Mitigation: That can decrease the impact. ?

Trench: A deep and narrow depression in the ocean floor. This is caused when two plates move towards each other. ? Mascarene Plateau: A long and wide plateau that rises from the sea floor and where the sea is relatively shallow. Mauritius lies to the south of the plateau. ? Horsts and grabens: An elevated block of rock followed by a depression.

Tsunamis in the Indian Ocean : a historical perspective

 1881 -A major earthquake (with estimated Richter magnitude of 7.9 ) occurred on December 31, in the Andaman Sea in the vicinity of Nicobar island. The quake generated a destructive tsunami which affected the entire Andaman and Nicobar island group and the entire Bay of Bengal region.

1883 –The 26 August explosive eruption and collapse of the Krakatoa volcano between Java and Sumatra ? generated the best known and documented tsunami in recorded history for the Indian Ocean. That particular tsunami killed 37,000 people in the islands of Java and Sumatra.

 1907- Villagers of Simeulue Island,off the coast of Sumatra, speak of a destructive tsunami that had killed thousands of people.

 1941- On June 26, a devastating earthquake (7.7 on Richter scale) occurred again in the Andaman Sea. The earthquake generated a tsunami in the Bay of Bengal. According to reports, more than 5,000 people were killed on the east coast of India alone. It is suspected that this tsunami caused many more deaths than what was reported.

1945 – An earthquake of magnitude 7.8 occurred in November off the Makran coast of Pakistan. The earthquake generated a train of 10 ? 12 meter high tsunami waves which affected Pakistan, the western coast of India, Iran and Oman, killing thousands of people and causing a great deal of destruction.

2005 -The March 2005 earthquake killed some 900 people in Java. Fortunately no tsunami waves were generated.

2006 – The 27 May earthquake (6.2 on Richter scale) off the coast of Java killed some 6,200 people and caused havoc among the people living in the tourist zone of the coastal town of Yogyakarta. Soon after hearing about the quake, thousands of panic stricken people fled in all directions fearing an incoming tsunami. This proves that they have not yet recovered from the trauma and nightmare of the December 26, 2004. Again on July 17 2006, an undersea earthquake (magnitude 6.8) rocked the land on the southern shores of Pangandaran, south of Java. This generated 2 metre waves which caused the sudden death of some 600 people. Authorities are still making body counts. This proves that nothing much is being done in the region to increase people’s preparedness for such disasters and that hard and soft technology have to go in pair to save lives.

Seismotectonics of the Indian Ocean Region and potential tsunami generating sources

For millions of years, the Indian tectonic plate has been drifting and moving in a north-northeast direction at a speed of 4 cm per year colliding with the Eurasian tectonic plate and forming the Himalayan mountain chains. As a result of such migration and collision with both the Australian and the Eurasian tectonic plates and sub plates, the Indian plate’s eastern boundary has become an active zone of seismicity and deformation, characterised by extensive faulting and numerous earthquakes that can generate destructive tsunamis. To the west, similar interaction of the Indian plate with the Arabian and Iranian microplates of the Eurasian block, has created an active subduction (*) zone along the Makran coast of Pakistan. A major fault in this region has produced several tsunamigenic earthquakes recently and in the distant geologic past. This major fault is of the same character as the West Coast fault along the coast of Maharashtra in India which is also a region that can produce tsunamigenic earthquakes. Further south, the western side of the Indian tectonic plate is bounded by the Central Indian and Carlsberg mid-ocean ridges. This is a region of shallow seismicity. To the east, subduction of the Indo-Australian Plates beneath the Burma and Sunda Plates has formed the extensive Sunda Trench ? a very active seismic region where large earthquakes and active volcanoes are frequent. The Burma plate encompasses the northwest portion of the island of Sumatra as well as the Andaman and the Nicobar Islands. The major tectonic feature in the region is the Sunda Arc that extends approximately 5,600 km between the Andaman Islands in the northwest and the Banda Arc in the east. The Sunda Arc consists of three primary segments: the Sumatra segment, the Sunda Strait segment and the Java segment. These locations represent the area of greatest seismic exposure, with earthquake of magnitudes of up to 7.75 or even more on the Richter scale ? as the 26 December 2004 proved. Finally, deltaic sediment accumulations of major rivers flowing into the Indian Ocean basin have also the potential of generating tsunamis by generating submarine landslides.

 

(*) Subduction happens when two tectonic plates converge i.e moves towards each other. When this happens, one plate move under the other causing much tension and fractures releasing explosive elastic energy causing tsunamigenic earthquakes and volcanoes.