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In case of a tsunami

Rajendra Prasad
Saturday, November 04, 2017

The UN General Assembly has declared November 5 as the World Tsunami Awareness Day each year to promote a global culture of tsunami awareness. The proposal for this came in 2015 from Japan which has experienced the greatest number of tsunamis to date.

The March 2011 Tohoku, Japan devastating event still looms in the minds of people across the globe.

The 2017 World Tsunami Awareness Day marks efforts to "reduce the number of affected people" when a tsunami strikes. This theme takes inspiration from Target (b) of the Sendai Framework for Disaster Risk Reduction, which aims at reducing the number of people affected globally by disasters.

Irina Bokova, director general of the United Nations Educational, Scientific and Cultural Organisation (UNESCO) says: "Education, community awareness of tsunami risk and preparedness are essential to act and react in case of a tsunami. All vulnerable regions need to adopt and develop more effective tsunami warning systems.

"A perfect warning is useless if people do not know what to do, cannot recognise natural warning signs or do not evacuate immediately to higher ground."

The Intergovernmental Oceanographic Commission (IOC) of UNESCO co-ordinates national and regional tsunami early warning services, raising global awareness about effective actions, policies and practices to reduce exposure to disaster risk through its four Tsunami Warning and Mitigation Systems for the Pacific, Indian Ocean, Caribbean, and North-Eastern Atlantic, Mediterranean and Connected Sea regions.

The IOC also assists member states through education programs and regular tsunami communication and evacuation exercises, increasing the coordination, readiness for and understanding of tsunamis among citizens and communities around the world.

The Pacific Tsunami Warning and Mitigation System (PTWS) has recently undergone substantial changes with countries now placed in the forefront in terms of making warning calls for their respective communities, instead of relying largely on the Pacific Tsunami Warning Centre (PTWC) to issue warnings and watches as was the case in the past.

The PTWC now provides tsunami threat messages accessible by all, and graphical and other enhanced products accessible to the national tsunami warning focal point(s) designated with IOC to assist authorities in further assessing tsunami threat and providing warning service to coastal communities.

Here at home, Government has taken a massive drive to improve co-ordination of tsunami warning and response with the launching of the Fiji Tsunami Response Plan during this year's National Disaster Awareness Week commencing October 13. The theme for this year's Disaster Awareness Week was appropriately chosen to "Be Tsunami Ready".

The Pacific's Ring of Fire is always active and so it is not a matter of "if", but "when" a destructive tsunami would occur.

The big tsunami of 1953 in the Suva Peninsula resulted from a 6.7 magnitude, shallow depth earthquake that caused a slip in the seabed in the harbour area. Typically, a near 7.0 magnitude and above earthquake has potential to cause a local tsunami while an earthquake of over 7.5 magnitude causes a tsunami that can travel regionally or long distances.

Besides magnitude, depth of an earthquake is a critical factor in causing a tsunami with the PTWC generally using less than 100km source depth to issue initial threat message. Notably, a number of recent near 7.0 magnitude earthquakes in the Southwest Pacific region have occurred at very shallow depth of around 10km to trigger a small tsunami. It includes the 7.0 magnitude earthquake which occurred 285 km west-southwest of Suva on January 3 this year, triggering a small tsunami. This should be of great concern as it is only a matter of time for a big one!

The 2011 Tohoku, Japan earthquake and 2004 earthquake off the west coast of Sumatra, Indonesia measured around 9.0 magnitude while the 1960 Chile earthquake measured 9.5 magnitude, the latter being the most severe earthquake recorded in recent history. All three events triggered major tsunamis which travelled long distances to cause severe destruction and loss of lives.

But then, events like the Kaikoura, New Zealand earthquake of November 2016 should make us realise that earthquakes triggered on land close enough to the sea can also cause a tsunami! In the Kaikoura case, a 7.8 magnitude earthquake with epicentre on land caused a series of complex faults that eventuated in a 2.5 metre recorded tsunami and a 5m observed tsunami run-up.

With all tsunami-prone areas now having a well-functioning regional tsunami warning and mitigation system co-ordinated by the IOC of UNESCO, the times of "lack of warning" or "over warning" should be virtually gone!

Continued improvement in seismic data and analysis techniques by regional tsunami services providers like the PTWC have enabled them to issue the initial tsunami threat messages within 7-8 minutes of earthquake occurrence. At the same time, the deployment of tsunami model forecasts allow them to place countries under the various threat categories; less than 0.3m waves, 0.3m to 1m waves, 1m to 3m waves and over 3m waves in subsequent bulletins.

More detailed forecasts, threat maps and other enhanced products are made available to country tsunami warning focal points, should enable them to make sound warning decisions and finetune forecasts of potential tsunami threat, and advise people on appropriate response measures including any necessary evacuation of coastal communities.

However, in the event of a strongly felt earthquake (where it is difficult to stand upright) one should immediately move to high ground as a formal warning may be too late!

The PTWC and International Tsunami Information Centre based in Hawaii have helped in running tsunami simulations for most potential tsunami sources in the Pacific Ring of Fire generally using 9.0 magnitude earthquake as the worst case scenario. The results of this exercise have been presented during regional and in-country training on improving tsunami warning and response organised by IOC of UNESCO with support from US NOAA, JICA, SPC and other partners.

In summary, the results indicate that:

* For a locally generated tsunami (severely felt earthquake within Fiji region) one should be at least 20m above normal high tide level;

* For a tsunami from a regional source (1-3 hours tsunami travel time, for Fiji the source would be Tonga, Samoa, Solomon Islands and Vanuatu), one should be at least 10m above normal high tide level; and

* For a long distance or tele-tsunami (tsunami travel time of more than 3 hours), one should be at least 5m above normal high tide level.

As already mentioned the results are for a magnitude 9.0 earthquake and do not take into consideration orientation of the coastline (bays, estuaries, etc.) and topography. A magnitude 9.5 earthquake like the 1960 Chile one, though rare, will have much more devastating potential! Nevertheless, the results should help planning for most occurrences and convey the message to people living at higher levels not to panic and cause unnecessary traffic and other congestions!

The challenge, however, is especially in atolls where there is no high enough ground to evacuate to. In such cases, vertical evacuation is the only option with evacuation centres recommended to be tall enough. But, it should be consoling to note that tsunami inundation in atoll situations is not expected to be as bad as in island with gradual rising coasts and shallow bathymetry (slope of seabed) which can result in piling of water and high runup as a result!

In conclusion, the threat of a tsunami is real and people living in or visiting coastal areas should heed both natural warning signs and formal alerts! At the same time, the results and thresholds mentioned above should help peoples' preparedness thus reducing unnecessary panic!

* Rajendra Prasad is the UNESCO/IOC program officer for DRR/TW. Views expressed are his and not of this newspaper. Email: rajendrap@spc.int or r.prasad@unesco.org.








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