Tonga is located along the Pacific “ring of fire”, an extremely active seismic zone.  In 2009, a tsunami generated near Samoa devastated the island of Niuatoputapu in Tonga killing people and destroying over half the houses.  Though the country’s main island of Tongatapu was not affected, this event raised serious concerns on the country’s ability to effectively respond. 

In light of the destruction from the tsunami in 2009, the Government, working with the Secretariat of the Pacific Community and Geoscience Australia, developed a tsunami inundation model in 2012 to help visualise the potential impact of large tsunamis generated from the Tonga Trench on the main island of Tongatapu. 

The scenarios used for the model included tsunamis generated from large magnitude earthquakes (8.7) occurring along the Tonga Trench as well as from an earthquake with characteristics similar to the 2011 Tohoku earthquake in Japan (9.0). Numerical modelling was used to show that such an event could cause substantial tsunami inundation in Nuku’alofa, having a maximum depth of 5m.

Using the initial sea surface displacement generated by a magnitude 8.7 earthquake along the Tonga Trench and high resolution surface models of both the terrain and nearshore bathymetry, the numerical model showed that at 20 minutes after the earthquake, the tsunami would likely flood most of the northern part or the CBD of Nuku’alofa to a depth of 5 metres or more. This large initial inundation is followed by subsequent inundation waves at about 40 and 85 minutes respectively. By assuming bare earth topography, discounting the effect of buildings and vegetation, the model provides a worst case scenario to guide the evacuation plans for Nuku’alofa.  The numerical tsunami model further shows four high spots within the CBD that is not inundated and could be used as evacuation zones.   

The NEMO has used this information to engage with communities in identifying and preparing evacuation routes and zones. Areas that have no road access are now being prioritised for implementation starting from 2014. 

High rise buildings within the CBD will also be considered as evacuation zones once they are certified to be structurally sound.  In addition to the community preparedness activities, tsunami sirens have also been installed in high risk areas.