Island events recap

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Date: October 2009 - POLYNESIA

TSUNAMI
I survived the tsunami red alert in Tahiti, after the Samoa earthquake

1. Events chronology
Following the earthquake that hit the islands of Samoa on September 29, a tsunami of about 90 cm (3 ft) was annouced in the morning in the Polynesian islands. The warning was quickly given around 9:30AM and sirens were heard in all archipelagos. .
It was namely in Tahiti, Moorea and in the Marquesas, that the usual precautions were best followed. In the small capital city, the telephone networks (mobile and land) were unfortunately made out of order, in a few minutes, something that caused a big mess in the Papeete urban zone: worried parents could not call their children nor their schools, and preferred run to rescue the kids, while forgetting the most elementary safety rules, rather than trust the schools' personnel.
A surrealistic rush to each district's hills then followed with a memorable slalom through lines of idling cars!

At the forecast time, around 10:40 AM, a first 25cm (10") wave reached the coasts of Tahiti and Moorea, followed a little later by two others of the same size. Local radios, the only means of communication still working, kept the population informed in real time. A little after noon, the warning was lifted and everybody could return to their place of work or their safe "home sweet home". The Marquesas were hit by waves two hours later, and there also, no damage and no victims were reported, despite higher waves (about 120 cm, 4ft) which caused a slight scare after a while to some islanders.

For public authorities, this warning, although small, was a full size rehearsal of what it would be the day when nature will really get mad. And since the overall recommendations had been followed, Polynesia can be proud of the exemplary reactivity of its population.
But how much ink flowed for so little water!

2. Better understanding what happened
Concerned public services still raise questions to better understand Why did a tsunami measuring 4 to 6 m (12 to 18 ft) in Samoa, and caused so many casualties in Apia and in Pago Pago, measure no more than a few centimeters when reaching French Polynesia? .

The most logical hypothesis might be the presence of coral reefs (around most parts of the Polynesian islands, except in the Marquesas) which considerably tempered the acceleration of the wave when it impacts the land.
In the high seas, it is practically impossible to see a tsunami waves, which look only like a slight set of swells.
It is when they approach land that tsunamis become dangerous - or not - in function of the terrain (more or less wide bays and coastal zones, mountains near the shore, deep and narrow valleys, etc.). A real wall of water can be created when reaching the coastline, a phenomenon created by the quick rising underwater bottoms when reaching the shore. But coral reef "break" such wall of water, under the condition however that the wave is perpendicular to the coast. When it arrives parallel to the shore, it causes little damage, if any.
This is why, during the Christmas 2004 tsunami, the coasts perpendicular to the quake's waves caused the tsunami (Sumatra, Thailand and Malaysia to the east, Sri Lanka, India and even Africa to the west) suffered considerable damages, while Bangladesh for example, even closer but to the north, was spared.

In French Polynesia, the major part of the coasts are protected by very impressive coral barriers which most likely had a protective role when they broke the (small) wave, like the Papeete dike would do also (3.2 km long and 9 m high) in case of a tsunami coming from the north. Furthermore, the height of this same wave when it reached the Marquesas, was measured between 1 and 2 meters, depending on the place, most likely because these islands' coasts don't have any reef. But other reasons, not well known, must also be taken into consideration.
Who knows today what is really happening inside a tsunami moving at 800 km/h (500 mph) through the Pacific Ocean?  

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