‘We know this will happen’: Inside France’s red alert plan for 15 minutes after tsunami

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Nestled on the outskirts of Paris, France’s tsunami warning center could hardly feel further from the ocean.

Yet CENALT is the very epicenter of the country’s tsunami warning network – and the place that could one day save thousands of lives.

While it’s highly unusual to see anything of the magnitude of the 2004 Indian Ocean tsunami, which killed an estimated 250,000 people, tsunamis are more common than many people realize – including here in Europe.

In the Mediterranean and in the seas connected to it, there have been around a hundred tsunamis since the beginning of the 20th century. This represents about 10% of all those recorded during the same period.

Although less frequent, tsunamis in the northeast Atlantic still represent about 5% of the total total. Among these, a particularly catastrophic event.

The Lisbon earthquake of 1755

“The most powerful tsunami we know of in the Atlantic was the one triggered by the Lisbon earthquake in 1755”, explains Hélène Hébert, national coordinator of CENALT – which stands for Tsunami Alert Center.

This earthquake had a magnitude close to 8.5, she explains, which is similar to what we see in the Pacific Ocean several times a century, but in the Atlantic it is very rare.

“While the French coasts were largely protected by the Iberian Peninsula, the tsunami destroyed most of Lisbon and Cadiz and parts of Morocco, with several thousand victims.

The resulting waves reached south west Cornwall and Ireland.

“It’s the kind of major tsunami that we can expect every three to five centuries. So it could happen tomorrow – or it could happen in the next century – but we know it will happen.

A tsunami warning in 15 minutes

Although the majority of tsunamis tend to be much smaller than this, they can still leave havoc in their wake – and lead to loss of life – which is why CENALT’s work is so important.

Celebrating its 10th anniversary this year, the center was created the day after the 2004 tsunami as part of a wider UNESCO initiative to establish tsunami warning systems around the world.

Installed in an ultramodern building operating 24 hours a day, 7 days a week, it is led by a team of specialists in geophysical data analysis from the CEA (the French Alternative Energies and Atomic Energy Commission).

In short, there are two main purposes: first, to detect any earthquake which could cause a tsunami and alert the relevant authorities within 15 minutes. Second, to let them know if there is a tsunami – and, if so, the arrival times and amplitudes.

The first information is collected from a network of several hundred seismic stations, each displayed on a giant screen in the main gallery, and which transmit data each time a tremor is detected.

The event then receives one of three threat levels depending on severity – yellow, orange or red – and the corresponding warning sent. Appropriate tide gauges are also monitored to determine the sea ​​levels.

How does tsunami detection technology work?

“When one of the seismic stations flashes red, it means that it detects a signal”, explains Pascal Roudil, technical coordinator at CENALT.

The station could simply pick up a strong wind or the passage of a truck. But if several stations start flashing simultaneously, it means that they are affected by a fairly strong seismic wave. In other words, a earthquake.

The system then tries to find the epicenter of these detections and adds the information to the map, in the form of a circle whose size represents the magnitude and the color indicates the depth.

“Our goal is to alert the relevant authorities within 15 minutes because tsunamis can arrive quite quickly,” he adds. “In the western Mediterranean, for example, if we have an earthquake near Algeria, it will cross the sea in less than an hour and 15 minutes. So that doesn’t leave a lot of time.

“Also, although our tsunamis aren’t as big as in the Pacific, you don’t need 30-meter-high waves to cause damage and injury. Even 50 cm can be dangerous for swimmers.

What is the tsunami risk in France?

Fortunately, at least so far, there have been no major incidents at CENALT.

Since its creation, the center has issued 84 “information level” alerts (no tsunami risk) and only two “alert level” alerts (waves of less than one meter on the coast). The last, in March 2021, caused the sea level in Toulon to rise by around five cm.

But we also know that it is only a matter of time.

“The French Riviera is probably one of the most dangerous areas in mainland France, with the greatest threat coming from the North African coast,” explains Hébert.

The cities and towns of the Côte d’Azur, very popular with tourists, have also mobilized to prepare the populations. Cannes has set up a digital transformation point, while Antibes is organizing evacuation drills.

“We don’t expect 20m waves, like in JapanChile or Sumatra, but rather one to two meters,” adds Hébert.

However, what is extremely dangerous is not only the altitude of the tsunamis, but also the currents and the water flows – and the flood enter the shore – which can damage beaches, harbors and streets.

Looking to the future, while analysts can be rightly proud of their work at CENALT over the past decade, they are far from standing still.

Although the technology has advanced considerably, they are eager to continue their own research, especially because the rising waters caused by the climate crisis could make matters even worse.

The effect of the climate crisis

“While the water level does not affect the tsunami per se – as an earthquake can occur regardless of sea level – it can certainly impact the coastline,” says Hébert.

“For example, if it is a small port and the waterfront is very low, as in walking sticks or Toulon, a tsunami could be more treacherous.

“So in the years to come it would be great to produce real-time forecasts – as we do now with weather – using numerical simulation. For example, to predict that in Nice we can expect 3.5m, or in Marseille around 0.5m etc, for a specific event.

One more reason, therefore, why CENALT will undoubtedly remain at the forefront of tsunami warning technology. And not just for the next 10 years, but for many more to come.

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