These are called “
mega-earthquakes
”.
These major earthquakes, with a magnitude greater than 8 on the Richter scale (the CNRS puts forward the figure of 8.5), return every 100 to 200 years, according to experts.
As two powerful earthquakes, with epicenters located in Gaziantep (magnitude 7.8) and Ekinozu (magnitude 7.5) hit Turkey and Syria, killing more than 45,000 people, fears of a mega-earthquake are rekindled in Anatolia.
Faced with these devastating natural phenomena, the problems of seismic standards and the preparation of the population are resurfacing in the world.
The question of preparing for mega-earthquakes is not new.
Already in 1755, the devastation of Lisbon by a mega-earthquake of about 8.5 magnitude shook Enlightenment Europe.
At the time, philosophers developed for the first time the idea that man, by settling in risk areas, was responsible for disasters.
Rousseau then exclusively accuses the man of having built a coastal city on an earthquake zone, while Voltaire accuses an unfortunate combination of circumstances.
Affected by an interior torment, the latter published a writing made famous:
Poem on the disaster of Lisbon
.
Since then, in the face of multiple disasters, countries have been preparing in a unique way: today, "
paraseismic standards change according to political calculation
", explains Didier Combescure, president of the French Association of Paraseismic Engineering (AFPS), “
with each new disaster, we change the codes and we assess the consequences on the economy.
Among the most exposed regions, Turkey, Japan and California are tackling the difficult equation between human protection and material damage, with disastrous consequences for the economy.
The peculiarities specific to each country change the propensity of a country to protect itself from risk, more or less strongly: socio-economic context, recurrence of earthquakes, vigilance of the real estate sector and above all the education of the population.
Turkey facing corruption
Caught between three tectonic plates (Eurasian, African and Arabian), Turkey raises questions because of its cruel lack of preparation, even its negligence in the face of repeated earthquakes.
The shadow of a mega-earthquake hangs over Istanbul.
The metropolis of 16 million inhabitants is located on the only segment of the North Anatolian fault, in the Sea of Marmara, which has not broken since 1766, or 253 years.
“
There will be considerable damage
,” predicts Éric Fouache, professor of geomorphology at Sorbonne University.
In Istanbul, only 30% of buildings have a building permit.
Urbanization there is uncontrolled, in particular because of the gecekondus, the Turkish slums.
The specialist nevertheless makes the difference between the old town “
with old houses more or less restored
" and the modern city "
with a higher quality of construction
".
The country is nibbled by "
corruption, informality and spontaneous habitats
", continues the professor.
Real estate developers are singled out in Turkey, where the real estate frenzy for twenty years has got the better of the seismic standards, yet mandatory since the Izmit disaster in 1999. For ten years, there has been an increase of almost 50% companies in the sector.
"
The problem is the control of the construction at all stages
", adds Didier Combescure who nevertheless remembers having seen, during a post-seismic mission in 1998, "
a clear desire to change things in the universities, with highly recognized scientific experts
".
Builders often go for the cheapest in Turkey with poor quality concrete or too thin steel rods.
Éric Fouache nevertheless tempers: “
The legislative framework is going in the right direction.
“After the decried anti-seismic tax of 2002, the State indeed seems to be changing its approach “
with the establishment of mandatory seismic insurance for all private buildings in 2021
”.
Panoramic view of the Bosphorus Strait, in Istanbul, Turkey.
Adobe stock - Mario Savoia
California at the awareness challenge
The fear of a mega-earthquake is also palpable in California, located at the junction of the North American and Pacific plates.
The terrible San Andreas fault could also wake up.
The northern and southern segments are particularly monitored and would directly affect San Francisco and Los Angeles.
The last major earthquakes in the two cities date respectively from 1906 (magnitude 7.9) and 1857 (magnitude 8.3), ie 117 and 166 years ago.
Faced with the "
Big One
", this powerful earthquake expected in the region, raising public awareness remains California's greatest challenge, which every year since 2008 has invited residents to an earthquake simulation exercise: the "Great California
Shakeout
".
Property damage is also taken seriously by the US state, especially since the financial sinkhole of the Northridge earthquake in 1994. At the time, the more than 30 billion euros in damage was a tipping point in California: "
In addition to the protection of men, the notion of economic protection has been introduced
", explains Didier Combescure, "
the reference earthquake has increased for all buildings: 475 years for homes, 975 years for hospitals, 3,000 years for nuclear power stations
”, that is to say the degree of resistance of a building.
The higher the "
reference earthquake
", the more the building has strict seismic standards.
Indeed, the later an earthquake returns, the more powerful it will be due to the accumulated energy.
The doctor-engineer also points to the “
enormous work on motorway bridges, rebuilt in reinforced concrete since the 1990s.
” In addition, unlike Europe, hospitals are subject to “
earthquake certification for each material
", he explains before finishing: "
The great heights are well prepared, unlike the old buildings not up to standard.
»
Panoramic view of skyscrapers and freeway bridges, in Los Angeles, California, USA.
Adobe stock - PixilRay
Japan facing the risk of the sea
Located on the “
Pacific Ring of Fire
” like California, Japan amazes with its quasi-military preparedness for disaster.
It is necessary for this archipelago shaken every five minutes, the very large majority being "micro
-earthquakes
".
At the crossroads of three tectonic plates (Eurasian, Philippine and North American), the country fears a major mega-earthquake in Tokyo and its region.
The Nankai Trough (that area where the Philippine Plate dips beneath the Eurasian Plate) has been dormant since 1923, or 100 years.
But on the Japanese archipelago, the Japanese fear the earthquake less than the tsunami it causes.
In 2011, the “
megatsunami
” caused by a mega-earthquake left more than 20,000 families in mourning.
At the time, the skyscrapers withstood earthquakes of magnitude 9
well
.
“
Old towns as we know them in Europe are more limited, due to reconstruction after the war or the various earthquakes.
»
The Land of the Rising Sun impresses with its ingenuity in earthquake-resistant constructions.
The Japanese have understood, especially since the Kobe shock in 1995, that an earthquake causes buildings to move horizontally, but also vertically.
Japanese buildings are therefore always subject to one of three seismic standards: "
Taishin
" for medium-sized buildings (minimum thickness of walls, beams and pillars), "
Seishin
" for large buildings (shock absorbers, such as rubber plates, between the ground and the foundations), “
Menshin
” for skyscrapers (isolation of the structure from the ground, thanks to layers of lead, steel or rubber).
Read alsoFukushima: the story of those days that shook the world
Japan also has a head start on warning residents.
It only takes a few seconds for the authorities to alert of a tremor detected on the archipelago.
“
Jishin desu!
» or in French «
There is an earthquake!
is an example of messages sent by the J-Alert automatic sending system to prevent violent disasters (tsunami, volcanic eruption, etc.) or urgent threats (war, terrorism, etc.).
It must be said that the Japanese are convinced that one day or another, a disaster will overwhelm their country.
In the meantime, all "
alert systems have been corrected since 2011, which had falsely detected two earthquakes instead of just one, and therefore had not seen the fifteen-meter waves coming
",
Panoramic view of skyscrapers in Tokyo, Japan.
Adobe stock - Nick
