Although it is impossible to predict when major earthquakes will occur, some regions of the world are still much more prone than others to these types of natural disasters. Mapping of the areas with the greatest seismic risks.

A variety of every day earthquakeearthquake happens all over the world. In France alone, around 600 earthquakes are registered every year. This activity is closely related to tectonic movements, more specifically to the stresses and strains caused by the movement of the plates. lithosphericlithospheric relative to each other. It is enough to look at a seismicity map of the world to see that the foci of earthquakes are indeed very much at the level of the plate boundaries.

A globally unequally distributed risk

The distribution of seismicity on Earth is therefore very uneven: regions that are on or near an active plate boundary are more prone to earthquakes, while other regions of the Earth that are far from the plate boundaries are relatively calm, such as . B. Siberia, Canada or Central Africa.

It is therefore mainly the tectonic context that determines the seismic hazard of a region and especially the proximity of a large region active erroractive error. But even here there are differences. Because not all plates move at the same time speedspeed still in the same way to each other. If some faults regularly absorb this deformation in the form of small, regular and frequent earthquakes, others will tend to block and accumulate the tectonic stress for a long period of time before suddenly releasing it and then producing a powerful earthquake. The assessment of the seismic hazard therefore requires taking into account the amount of deformation to which the rocks are subjected, the return time of the earthquakes on the faults concerned, but also the geologygeology The side.

Subduction zones are the regions most at risk from strong earthquakes

In general, therefore, the most violent earthquakes occur at the level of subduction zones, where two plates collide, one sinking below the other. This convergent dynamic involves an intense accumulation ofenergyenergy at the level of zones, often blocked at different depths. The periphery of the Pacific Plate, which is characterized by numerous subduction zones, is therefore exposed to a particularly high seismic hazard.

Mountain ranges formed by the collision of two crowdscrowds Continents also experience severe earthquakes after a subduction episode. These highly compressive regions are riddled with numerous faults that regularly produce strong earthquakes. SizeSize. The Himalayan mountains and their surroundings are particularly affected.

In other parts of the world, the plates do not sink under one another, but move laterally relative to one another. These are the limits of leaf slip or thrust plates. The amount of strain accumulated along these large faults can be enormous and the earthquakes, usually shallow, also very strong.

Earthquake risk: the importance of construction quality

However, if the tectonic context takes precedence in determining the seismic hazard of a region, the assessment of the seismic risk in turn requires taking into account the vulnerability of the population faced with the occurrence of an earthquake. The population density, but above all the quality of the buildings and the ApplicationApplication standards of constructionconstruction paraseismic are then essential.

Japan and Indonesia

Despite draconian and proven seismic standards, Japan and in particular the city of Tokyo with its very high population density remains one of the regions with the highest seismic risk in the world. Its east coast is particularly threatened by tsunamis. With a magnitude of 9.1, the 2011 earthquake killed around 16,000 people, mainly due to the vaguevague generated tsunamis. Thanks to his politics preventionpreventionthe country manages to significantly limit the number of victims involved breakdownsbreakdowns.

Just like Japan, Indonesia sits on the boundary of three tectonic plates that are undergoing subduction. Their displacement here is enormous, often producing earthquakes greater than magnitude 8 which can produce devastating tsunamis, as in 2004. The magnitude 9.1 earthquake then caused 230,000 deaths due to the very high population density at the coastal level rather than one proper installation to fight against the tidal waves.

The Antilles and the Mediterranean

The West Indies is a complex and very active tectonic zone and a region where the seismic risk is also very high, mainly due to the vulnerability of the population, especially the lack of paraseismic standards. For example, in 2010, Haiti experienced absolute tragedy after a 7.3 magnitude earthquake that claimed the lives of more than 220,000 people. The reason: collapse of buildings.

Although the tectonic context is completely different, Turkey presents the same problem. The country has been affected by several major land faults, on which very densely populated cities are located and whose buildings cannot withstand strong shaking, as the events of February 2023 sadly showed. movementsmovements Tectonics make the Mediterranean a zone of high seismic risk, particularly from tsunamis. In addition to Turkey, the populations of Greece and southern Italy are particularly threatened.

California

Other kickback zonekickback zone Major, the San Andreas Fault in California, on which two important cities are located: San Francisco and Los Angeles. With stresses accumulating since the last major earthquake in 1908, the west coast of North America is preparing for a very large seismic event, commonly referred to as the “Big One”. However, it is not possible to predict exactly when it will take place.

The Andes and regions of the Himalayan Mountains

Despite some efforts to bring buildings up to seismic standards, Chile and Peru remain countries at significant seismic risk due to subduction of the Pacific Plate beneath the South American Plate. In 1960, Chile recorded the strongest earthquake in modern history with a magnitude of 9.5!

On the Himalayan chain region side, it is the city of Kathmandu, the Nepalese capital, that faces a very high seismic risk. Studies fear a potential magnitude 9 earthquake in this densely populated region of substandard buildings.