[Un article de The Conversation écrit par Dee Ninis – Sismologue, Université Monash – & John Townend – Professeur de géophysique, Te Herenga Waka — Université Victoria de Wellington]
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Tsunami alerts and evacuations have been launched in Russia, Japan and Hawaii, and opinions were issued for the Philippines, Indonesia and even New Zealand and Peru.
The whole Pacific region is very exposed to powerful earthquakes and the resulting tsunamis, because it is located in the “fire belt”, an intense seismic and volcanic activity area. The ten most powerful earthquakes ever recorded in modern history have all occurred in the belt of fire.
Here is why the tectonics of the plates makes this part of the world so unstable.
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Why is Kamthatka affected by such violent earthquakes?
Off the Kamchatka peninsula is the Kouriles pit, a tectonic border where the peaceful plate is pushed under the Okhotsk plate.
While the tectonic plates move continuously compared to each other, the interface between the tectonic plates is often “blocked”. The tension linked to the movement of the plates accumulates until the resistance of the interface is exceeded, then is released in the form of a sudden rupture: an earthquake.
Due to the large area of the interface at the borders of the plates, both in length and in depth, the rupture can extend over large areas on the border of the plates. This gives rise to some of the most important and potentially destructive earthquakes in the world.
Another factor that affects the frequency and intensity of earthquakes in subduction areas is the speed at which the two plates move with respect to the other.
In the case of kamthatka, the peaceful plate moves to around 75 millimeters per year compared to the Okhotsk plate. This is a relatively high speed for tectonic plates, which explains that earthquakes are more frequent there than in other subduction zones. In 1952, an earthquake of magnitude 9.0 occurred in the same area of subduction, only about 30 kilometers from the 8.8 magnitude earthquake today.
Among the other examples of earthquakes on the border of a subduction plaque, we can cite the 9.1 magnitude earthquake which struck the Tohoku region in Japan in 2011 and the magnitude 9.3 earthquake which struck Sumatra and the Andaman islands in Indonesia on December 26, 2004. These two earthquakes started at a relatively low depth and caused a break in the limit of the plates surface.
They raised one side of the seabed compared to the other, moving the entire column of water from the ocean above and causing devastating tsunamis. In the case of the Sumatra earthquake, the breakdown of the seabed occurred over a length of approximately 1,400 kilometers.
What will happen now?
As we write these lines, around six hours after the earthquake, 35 replicas of magnitude greater than 5.0 have already been recorded, according to the US seismic surveillance service (the United States Geological Survey, USGS).
The replicas occur when tensions in the earthly crust redistribute after the main earthquake. They are often of lower magnitude from one point to that of the main earthquake. In the case of today's earthquake, this means that replicas of magnitude greater than 7.5 are possible.
For an earthquake of this magnitude, replicas can continue for weeks, even months, but their magnitude and their frequency generally decrease over time.
Today's earthquake has also caused a tsunami that has already touched the coastal communities of the Kamchatka, Kourile Islands, and Hokkaido islands in Japan.
In the coming hours, the tsunami will spread across the Pacific, reaching Hawaii about six hours after the earthquake and continuing to Chile and Peru. [ndlt : à l’heure où nous effectuons cette traduction, les alertes à Hawaï ont été réduites, et annulées aux Philippines. Les vagues ont atteint la côte ouest des États-Unis, jusqu’à un mètre de hauteur en Californie et dans l’Oregon.]
Tsunamis specialists will continue to refine their models from the effects of tsunami as it spread, and the civil protection authorities will provide authority on the expected local effects.
What lessons can we learn from this earthquake for other regions of the world?
Fortunately, earthquakes of such magnitude are rare. However, their local and global effects can be devastating.
In addition to its magnitude, several aspects of the earthquake that has struck Kamthatka today will make it a particularly important research subject.
For example, the region has experienced a very intense seismic activity in recent months and an earthquake of magnitude 7.4 occurred on July 20. The influence of this previous activity on the location and the moment of today's earthquake will be a crucial element of this research.
Like kamthatka and northern Japan, New Zealand is located above a subduction area, and even two subduction zones. The largest, the Hikurangi subduction area extends off the east coast of the northern island.
According to the characteristics of this tectonic interface and the geological archives of past earthquakes, the Hikurangi subduction zone is likely to produce earthquakes of magnitude 9. It has never happened in history, but if it happened, it would cause a tsunami.
The threat of a major earthquake in a subduction area is never dismissed. The earthquake that has occurred today in Kamchatka is an important reminder for all those who live in seismic areas to remain cautious and take into account the warnings of the civil protection authorities.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
