The Earth shakes when there is an earthquake because, under the effect of plate tectonics, tensions accumulate in the rocks of the Earth's crust. When these tensions are suddenly released, there is a release of energy in the form of seismic waves, causing the earthquake.
Our Earth's crust is not a fixed structure; it is divided into gigantic pieces called tectonic plates. These plates move slowly across the surface of the Earth's mantle, thanks to very subtle but constant movements of the semi-fluid rock located just beneath. Sometimes the plates gently move apart, other times they collide or slide past each other. Their movements generate enormous tensions that, when released all at once, cause a violent motion at the surface: this is what is called an earthquake.
Due to the continuous movement of tectonic plates, giant pieces of rock are constantly sliding, pressing, or getting stuck. But sometimes they remain trapped: pressure gradually builds up over huge periods of time, creating an accumulation of elastic energy. When the tension becomes too great, it suddenly releases. Imagine a rubber band that is stretched for a long time and then suddenly released: all that accumulated energy is released abruptly. This release of energy causes seismic waves to propagate in all directions from the seismic focus, where everything has given way. It is this sudden release of energy that makes the ground shake during an earthquake.
When an earthquake violently releases its accumulated energy, it generates seismic waves that propagate through the Earth somewhat like waves in water when you throw a stone. These waves exist in two main forms: P (primary) waves, which are very fast and compress and stretch the rocks, and S (secondary) waves, which are slower and shake the ground transversely. As they pass through different layers of the Earth, each wave travels at its own speed and slightly alters its trajectory depending on the materials encountered. When these underground waves reach the surface, they create the tremors that we feel directly under our feet, stronger or weaker depending on your distance from the earthquake's epicenter.
The Earth's surface is made up of large rigid plates, somewhat like a giant puzzle called tectonic plates. These plates move slowly but surely, generally only a few centimeters per year. They can pull apart, come together, or slide against each other. When two plates rub against each other or collide without being able to slide smoothly, it gets stuck, pressure builds up, and energy is slowly stored. At some point, it suddenly releases: this sudden movement causes earthquakes. Most earthquakes occur at the boundaries of these plates.
When it shakes hard, the first affected are often the human constructions, with many houses, buildings, roads, or bridges damaged or even destroyed. These destructions unfortunately often lead to injuries or loss of life, particularly in densely populated areas. The natural environment also takes a hit: landslides, cracks in the ground, collapses, and sometimes even tsunamis or significant damage to vegetation and animal habitats. Essential service networks such as drinking water, electricity, or gas can be severely disrupted, complicating crisis management. Finally, earthquakes often leave psychological scars on people, with lasting stress and fear.
Seismic waves generated by earthquakes are used by scientists to study the inaccessible interior of the planet, thereby enhancing our understanding of the various internal layers of the Earth.
Animals sometimes seem to detect an earthquake before humans do. Some researchers believe they can sense the initial vibrations or perceive subtle changes in the Earth's electromagnetic field, thereby alerting to the impending phenomenon.
After a major earthquake, the Earth often continues to shake: these secondary tremors, known as aftershocks, can last for hours, or even days, and in some cases, even several weeks after the main event.
On average, around 500,000 earthquakes are detected each year worldwide, but only a small fraction—about 100,000—are strong enough to be felt by humans.
During an earthquake, it is recommended to take shelter under a table or a sturdy piece of furniture, to avoid windows and objects that may fall, to refrain from rushing outside, and once the shaking has stopped, to calmly evacuate the building in case of obvious structural damage.
The focus (or hypocenter) of an earthquake is the precise point underground where the seismic rupture begins. The epicenter, on the other hand, is the point located on the Earth's surface directly above the focus, where the effects of the earthquake are typically felt the most.
Aftershocks are secondary earthquakes that occur following the main earthquake as the Earth's crust gradually stabilizes. They result from the readjustment of stresses in the Earth's crust after the main event.
A magnitude scale measures the energy released by an earthquake. The most commonly used scale is the Richter scale, which is logarithmic, meaning that an earthquake with a magnitude of 6 releases approximately 30 times more energy than an earthquake with a magnitude of 5. Thus, each increase of one unit represents a significant additional release of energy.
No, it is currently not possible to accurately predict the exact date and time of an earthquake. However, scientists can identify at-risk regions and estimate probabilities over the medium or long term.
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