Could someone explain the phenomenon of earthquakes?
The continents move
The earth shakes hundreds of times every day. Such tremors are evidence of the enormous forces that reign inside our planet. But why is the earth shaking at all?
Earthquakes have always been scary to people. The ground beneath your feet begins to shake, cracks open and houses collapse. For millennia, the wrath of the gods was blamed for this. Many modern scientists have also invented theories, some of which seem quite ludicrous, of how earthquakes could arise.
It was not until the beginning of the 20th century that the solution was taken that decisive step closer. The German geoscientist Alfred Wegener proposed in 1912 that the individual continents are not firmly anchored on the earth's crust, but move.
At first, Wegener was ridiculed for this by his colleagues. But with his considerations he laid the foundation for the theory of plate tectonics, which was not decisively further developed until the 1960s.
The mantle is the engine
It is now known that the top layer of the earth, the earth's crust, consists of several large and numerous smaller crustal plates.
These plates move away from each other, rub against each other, or one plate is pushed under the other, so that the continents lying on them also move. The so-called continental drift is caused by the material flows in the softer earth's mantle, which is below the earth's crust.
The material flows in the earth's mantle are so-called convection flows. They ensure that hot, liquid magma rises to the surface and thus drives the plates.
You can also see something like this when you pour milk into a cup with very hot coffee. Small convection cells immediately arise from rising and falling milk. The rock masses move according to the same principle in the earth's mantle.
Tensions at the plate boundaries
At the boundaries of the plates, the hot magma from the earth's interior can break up the earth's crust and find its way to the earth's surface.
In the area of the mid-ocean ridges, which span the earth for more than 60,000 kilometers, the plates are drifting away from each other. Liquid magma moves up from the earth's mantle and forms a new oceanic crust. The earth grows.
When the oceanic crust is rigidly connected to a continent, it pushes it forward. Since the earth always remains the same size in its expansion, it has to "shrink" again in other places. Geoscientists call these places subduction zones.
The immense pressure that is exerted on the joints between the panels pushes one panel under another. Due to faults in the rock, the plates in these zones often get caught in one another.
But since they keep moving, the pressure eventually becomes too great and the rock breaks suddenly. The earth is shaking. Nine of the ten strongest earthquakes in the 20th century took place in these subduction zones.
Continental plates can, however, simply move past each other due to so-called transform faults. This is the case, for example, in the San Andreas Trench, which also runs under the California city of San Francisco.
There tensions build up again and again due to the forces acting in the interior of the earth, the plates interlock. Until the pressure becomes too great, the rock suddenly breaks and a slab jumps forward.
Then the offset can be up to a few meters in one fell swoop. The consequences are often severe earthquakes, such as the famous San Francisco earthquake in 1906.
Earthquakes can also occur when two continental plates collide head-on. The plates move the rock masses in these regions and arch up mountains. Most of the earthquakes in India and China are caused by the collision of two continental plates.
Consequences of earthquakes
The quakes themselves are not as destructive as is generally believed. Rather, it is the subsequent processes of earthquakes that bring great suffering to people in earthquake-prone areas.
Huge tidal waves, the so-called tsunamis, are triggered, for example, by violent seaquakes. These then roll towards coastal areas and flood the areas at sea level.
In 2004, a seaquake off Sumatra in the Indian Ocean triggered a massive tsunami that killed hundreds of thousands of people in eight Asian countries. Landslides triggered by an earthquake can have similar consequences.
However, it is often people themselves who are responsible for the devastating consequences of earthquakes. In many earthquake-prone areas, people only die because the necessary standards were not met in the development.
This is by no means only the case in third world countries. During the earthquake in Abruzzo, central Italy, in retrospect, it came to light that the construction of new houses had been tampered with for years.
Despite the constant technical development of measuring instruments, earthquakes cannot be reliably predicted in the foreseeable future. That is why, when it comes to prevention, priority is given to safe building.
The USA and Japan are pioneers in the development of new technologies for earthquake-proof construction. In Alaska, for example, an oil pipeline survived a violent quake because it was built according to the latest findings.
And the strong earthquake in the Japanese metropolis of Kobe in 1995 "only" claimed a little more than 6,000 deaths due to safe development. This quake went down in history as one of the most expensive because of the damage to buildings and roads, but not as one that killed many people.
When the earth shakes anywhere in the world, scientists shortly afterwards try to measure the strength of the quake and classify it on a scale. As early as 1902, the Italian volcanologist Guiseppe Mercalli developed a scale which was later named after him and which is based on the effects of an earthquake.
The Mercalli scale, which has been modified several times, assigns tangible vibrations and visible damage to buildings or in the landscape to individual classes, ranging from "barely noticeable" to "completely devastating".
The advantage of this purely observational method is that the intensity can be determined even in regions where there are no measuring instruments. If there are good eyewitness accounts, they can even be used to classify historical earthquakes.
This method is too imprecise for geoscientists because it is only based on observations. For this reason, the American seismologist Charles Richter developed a scale in 1935 that is still the best known today. In contrast to the Mercalli scale, it is based on physically measurable factors.
Using the maximum deflection in a seismogram, the graphic representation of earthquake waves and the distance to the source of the quake, Richter calculated the strength of an earthquake.
The Richter scale is no longer used in science because it cannot capture all tremors precisely enough. In common parlance and in news broadcasts, however, it is still an integral part.
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