What happens before an earthquake
Causes of earthquakes
When do we speak of an earthquake? A definition
An earthquake is a usually naturally occurring, measurable vibration in the uppermost layers of the earth. Measurable means: Most earthquakes generate vibrations that are so low that we humans cannot feel them. These earthquakes are only registered and recorded by sensitive measuring devices, the seismograph.
The causes of the vibrations can be very diverse. The following are possible:
- Tectonic shifts. Here, tectonic plates move against each other in such a way that cracks and sudden stress relief occur in the earth's crust. These so-called tectonic tremors are by far the most common type of earthquake. The heaviest tremors in human history are all of this type. The formation of tectonic earthquakes is explained in detail in the chapter plate tectonics.
- Volcanism. In some regions of the world magma rises from the depths. When this liquid rock reaches the surface of the earth (which does not always have to happen), it forms a visible volcano. When it rises, the magma needs space, displaces the existing rock - and in this way sometimes creates a local earthquake. Gas explosions in a volcanic vent can also lead to earthquakes. Volcanic earthquakes only account for around seven percent of global earthquakes and rarely lead to supraregional destruction. These earth tremors are important for the researchers, as they often precede a volcanic eruption as a “warning”.
- Extraction of raw materials. Human activities also sometimes cause earthquakes. If crude oil or natural gas is extracted from a depth of several kilometers, rock masses can shift or subterranean cavities collapse. Similar phenomena are also known from hard coal and lignite mining. Such earthquakes are even possible in Germany: especially in the Ruhr area and Saarland, coal mining has left countless underground cavities that can collapse and shake the earth.
- Frost quake. In rare exceptional cases, even the onset of frost can cause slight earthquakes. When a layer of earth freezes, it expands as the water it contains turns into ice and takes up more space. If this process happens very quickly (with an extreme drop in temperature), enormous forces can arise. Sometimes adjacent rock is broken, which can be felt as an earthquake.
- Meteorite impact. Admittedly, this is a rather exotic cause. The common meteorites that regularly hit the ground today don't have nearly enough energy to cause an earthquake. Viewed in geological terms, however, there were certainly impacts that could trigger tremendous tremors. About 200 million years ago a huge meteorite hit what is now France (Rochechouart-Chassenon crater). The result was a catastrophic mega-quake of magnitude 11 and global tsunamis.
Distribution of the types of earthquakes
Tectonic quakes occur in the numerous fracture zones of our earth, in which large continental plates or smaller clods of earth adjoin one another. The rarer and less violent volcanic quakes can also be expected in the same areas. Volcanism and plate movements often have the same geological causes. Which regions of the world are particularly affected is described in the chapter on earthquake areas.
Mining earthquakes can occur wherever mineral resources are extracted underground or in large open-cast mines. The very rare frost quakes are observed in Canada and the USA, among others.
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