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Earthquakes

An earthquake is a vibration or abrupt movement of a portion of the Earth's crust. The Earth's crust is slowly moving through plate tectonics. The movements of parts of the Earth's crust can build up stress along fault zones. The large friction on the fracture surfaces prevents a gradual movement in the majority of cases. When the stress becomes too high, a sudden movement occurs along a fault causing an earthquake. Hereby (a part of) the stress is discharged.

Therefore earthquakes in some areas occur with a certain regularity. However, this regularity is not precise enough to allow the precise prediction of earthquakes. Unfortunately, precisely predicting earthquakes is not yet possible, but the areas with elevated seismic risk are known. Due to the discharge of stress at a certain spot, stress is often increased in nearby areas by the displacement during the earthquake. This causes often several aftershocks after a major earthquake.

 GSHAP Global Seismic Hazard Map (T. Hengl)



Most earthquakes occur in areas that have a lot of tectonic activity. Mostly along plate boundaries, in subduction or collision zones of tectonic plates, at volcanoes and at active faults. However, there are also earthquakes caused by man, for example by gas extraction. Research has recently indicated that fracking in search of shale gas can cause tremors. 

Some known earthquakes from history

Strength

The place on the surface where the earthquake was most severe in the subsurface is called the epicenter. Further away from the epicenter the strength of the earthquake decreases. The intensity of an earthquake at the epicenter is usually measured and communicated using the Richter scale. This is a logarithmic scale of the amount of energy released by an earthquake. In an earthquake, for example 5 on the Richter scale, 10 times as much energy is released as a tremor of 4 on the Richter scale. Earthquakes from 3 can be felt, from 5 there may be extensive damage, not to mention earthquake measuring 9 on the Richter scale. The amount of damage depends not only on the strength of the quake, but also on the depth of the quake, the condition of the buildings, and the type of sediments at the surface.

Due to excessive vibration of the Earth's crust, buildings may collapse and cracks can occur in the ground. If the quake takes place under the sea, then a tsunami (tidal wave) caused by the sudden movement of the water column can occur. This may cause massive destruction in coastal regions and take many casualties.

The epicenter can be determined with the aid of seismometers. There are two types of earthquake waves in the Earth's crust. The so-called P-wave (primary wave) are longitudinal compression waves that
propagate approximately 6 km per second. The S-waves are transverse and travel 3.5 km per second through the Earth's crust. Due to the different speeds of these waves, the distance can be calculated from the seismometer to the epicenter. By combining data from multiple seismometers in various locations, the exact position of the epicenter can be determined.

Strength of earthquakes and their consequences

The lightest earthquakes on the Richter scale are very common and the most severe earthquakes only occur once every few years.


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