{"id":15899,"date":"2024-05-21T13:53:45","date_gmt":"2024-05-21T12:53:45","guid":{"rendered":"https:\/\/besafenet.net\/naturgefahren\/earthquakes\/"},"modified":"2024-06-11T13:53:52","modified_gmt":"2024-06-11T12:53:52","slug":"earthquakes","status":"publish","type":"page","link":"https:\/\/besafenet.net\/de\/naturgefahren\/earthquakes\/","title":{"rendered":"Earthquakes"},"content":{"rendered":"\t\t
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Naturgefahren<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Earthquakes<\/h1>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t
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Prepared by CERG – European Centre on Geomorphological Hazards – Strasbourg, France & the Editorial Board<\/em><\/h6>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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<\/path><\/svg><\/span><\/a><\/path><\/svg><\/span><\/a><\/div>
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An earthquake is a shaking and vibration of the ground caused mainly by the sudden breaking and movement of the tectonic plates in which the Earth’s crust is subdivided or by volcanic activity. Tectonic plates can include continental crust, oceanic crust or both; the earthquakes are usually concentrated along the edges of plates.<\/p>\n

Potential damage in an earthquake can be produced by the following main hazards: ground shaking, ground displacement, tsunamis, flooding, landslides, fires etc.<\/p>\n

Today, earthquakes prediction is not possible; nonetheless, scientific studies include activities aimed at understanding the mechanisms of the seismicity and assessing the probability of an earthquake.<\/p>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/i><\/span>\n\t\t\t\t\t\t\t\t<\/i><\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t1. What is an earthquake?<\/a>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t

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An earthquake is a shaking and vibration of the ground caused mainly by a sudden release of energy in the Earth’s crust due to constraints suffered by rocks situated in deeper layers of the Earth, namely the solid mantle (included in the litosphere) and the molten mantle (the asthenosphere).<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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The earthquake’s energy release is at the Focus, some hundreds of meters below the Earth’s surface, on the fault plane. The Focus is the place where the first rupture is reached by the two portions of the crust, sliding one on top of the other (see the two black arrows) along the fault plane. The Epicenter is the vertical projection of the Focus on the surface. An earthquake’s energy waves propagate with a 3D spherical pattern in all directions (coloured circles). Source: http:\/\/earthquakesandplates.wordpress.com\/<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Earthquakes are characterised by their magnitude (seismic energy measured on the Richter scale) and their hypocentre (the underground fault movement location) and the epicentre (the surface location situated just over the hypocenter).<\/p>

Considering two earthquakes releasing the same seismic energy, in general the deeper the hypocenter, the wider is the impact areas on the surface; large seismic faults with very high depths can produce very destructive earthquakes over hundreds of square kilometres.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/p>\n<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t\t

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Earthquakes can only be differentiated with respect to their origin:<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Tectonic earthquakes<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Large faults within the Earth’s crust accommodate differential or shear motion produced by the inner forces (called tectonic forces) responsible for the plate movements. When the energy accumulation along one or more faults becomes too high, rocks break suddenly, releasing energy through a rapid motion (the earthquake). Such faults can be very large (many kilometers) and they are responsible for the adjustment of movements between the various plates composing the Earth\u2019s crust.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Different types of Plate Boundaries. This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an agency of the United States Department of Interior. For more information, see the official USGS copyright policy. Source: Wikipedia<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Volcanic earthquakes<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Earthquakes can also occur in active volcanic areas as they are continuously deforming: such deformation is mainly produced through large and little fractures. In general, volcanic earthquakes release low energy and have a shallow focal depth.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Most earthquakes associated with young volcanoes are related to volcanic processes and may indicate that a quiet volcano is becoming active. Although large eruptions are often preceded by several significant earthquakes and many small rock breaking quakes, there is also a continuous release of seismic energy associated with the underground movement of magma. Volcanic tremor is a seismic vibration caused by the pulsing of pressurized magma and gas. The seismogram is of longer duration and more continuous than rock-breaking earthquakes of the same amplitude. Earthquake swarms recorded by seismometers and the ground deformation monitored by tiltmeters, help scientists determine the depth and location of flowing magma beneath the volcano, which in turn, provides scientists with information to issue hazard warnings. Source: USGS – http:\/\/gallery.usgs.gov\/videos\/660<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Anthropogenic earthquakes<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Anthropogenic earthquakes occur where human activity has played some part in bringing the system to failure. They can be classified in:<\/p>

a)\u00a0Induced earthquakes,<\/strong><\/em>\u00a0where human activities produce stress changes, which are sufficiently large as to produce an earthquake. The rocks may not necessarily have been in a stress-state that would have led to an earthquake in a reasonably near future (in a geological sense!).<\/p>

b)\u00a0Triggered earthquakes,\u00a0<\/strong><\/em>where a small perturbation generated by anthropogenic activities has been sufficient to move the rocky system from an almost-critical state to an unstable state. The seism would have eventually occurred anyway although probably at some later time. That is, these activities have advanced the earthquake clock. In this case, the additional perturbation stress is often very small in comparison with the pre-existing stress system. In recent years there has been an increasing attention toward earthquakes related to some kinds of human activities, such as traditional drillings for oil exploitation, the hydraulic fracturing (known as \u201cfraking\u201d) procedure to extract oil from shales or the over-exploitation, through years, of deep underground water. There is still no clear evidence of correlation between high magnitude earthquakes and such human activities; nevertheless, it is known that any kind of surface and \/ or underground perturbation is able to produce seismic waves, although of small intensities.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Example of how „fracking“ works. Source: http:\/\/savesomegreen.co.uk\/definition-frack\/<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/p>\n<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t\t
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Plate tectonics, a scientific theory created in the second half of the 20th century, states that the rigid earth\u2019s crust (also called lithosphere) is broken up into „plates“ moving on a low-viscous layer with a fluid mechanic behaviour (also called the asthenosphere). The very cause of earthquakes is thus the movement of such large plates which compose the outer shell of the Earth (earth\u2019s crust).<\/p>

When tectonic plates move under, over or slide past each other, earthquakes happen. Such motion can be either of millimeters, producing small shakings, or of meters, producing very violent earthquakes. Heat from the layers below the litosphere and the difference between the light density of lithosphere and heavy density of the underlying asthenosphere, explain such movements and are viewed as the most important source of energy driving plate tectonics. Difference in density also allows the asthenosphere to sink into the deep mantle at subduction zones.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Different types of Convergent Boundaries and subduction zones. Source: Wikipedia.<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Tectonic plates can include continental crust, oceanic crust or both. Their movement is mostly due to the relative high density of lithosphere and the relative fluid mechanic behaviour of the asthenosphere.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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This map by National American Space Administration shows the world areas in which tectonic late motion is more active. Consequently, in such areas Convergent, Divergent and Transform boundaries may be found where the main seismic and volcanic activity is registered. Source: Wikipedia, NASA<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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As it has been indicated in question 2, small-magnitude earthquakes can be also induced by human activities (ref. to Question 2 for the most common types), although no validated data is available so far to the scientific community, to identify some kind of correlation between those activities and big-magnitude quakes.<\/p>\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/p>\n<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t\t

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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/i><\/span>\n\t\t\t\t\t\t\t\t<\/i><\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t4. Where do earthquakes occur?<\/a>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t

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Earthquakes are strongly linked to the geological forces that build mountains and create oceans. Where plates meet each other, wide fault and fracture zones are concentrated. Earthquakes are thus usually concentrated along the edges of plates. Therefore, seismic risk mainly occurs in places where human settlements are located in:<\/p>