As earthquake measurable vibrations of are terrestrial body designated. They arise through mass shifts , mostly as tectonic quakes as a result of shifts in the tectonic plates at fractures in the lithosphere , to a lesser extent also through volcanic activity, collapse or subsidence of underground cavities, large landslides and landslides as well as blasts. Earthquakes whose focus is below the sea floor are also called seaquakes or undersea earthquakes . These differ from other earthquakes partly in the effects such as the occurrence of a tsunami , but not in their development.
Almost as a rule, earthquakes do not consist of a single shock, but usually result in more. In this context, one speaks of foreshocks and aftershocks with reference to a stronger main tremor . If earthquakes occur more frequently over a longer, limited period of time, one speaks of an earthquake swarm or swarmquake . These occur mainly in volcanically active regions. In Germany there are occasional swarms of earthquakes in the Vogtland and on the Hochstaufen .
The vast majority of recorded earthquakes are too weak to be perceived by humans. Strong earthquakes can destroy buildings, tsunamis , avalanches , rockfalls, landslides and landslides and kill people in the process. They can change the shape of the earth's surface and are natural disasters . The science that deals with earthquakes is called seismology . With one exception, the ten strongest earthquakes recorded since 1900 all occurred in the subduction zone around the Pacific , the so-called Pacific Ring of Fire (see list below).
According to an analysis of more than 35,000 natural disaster events from 1900 to 2015 by the Karlsruhe Institute of Technology (KIT), a total of 2.23 million people worldwide were killed by earthquakes during this time.
Even in ancient times , people wondered how earthquakes and volcanic eruptions arise. These events are frequently wrote gods (in Greek mythology, the Poseidon ). Some scientists in ancient Greece believed that the continents floated on water and rocked back and forth like a ship. Other people believed earthquakes erupted from caves. In Japan there was the myth of the dragon that made the ground tremble and breathed fire when angry. In the European Middle Ages, natural disasters were attributed to God's work. With the discovery and research of magnetism , the theory emerged that earthquakes can be derived like lightning. Earthquake arresters like the first lightning rods were therefore recommended .
It was not until the beginning of the 20th century that the now generally accepted theory of plate tectonics and continental drift came about by Alfred Wegener . From the middle of the 20th century, the explanatory patterns for tectonic earthquakes were widely discussed. Until the beginning of the 21st century, however, it was not possible to develop a technology for reliably predicting earthquakes.
Measurement, research, causes and consequences of earthquakes
Dynamic processes in the earth's interior
Earthquakes are mainly caused by dynamic processes inside the earth. One consequence of these processes is plate tectonics, i.e. the movement of the lithospheric plates , which extend from the surface of the earth's crust to the lithospheric mantle .
Especially at the plate boundaries, where different plates move apart (" spreading zone "), towards each other (" subduction " or " collision zone ") or past each other (" transform fault "), mechanical tensions build up within the rock when the plates get caught in their movement and tilt. If the shear strength of the rocks is exceeded, these tensions are discharged through jerky movements of the earth's crust and a tectonic quake occurs. More than a hundred times the energy of a hydrogen bomb can be released. Since the tension built up is not limited to the immediate vicinity of the plate boundary, the relief fracture can in rare cases also occur inside the plate if the crustal rock has a weak zone there.
The temperature increases steadily towards the interior of the earth, which is why the rock becomes more and more easily deformable with increasing depth and is no longer brittle enough to break even in the lower earth crust. Earthquakes therefore usually have their origin in the upper crust of the earth, at a depth of a few kilometers. Occasionally, however, earthquakes with herds up to a depth of 700 km are detected. Such "deep hearth tremors" occur primarily in subduction zones. There two plates move towards each other, whereby the denser of the two is pushed under the one with the lower density and dips into the earth's mantle . The submerged part of the plate ( slab ) heats up relatively slowly in the mantle, so that its crust material is fragile even at greater depths. The hypocenters of earthquakes that occur within a slab thus enable conclusions to be drawn about the position of the slab in depth (" Wadati-Benioff zone "). One of the triggers for these deep-hearth quakes is the change in volume of the slab rock as a result of mineral transformations under the temperature and pressure conditions in the mantle.
So-called tsunamis can arise in the event of undersea earthquakes, the eruption of oceanic volcanoes or the occurrence of undersea landslides . Sudden vertical displacement of large parts of the ocean floor creates waves that travel at speeds of up to 800 kilometers per hour. Tsunamis are barely noticeable in the open sea ; However , if the wave ends in shallower water , the wave crest steepens and in extreme cases can reach a height of 100 meters on the shore. The most common tsunamis are in the Pacific . That is why the states bordering the Pacific have an early warning system , the Pacific Tsunami Warning Center . After around 230,000 people died on December 26, 2004 after a devastating earthquake in the Indian Ocean , an early warning system was set up there too.
Very shallow earthquakes that can only be felt locally can be triggered by frost if large amounts of water in the ground or in the rock substrate freeze and expand in the process. This creates tensions that are discharged in smaller vibrations, which are then perceived on the surface as "earthquakes" and rumbling noises. The phenomenon usually occurs at the beginning of a severe frost period, when temperatures have dropped rapidly from values above freezing point to values well below freezing point.
Earthquake due to human activity
In addition to naturally triggered earthquakes, there are also anthropogenic , i.e. man-made. This induced seismicity is not necessarily intentional or knowingly brought about; B. in the case of active seismics or as a result of nuclear weapons tests , but it is often events that occur as unintended "side effects" of human activities. These activities include the extraction of fossil hydrocarbons ( crude oil and natural gas ), which changes the stress conditions in the rock of the deposit by changing the pore pressure , or the (attempted) use of geothermal energy (→ geothermal energy ).
Anthropogenic earthquakes also occur when underground cavities collapse ( rock blows ). The magnitude of these earthquakes is in the vast majority of cases in the range of microquakes or ultra-microbial earthquakes. It rarely reaches the value of noticeable tremors.
Some of the strongest anthropogenic earthquakes occurred as a result of the accumulation of large amounts of water in reservoirs due to the increased load in the subsurface near large faults. The Wenchuan earthquake in China in 2008 (magnitude 7.9), which killed around 90,000 people, is considered to be the candidate for the world's strongest reservoir-triggered earthquake to date.
Earthquakes produce earthquake waves of various types that propagate over and through the entire earth and can be recorded in seismograms by seismographs (or seismometers) anywhere on earth . The destruction of the earth's surface associated with strong earthquakes (crack formation, damage to buildings and transport infrastructure, etc.) can be traced back to the "surface waves" that propagate on the earth's surface and trigger an elliptical ground movement.
The speed of propagation of an earthquake is normally around 3.5 km / s (not to be confused with the wave speed given above for seaquakes). In very rare cases, however, the quake spreads at supersonic speed, with propagation speeds of approx. 8 km / s already being measured. In a supersonic quake, the crack propagates faster than the seismic wave, which is usually the other way around. So far only 6 supersonic earthquakes have been recorded.
By recording and evaluating the strength and duration of earthquake waves in observatories around the world , one can determine the position of the earthquake focus, the "hypocenter". This also results in data about the interior of the earth . As measurements on waves, position determination is subject to the same blurring that is known for waves in other areas of physics . In general, the uncertainty of the location determination increases with increasing wavelength. A source of long-period waves cannot be localized as precisely as that of short-period waves. Since severe earthquakes develop most of their energy in the long-period range, the depth of the source in particular cannot be precisely determined. The source of the seismic waves can move in the course of an earthquake, for example in the case of severe earthquakes that can have a fracture length of several hundred kilometers. According to international agreement, the first measured position is called the hypocenter of the earthquake, i.e. the place where the earthquake began. The place on the earth's surface directly above the hypocenter is called the epicenter . The point in time at which the break begins is referred to as the "hearth time".
The fracture surface that triggers the earthquake is referred to in its entirety as the "hearth surface". In most cases, this fracture surface does not reach the surface of the earth, so that the focus of the earthquake is usually not visible. In the case of a larger earthquake, the hypocenter of which is only shallow, the focal area can reach to the surface of the earth and lead to a significant offset there. The exact course of the fracture process determines the "radiation characteristics" of the quake, i.e. how much energy is radiated in the form of seismic waves in each direction of the room. This rupture mechanism is known as the hearth process . The sequence of the hearth process can be reconstructed from the analysis of initial deployments at measuring stations. The result of such a calculation is the stove surface solution .
Types of earthquakes
There are three basic types of earthquake events, which reflect the three types of plate boundaries: In spreading zones, where the tectonic plates drift apart, tensile stress acts on the rock ( extension ). The blocks on both sides of the hearth area are thus pulled apart and there is a removal (engl .: normal fault ), in which the block is placed above the fracture surface downward. In collision zones where plates move towards each other, on the other hand, compressive stress acts. The rock is compressed and there is, depending on the inclination angle of the fracture surface, to an up or thrust (engl. Reverse fault or thrust fault ), in which the block offset above the fracture surface upward. In subduction zones , the plunging plate can get caught over a large area, which can lead to a massive build-up of tension and ultimately to particularly severe earthquakes. These are sometimes referred to as " megathrust earthquakes ". The third type of oven is called the " strike-slip (engl." Strike-slip fault ) indicates that occurs in "Transform faults", where the plates involved slide laterally past one another.
In reality, however, the forces and tensions mostly act at an angle on the rock, as the lithospheric plates can tilt and turn. The plates therefore normally do not move straight towards or past one another, so that the hearth mechanisms usually represent a mixed form of upward or downward displacement and a sideways movement of the blades. One speaks here of an " oblique fault " or " oblique fault ".
The spatial position of the hearth surface can be described by the three angles Φ, δ and λ:
- Φ denotes the strike (ger .: strike ) of the fault plane. This is the angle between the geographical north direction and the intersection of the incident hearth surface with the horizontal ( stroke line ). The stroke can take on values between 0 ° and 360 °; a hearth area dipping to the east would be marked by a north-south stroke line and would thus have a stroke of Φ = 0 °.
- δ denotes the falling , i.e. the dip of the hearth surface. That is the angle between the horizontal and the hearth surface. It can assume values between 0 ° and 90 °; a fracture surface running exactly vertically would have an inclination of δ = 90 °.
- λ denotes the direction of the offset ( rake ), which is determined in the plane of the offset. This is the angle between the stroke of the stove surface and the direction vector of the offset, which can assume values between 0 ° and 360 °. Is z. B. the hanging wall, i.e. the block above, shifted exactly upwards, would be λ = 90 °. If the stove surface is exactly vertical, looking in the direction of the stroke, the right block is defined as the "hanging wall". For a left lateral shift, λ = 0 °, for a right lateral shift, λ = 180 °.
In order to be able to compare earthquakes with one another, it is necessary to determine their strength. Since a direct measurement of the energy released by an earthquake is not possible due to the depth of the focal process alone, different earthquake scales have been developed in seismology.
The first earthquake scales, which were developed at the end of the 18th to the end of the 19th century, could only describe the intensity of an earthquake, i.e. the effects on people, animals, buildings and natural objects such as bodies of water or mountains. In 1883 the geologists M. S. De Rossi and F. A. Forel developed a ten-point scale for determining the intensity of earthquakes. However, the twelve-part Mercalli scale introduced in 1902 became more important . It is based solely on the subjective assessment of the audible and tangible observations as well as the damage effect on the landscape, streets or buildings (macro-seismic). In 1964 it was further developed into the MSK scale and later the EMS scale .
Intensity scales are still used today, with various scales that are adapted to the construction and soil conditions of the respective country. The spatial distribution of the intensities is often determined by questionnaires from the responsible research institutions (in Germany, for example, nationwide by the BGR using an online form) and displayed in the form of isoseist maps . Isoseists are isarithms of equal intensities. The possibility of recording intensities is limited to relatively densely populated areas.
The development and constant improvement of seismometers from the second half of the 19th century made it possible to carry out objective measurements based on physical quantities, which led to the development of the magnitude scales. Using empirically found relationships and physical laws, these enable conclusions to be drawn about the strength of an earthquake from the location-dependent amplitude values recorded at seismological measuring stations .
There are several methods of calculating the magnitude. The magnitude scale most commonly used by scientists today is the moment magnitude scale (Mw). This is logarithmic and ends at Mw 10.6. It is assumed that at this value the earth's solid crust breaks completely. The increase by one magnitude corresponds to a 32 times higher energy release. The Richter scale introduced in the 1930s by Charles Francis Richter and Beno Gutenberg , which is also known as the local earthquake magnitude , is cited most frequently by the media . Seismographs, which should be 100 km away from the epicenter of the earthquake, are used to precisely measure the strength of the earthquake. With the Richter scale, the seismic waves are measured in a logarithmic scale. It was originally used to quantify earthquakes in the California area. If an earthquake measuring station is too far away from the earthquake focus (> 1000 km) and the strength of the earthquake is too great (from around magnitude 6), this magnitude scale cannot be used or can only be used to a limited extent. Due to the simple calculation and the comparability with older earthquake classifications, it is often still in use in seismology.
According to a publication from 2017, slight fluctuations in the earth's gravitational field can be detected in the seismometer records during strong earthquakes , which are triggered by the mass displacement. These signals propagate through the earth's body at the speed of light , i.e. significantly faster than the primary earthquake waves (P waves), which are usually the first to be registered by the seismometers and can reach a speed of no more than 10 km / s. In addition, they should enable a more precise determination of the magnitude of an earthquake, especially at measuring stations that are relatively close to the earthquake focus. Both of these mean a significant improvement in earthquake early warning .
According to the current state of science, it is not possible to predict earthquakes precisely in terms of time and space. The various determining factors are largely understood qualitatively. Due to the complex interplay, however, an exact quantification of the focal processes has not yet been possible, only an indication of the probability of an earthquake occurring in a certain region.
However, we know precursor phenomena (Engl. Precursor ). Some of these express themselves in the change in geophysically measurable quantities, such as B. the seismic speed, the slope of the ground or the electromagnetic properties of the rock. Other phenomena are based on statistical observations, such as the concept of seismic calm , which sometimes suggests a major event ahead.
There have also been repeated reports of unusual behavior in animals shortly before major earthquakes. In the case of the Haicheng earthquake in February 1975, this enabled the population to be warned in good time. In other cases, however, no abnormal behavior was observed in animals prior to an earthquake. A meta-analysis , in which 180 publications were considered, in which more than 700 observations of conspicuous behavior in more than 130 different species in connection with 160 different earthquakes are documented, showed in comparison with data from the global earthquake catalog of the International Seismological Center (ISC- GEM) that the spatio-temporal pattern of the behavioral anomalies noticeably coincides with the occurrence of foreshocks. According to this, at least some of the behavioral anomalies could be explained simply by the foreshocks that can be perceived by the animals, which are often equipped with more sensitive sensory organs, at greater distances from the epicenter. Although many studies looked at unusual behavior, it was unclear what unusual behavior actually is and which behavioral anomalies are considered to be precursors. Observations are mostly anecdotal and there is a lack of systematic evaluations and lengthy series of measurements. So there is no evidence that animals can reliably warn of earthquakes.
All known precursor phenomena vary greatly in time and order of magnitude. In addition, the instrumental effort that would be required for a complete recording of these phenomena would not be financially and logistically feasible from today's perspective.
In addition to the "conventional", noticeable and sometimes very destructive earthquakes, there are also so-called "unconventional" or "slow" earthquakes, the sources of which are not below but on the surface of the earth and which emit very long-period ( period approx. 20 to 150 s) surface waves . These waves must be filtered out of globally or continent-wide recorded seismic data using special algorithms and can be assigned to certain sources based on their characteristics and sometimes other criteria. Such unconventional earthquakes include the glacier quakes , which are triggered by calving processes on large polar glaciers, as well as the stormquakes which , under certain circumstances , occur in strong storms ( hurricanes, etc.) through the interaction of storm-induced long-period ocean waves with larger shallows in the area of the shelf edge be generated.
Strongest recorded earthquakes
The following list was compiled according to the USGS . Unless otherwise stated, the values relate to the torque magnitude M W , whereby it must be taken into account that different magnitude scales cannot be directly compared with one another.
|1.||Valdivia earthquake in 1960||Chile||May 22, 1960||9.6||1,655 dead|
|2.||Good Friday quake 1964||Alaska||March 27, 1964||9.3||Tsunami with a maximum height of about 67 meters|
|3.||2004 Indian Ocean earthquake||before Sumatra||Dec 26, 2004||9.1||Around 230,000 people died as a result of the quake and the subsequent tsunami . Over 1.7 million coastal residents around the Indian Ocean have become homeless.|
|4th||2011 Tōhoku earthquake||east of Honshū , Japan||March 11, 2011||9.0||The “most expensive earthquake ever”: 18,500 people died, 450,000 people became homeless, and there was direct damage of around 296 billion euros.
As of April 7, 2011, 12,750 dead and 14,706 missing were counted who were victims of the quake and the subsequent tsunami. Because of the tsunami, the Fukushima disaster also occurred for the nuclear reactor blocks of the Fukushima Daiichi nuclear power plants . The Fukushima Daini , Onagawa and Tōkai power plants were also hit, but only suffered minor damage. There have been hundreds of fires and long-term power outages in millions of homes.
|5.||Kamchatka earthquake in 1952||Kamchatka , Russia||Nov 4, 1952||8.9|
|6th||Earthquake in Chile 2010||Chile||Feb. 27, 2010||8.8||521 dead, 56 missing|
|6th||Earthquake Ecuador-Colombia 1906||Ecuador / Colombia||Jan. 31, 1906||8.8||1000 dead|
|7th||Earthquake in the Council of Islands, 1965||Council Islands , Alaska||Feb. 4, 1965||8.7|
|8th.||Earthquake off Sumatra 2012||off the coast of Sumatra||Apr 11, 2012||8.6|
|8th.||Earthquake off Sumatra in 2005||off North Sumatra||March 28, 2005||8.6||Over 1000 dead|
|8th.||Araucanía earthquake in 1960||Araucanía||May 22, 1960||8.6|
|8th.||Earthquake near the Andreanof Islands in 1957||Andreanof Islands , Alaska||March 19, 1957||8.6|
|8th.||Assam earthquake in 1950||China-India border region||Aug 15, 1950||8.6|
|8th.||Earthquake in the Aleutian Islands in 1946||with the Aleutians||Apr 1, 1946||8.6|
The extent of the damage caused by an earthquake depends primarily on the strength and duration of the earthquake as well as on the population density and the number and size of the structures in the affected area. The earthquake resistance of the structures is also essential . The European standard EC 8 (in Germany DIN EN 1998-1) defines the basics for the design of earthquake effects for the various types of construction wood, steel, reinforced concrete, composite construction, masonry, design criteria.
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