Seismology - SpeedyLook encyclopedia

History

Disciplines

Sismogénèse

The sismogénèse studies the mechanisms causing the earthquakes. This discipline tries to also include/understand not only what occurs at the time of an earthquake on the Faille S implied, but tries to apprehend (if they are appréhendables) the conditions associated with release (the technical term is nucleation) with a Earthquake in time and space.

In its most extreme simplification, the source of a seism can be regarded as a point representing the position of nucleation (also called hearth or Hypocentre). Work consisting in finding the position of this point is called localization. The diagram of the radiation of low frequency energy of a seism corresponds to that of a double couple of force of which one of the two nodal plans corresponds to the Fault plane . The space orientation of this double couple is called Mécanisme with the hearth. This last makes it possible to know if it is about a Faille reverses, normal or taking down. The first stage of the study of a seism is thus to find the localization and the mechanism with the hearth. The availability of the seismological data in real-time with the planetary scales makes it possible to very quickly obtain this information after an event (less than one hour for the major seisms).

But the source of a earthquake is not a point. The greatest seisms are generated by ruptures of Faille S of several hundreds of Kilomètre S. the seismologist speaks about extended source when it describes either the Séisme like a simple point but like a more or less complex Surface two-dimensional.

The sismogénèse uses two types of representation of the seismic source which gradually tend to meet. The kinematic approach represents the seism starting from the difference of the state of the Faille before and after the rupture. The seismic source is then described mainly by the Speed (and its variations) of the slip of a point on the fault (about the m.s^-1) at the time of the seism and by the speed to which the rupture is propagated on this same fault (about some km.s^-1). The second representation is dynamic. This representation starts from an initial state of the Faille which is carried in a critical condition where the rupture starts (nucleation). The rupture develops according to constitutive laws (for example the law connecting the speed of slip to friction). The dynamic representation has surely more physical direction than the kinematic representation but is much more complex to handle. One can in the majority of the cases deduce a kinematic representation of a dynamic representation (the opposite is not possible).

To include/understand the seismic source is fundamental to be able one day to hope to envisage the seisms. Certain groups of researchers estimate that it is possible to predict certain seismic events but this research does not have the consensus of all the seismological community and is often the origin of very ignited debates.

Sismotectonique

The sismotectonique one is the branch of geology and the geophysics which studies the structures and the movements Tectonique S thanks to the seisms, as well as the relationship between the seisms and tectonics. Indeed, the spatial distribution of the earthquakes (seismicity) is not random. By looking at the seismicity with the planetary scales, the major part of the Séisme S is at the borders of the tectonic plates. The variation depth of the Hypocentre S underlines the presence of the zones of Subduction.

This simple analysis on a scale it sphere can be carried out on all the scales. Using different seismic stations distributed around a seism, it is possible to find the physical parameters of a seism, like the coordinates of the seism, its depth (often difficult to determine), and the mechanism with the hearth of the seism; thus one determines the type of Faille concerned. Starting from the simple analysis of seismograms having recorded a jolt, there always remains a doubt about the orientation of the principal fault, the distinction between the fault plane and the nodal Plan (plane theoretical directed perpendicular to the fault plane) which can be obtained only by geological knowledge and/or the study of the counterparts of the principal seism. The mechanisms with the hearth (geometrical parameters of the rupture) are related on the orientation and the variations of the field of Contrainte in the crust.

The precise localization of the Séisme S requires a rather detailed knowledge of the variations the speed of the seismic waves in the basement. These speeds are directly related to the elastic and physical properties of the medium. In general, the variations speed in the Ground are function depth. This is the reason for which, in first analysis, the medium in which the Onde S are propagated (propagation medium) is often compared to a horizontal laminated medium (stacking of horizontal layers, the technical term is monodimensional medium ). But the taking into account of three-dimensional complex mediums is current practice today. Thus the determination of the propagation medium and the localization of the seisms are obtained jointly by techniques of Tomographie known as passive (the sources are natural).

A Séisme is always the testimony of the presence of a Faille (if one excludes certain very particular sources). But a Faille always does not produce Séisme S. One will speak then about inactive fault if this one does not cause any deformation. On the other hand a fault, or a segment of fault, can be active but not generate any Séisme (or a diffuse seismicity of very weak magnitude). The fault is then known as asismic. The movement on the fault is done then very slowly (a few millimetres per annum). The technical term is “creeping” (English word meaning “creeping literally”). This deformation can be highlighted only by given geodetic (for example of measurements GPS or images InSAR). This same type of data made it possible to recently detect slips on faults having very long durations (several weeks in several months). These events are called “slow seisms”.

The relation between seismic activity and Faille is important for the seismic forecast. In a simplified vision, the deformation due to the Tectonique increases the Contrainte S on the fault. Arrived at a certain threshold, a rupture starts and the fault generates a Séisme slackening the accumulated constraints. The fault is then ready for a new cycle of accumulation. Thus, on a fault system where the load in constraint is homogeneous, the fault or the segment of fault not having undergone strong earthquakes for a long time becomes a good candidate for the next seism. This candidate is called “gap” seismic. This simplification is not often checked because the stress field is not homogeneous and the geometry of the faults is complex.

Seismic risk

The analysis of the seismic risk studies the occurrence of the earthquakes and the strong movements of the ground which result from this. Two distinct approaches in general are distinguished: the probabilistic Analysis of the seismic risk (in English PSHA for Probabilistic Seismic Hazard Analysis) and deterministic approach. These two approaches are complementary and are often used together.

The deterministic approach makes it possible to make studies of scenario when the majority of the parameters of the problem are fixed. In practice, it makes it possible to answer requests of the type: “Which would be the accelerations of the ground awaited Aix-en-Provence in the case of a seism magnitude 6 on the Fault of Trévarese? ”. The answer to this question is often based on the knowledge acquired thanks to the historical seismicity. If the scenario is new and does not have answer in the databases, then a digital simulation of the problem is necessary.

The probabilistic approach utilizes the concept of time and occurrence. It requires the knowledge of the variation of the rate of seismicity on the territory. The typical request is the following one: “Which are the chances to exceed an acceleration of the ground of 2 m.s^-2 in Aix-en-Provence in the 50 next years? ”. This approach also makes it possible to carry out a chart of the seismic risk when the question is slightly modified: “Which is the acceleration of the ground in this point having 10% of chance to be exceeded in the 50 next years? ”. It is necessary to make the distinction between the seismic risk and the seismic Risque. Indeed the seismic risk is the impact of the seismic risk on the human activity in general. Thus one speaks about a high seismic risk for an area having an important seismic activity. But to a high seismic risk forcing does not correspond a high seismic risk if the area is deserted and does not comprise construction. On the other hand even a zone having a moderate seismicity can be considered high-risk because of density of the population, the importance of built or of the presence of sensitive buildings (nuclear plants, chemical plants, fuel depots,…).

Total seismology

Total seismology studies the structure of the Earth by using the recordings of the waves produced by the seisms with very large distances. Indeed, when the magnitude of the seism is sufficient (higher than 5), the waves which it emits can be measured on all the surface of the Earth.

The waves of volume, primary educations and secondaries (known as waves P and waves S ), cross the Earth and are reflected on major discontinuities (interface core-coat, Moho, surface of the ground). Each reflection produces various phases and the study of their run time between the source and the seismometer gives information on the crossed structure. For example, the absence of wave of shearing S passing by the external core made it possible Richard Dixon Oldham to conclude that it was liquid.

The first model of reference was precisely deduced from the study of run times of the seismic waves. It is about a monodimensional model defining the variation the speed of the seismic waves and the density according to the depth.

But the approximation of parameters not depending that depth is only of first order. The three-dimensional variability of the internal structure from the seismological point of view has multiple causes. The main cause is heterogeneity associated with discontinuities major. Their geometry is complex. They are also zones of exchanges creating of the important variations of the physical parameters which are sensitive the seismic waves. For example, the study of the phases thought of the border enters the core and the coat provides information not only on its topography but also on its behavior, which is very important for the dynamics of the planet Ground. By using the tool tomographic, the last studies show increasingly clear images of the coat, zones of subduction and propose answers on the origin of the mantellic feathers.

The waves of volume are not the only ones with being sensitive on a sphere scale. At the time of the great earthquakes, the waves of surfaces can make several times it tower of the Earth. The use of these types of data is also used for knowledge of the structure of the Earth in the first hundreds of kilometers. Indeed, the amplitude of the waves of surface attenuates with the depth.

Lastly, the Earth is a finished volume and can resound. For the most important seisms, the constructive interaction of the waves of surface making it tower of the Earth excites its own modes. The Earth is then put to sound like a bell. The sound low emitted by the ground one period of approximately 53,83 min has. This sound lasts several days before attenuating. The period of the various modes is directly connected to the internal structure of the Earth. The model of reference more used until now is called the PREM of English Preliminary Reference Earth Model. Today several other models very slightly different are also used.

Seismic of exploration

The projections of the Sismique of exploration are closely related to the prospection oil and with the monitoring of the layers. However, developed techniques in this field are also employed for the knowledge of the structure in general scale of the laboratory until the scale of the Earth's crust.

Often this activity is also called Sismique activates because the sources used are generally artificial (blow of hammer to the nuclear Explosion). It should be noted that the seismic one of exploration is carried out more and more with natural and/or induced sources in the case of tanks.

Configurations of the device source - receiver are fundamental in this field. They indeed will define the type of data obtained and thus the type of method to be used and the type of awaited result. The first distinction is the dimensionality of acquisition. It can be 1D (an aligned source and several sensors or opposite), 2D (the sources and the receivers are contained in a plane in general vertical), 3D and 4D (study of the variation of the 3D problem in time). Each passage of dimension imply a substancielle increase in the cost of acquisition but also the cost of data processing and of their interpretation.

The other important characteristic of the configuration is the type of Déport (distance source - sensor) used. When the offsets are small, the energy recorded on the sensor comes mainly from the reflection of energy on discontinuities of impedance medium. one speaks about Sismique reflection. When the offsets are large, energy recorded comes from the seismic phases crossing the medium or skirting discontinuities (refracted waves). One speaks then about Sismique refraction.

These two concepts are especially related to the prospection at sea. For the seismic reflection the boat while progressing drag a line of sensors called flute while emitting energy (shootings) thanks to air guns. In the case of the seismic refraction, the sensor is fixed and the boat moves away from there in tie. These acquisitions are mainly 2D or 3D in the direction of multi 2D. Moreover in more prospections now mix these two concepts in only one acquisition (seismic reflection with great angle). The data acquisition to ground is much more expensive and the mediums are in general more difficult to interpret.

Space seismology

Seismology and its tools are not confined any more with blue planet since the end of the Années 1960 thanks to the Programme Apollo. At the time of the mission Apollo 12, the first extraterrestrial seismometer is installed on the Moon the November 19th 1969. At the time of each of the three landings according to (Apollo 14, 15 and 16), a seismometer is installed. These instruments formed the first (and single for the moment) extraterrestrial seismological network. The experiment ended the September 30th 1977.

The seismic sources recorded on the moon are of five different types:

impacts of Meteorite S;
artificial impacts;
very surface thermal sources caused by the variation day laborer of temperature on the surface;
surface seisms high frequency due to thermal cooling (magnitude observed up to 5 - many observations: 28);
major seisms (called Moon tremor) (number: 3145) caused by the lunar Tide. They are localized between 800 and 1200 km of depth.

The analysis of these single data made it possible to show that the structure of the Moon is differentiated (existence of a crust, a coat and a hypothetical core). Speeds of the seismic waves added constraints on the chemical composition and mineralogical, compatible with the assumption of a collision between two stars. The recordings of the Moon tremors last very a long time (up to one hour). This characteristic is explained by great dispersion (great heterogeneity) and by the weak attenuation in the lunar crust.

The Apollo program was not the first to try to put a seismometer on the Moon. The Program To arrange tried in 1962 to deposit an instrument with the probes Ranger 3 and 4. Unfortunately the first missed the Moon and the second was crushed there. With regard to Mars, the Sonde Viking installed successfully a seismometer in 1976. A defect of adjustment of the instrument associated with the strong winds Martians made these data not exploitable. Within the framework of the mission March 96, the two Optimism seismometers planned for an installation over Mars were lost with the launcher the November 16th 1996.

Seismology was also applied to the nonsolid stars. The impact of the Comet Shoemaker-Levy 9 on Jupiter in 1994 generated seismic waves of compression and observable waves of surface on the images Infrarouge S. Moreover the study of the waves P, of surface and gravity observed on the Sun is now an established discipline which is called the Héliosismologie. These waves are generated by the turbulent movements convectifs inside star.

The future space programs speak about new seismological measurements on the Moon, to send seismometers on a comet (Sonde Rosetta for a “ acomètissage ” in 2016) and on Mercure (mission BepiColombo in project). The first seismometer Martian as for him is awaited for 2013 with the mission ExoMars.

Seismic waves

The earthquakes produce various types of seismic waves. These waves, while crossing the ground and while being reflected or diffracting themselves on principal discontinuities of physical properties of the rocks, provide us useful informations to include/understand not only the seismic events but also the underlying structures of the Ground.

Measurement in seismology

Some famous seismologists and their contributions

In 1893, the bond between seism and fault is highlighted by Bunjiro Koto.
In 1902, Giuseppe Mercalli creates a scale intensity which will be the reference until the introduction of the concept of magnitude.
In 1906, Richard Dixon Oldham deduces that the core of the Earth is liquid.
In 1909, Andrija Mohorovičić discovers a seismic discontinuity at the base of the Earth's crust: this discontinuity bears, in its honor, the name of Moho.
In first half of the 20th century, Beno Gutenberg, in addition to this famous work on the magnitude, notices that the number of the seisms on a total scale follows a law.
In 1935, Charles Francis Richter develops a scale founded on the magnitude (released energy) of the jolt, and not on its intensity (effects felt or observed). The systematic use of this scale, in particular by the journalists, makes of Richter the only largely known seismologist of the general public.
In 1936, the presence of a solid seed in the core of the Earth is discovered by Inge Lehmann.
In 1966, Keiiti Aki introduced the seismic Moment.
In 1977, Hiroo Kanamori proposes a M_w magnitude based at the seismic time.

Random links:

Junkers | Law of Pareto | Invincible The Fist | Characteristic of Euler | Canton of Saint-Hilaire