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RTD info logoMagazine on European Research N° 43 - November 2004   
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Title  The mechanics of tectonics

When the plates of the earth’s crust exert pressure on one another as they move, the rocks that make up these plates are subject to extreme stress. During a latent period, this stress is absorbed by a certain elasticity. But beyond a particular point, a sudden rupture occurs, either along an existing fault or by creating a new fault. The energy accumulated during the elastic deformation is then released, giving off heat and producing mechanical effects of movement along the fault. This rupture movement is transmitted through the lithospheric mass in all directions, in the form of seismic waves that produce elastic deformations of the rocks in their path.

Typical fault structure: at the breaking point between two host rocks, the real core of the fault is a narrow zone consisting of crushed or cataclastic rocks (especially in the form of ‘breccia’). On either side of this is a ‘damaged’ zone characterised by many dispersed fissures.
Typical fault structure: at the breaking point between two host rocks, the real core of the fault is a narrow zone consisting of crushed or cataclastic rocks (especially in the form of ‘breccia’). On either side of this is a ‘damaged’ zone characterised by many dispersed fissures.
There are three kinds of rupture mechanisms produced at the ‘focus’ of an earthquake, the point where a fault already exists or is produced. Movements of the plates in opposing directions, produced in an oblique vertical plane, cause a shearing of the rocks on a horizontal plane (divergent faults), or, conversely, a tightening of the rocks creating overlaps (convergent faults). The third variety is where two parallel plates move in opposiing directions on a horizontal plane (sliding fault). The surface effects are, of course, dependent upon the type of movement.

Several series of waves emanate from the focus of an earthquake. They have different speeds and produce different effects as they travel through the lithosphere. The fastest are the body waves, divided into two categories: primary or P waves and secondary or S waves. P waves are longitudinal and correspond to the successive expansion and contraction of the rocks through which they pass – like the rectilinear movements of an earthworm – and it is these waves that are felt first. The S waves travel much more slowly and are characterised by the movement of the rock mass along a line that is vertically perpendicular to the direction of the movement, rather like the swelling of the sea as the waves break on the shoreline.

Lastly, when the earthquake focus is not too deep, i.e. in the upper layers of the lithosphere, it creates surface waves that have a more complex and destructive deforming force. Two types are distinguished, Love or L waves that cause an elastic movement of the rocks on a horizontal plane perpendicular to the wave direction (like a snake along the ground), and Raleigh or R waves that combine this kind of deformation in the horizontal and vertical direction.

From top to bottom: P Waves - S Waves - Love Waves
From top to bottom: P Waves - S Waves - Love Waves
Epicentre and magnitude
The advanced technology of today’s seismometers permits a detailed wave analysis, determining their nature, amplitude, speed and speed variations (slowing or accelerating). As soon as several devices (at least three) record the arrival of P and S waves, seismologists are able to calculate mathematically the exact focus of the earthquake, the ‘epicentre’ being the point on the earth’s surface directly vertical to it

Using the data supplied by waves, seismologists developed a common calibration system that makes it possible to calculate an earthquake ‘magnitude’. This notion refers to the energy released at an earthquake’s focus, independently of the place where it is recorded and the type of seismometer used. It was introduced by the US geophysicist Charles Richter who created the logarithmic scale that today bears his name. 

Use of a scale of this kind permits a more precise classification of the vast range of ruptures that the lithosphere can experience and that seismometers are able to record. A change of one on the Richter scale represents a tenfold increase in the amplitude of the seismic waves at the focus. The most violent earthquake ever recorded registered 9.5 and struck Chile in 1980. Readings in excess of 9 are extremely rare.

Intensity and site effect
Magnitude, which is a precise measurement of the focus, does not correspond directly to the damage an earthquake causes at a given site. In the latter case, one speaks of the earthquake intensity at the surface, a quite subjective notion linked to the site in question. This intensity is classified on the basis of ex-post estimates, first the perceptions of inhabitants and then the scale of the destruction, on the basis of a number of general criteria (but not the number of victims, as this is, of course, linked to the population density). The oldest classification, which dates back to the beginning of the last century, is the Mercalli scale that has 12 levels. The European Union recently adopted a new EMS 98 (European Macroseismic Scale) that takes better account of present-day construction materials and methods.

By comparing studies of intensity with soil geology, it has been possible to build up an ever-increasing knowledge of what are known as site effects, which has become an essential tool in preventing potential earthquake damage (see "Dealing with the local ‘site effects’ of an earthquake ").

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