Earthquakes - How the Earth changes its shape

Earthquakes are a clear testimony to the vitality of the planet, of course along with volcanoes, are the most striking effect of the fact that our planet is alive and this is mainly due to the movement who suffer the plaques above the Earth's mantle.

The earthquakes are sudden and rapid movements of the earth's crust, caused by the release of energy in an internal point, said hypocenter; here, a series of elastic waves, called “seismic waves”, propagate in all directions, also within the Earth itself, the place of the earth's surface located on the vertical hypocenter is called the epicenter and is generally most affected by the phenomenon. The majority of tectonic earthquakes generate sudden opening of large cracks in the rocks. Less important are the earthquakes of volcanic origin, caused by the rise of molten material. Every day on Earth occur thousands of earthquakes; only a few dozen are perceived by the population, and most of the latter cause little or no damage. The average duration of a shock is far below the 30 seconds for the strongest earthquakes, however, can reach up to a few minutes. An earthquake can be accompanied by loud noises, these sounds are due to the passage of the seismic waves to the atmosphere and are more intense in the vicinity of the epicenter. Earthquakes are natural events by far the most powerful on earth, earthquakes can release energy in a few seconds more than thousands of nuclear bombs. In this regard, considering that an earthquake can move in a few seconds volumes of rock of hundreds of cubic kilometers.

The earthquakes of greater magnitude are usually accompanied by secondary events that follow the main shock and define replicas. When more events occur simultaneously, or nearly so, it may be induced earthquakes. According to the elastic rebound theory, the rocks that form the lithospheric plates are subjected to high stresses that cause their deformation. In fact, the two blocks are free to slide, because “held back” by the friction and the weight of the overlying rock. Elastic energy accumulates inside the rocks and increases more and more, when the rocks reach their breaking point, they break along the fault plane and the two blocks slide suddenly. An earthquake, is a sudden vibration of the soil produced by a sudden release of energy that occur only in certain sections of the earth's surface and said seismic energy that propagates in all directions (as a sphere) in the form of waves . The point where it generates the breaking of rocks is said hypocenter. The perpendicular meets the Earth's surface at a point, called the epicenter, where the earthquake reaches maximum intensity.

What is this energy?

It's like to have in my hands a wooden stick: if you start to bend it offers resistance to bending which is expressed in the form of elastic energy; rocks behave in the same way. The seismologist Reid in 1906 he realised what are the conditions for which the deformations occur at the origin of earthquakes. The seismologist came to the conclusion that the rocks behave elastically deformed, gradually up to the breaking point: it creates a fault line (the line of least resistance of the rock under pressure and consequent breakage occurs along this line), the two parts of the original rock react elastically, reclaiming of their volume (which had been compressed) and of their position with a series of shock. Reid spoke of elastic rebound: when a block is subjected to crustal efforts, it behaves elastically deforms slowly and accumulates elastic energy. Persisting the stress, the stored energy reaches a critical point (tensile strength), and the rocks are split suddenly. The accumulated elastic energy is released suddenly in the form of intense vibrations which propagate in all directions. That is, if a portion of the rock begins to deform, it will offer a certain resistance (which changes depending on the type of rock), but when the forces that hold together the rock are exceeded from those that deform then this is broken and has a abrupt shift of the two parties that release the energy that had accumulated during the deformation and return to the undeformed state. It moves both vertically and horizontally. The movements at the hypocenter generate various types of deformation, given the complex structure of the earth, in the waves which propagate are formed phenomena of refraction and reflection, for which only some waves may arrive at the surface. There are three types of waves:

  • Longitudinal waves or compression
  • Transverse waves or shear
  • Surface waves (Rayleigh and Love waves)

Longitudinal waves: spread compression and expansion, causing volume changes. The particles that make up the rock vibrate in the direction of propagation of the wave itself. Fast waves are said longitudinal P-waves (detected for the first seismograph), move in each half at a speed of 4/8 Km / s. Transverse S-waves: you have when rock masses slide along the plane of the fault causing shear deformations. The vibrations of the particles take place in planes perpendicular to the propagation direction. Shake the ground either horizontally or vertically; said waves are S and move at the speed of 2.3/4.6 Km / s; these can not propagate in the fluid state. P waves and the S waves are called volume. Among the surface waves are the waves R, in which the particles perform elliptical orbits in a vertical plane along the direction of propagation, and the waves L, which have a radial movement on the Earth's surface and dampen in depth, and that, as the S, they move transversely to the direction of propagation, but only in the horizontal plane. Surface waves are slower and longer than the inner ones, but they can travel long distances. Usually these breaks, and the resulting displacements, are long lines preferential called faults, and the precise point from which propagates the earthquake hypocenter is said, while the same point, carried vertically on the Earth's surface is called the epicenter.

What are these faults?


A fault is essentially a fracture in the soil, even several kilometers deep, along which occur the ground motions. In fact, a fault is nothing more than a line of lower resistance of the rock under pressure, and then the rupture occurs along this line. There are various types of faults also very different, but they all have in common the fact that along that line there is a relative movement of the rocks. Interesting is the case of the famous San Andreas fault that runs along the west coast of the United States. This is a type of strike-slip fault, that is, the ground motions always occur on the horizontal plane, (for example a side goes towards North while the other goes to the South) and slowly approaching the city of Los Angeles to that of S.Francisco at a speed of about 2 inches per year. This may seem a very small number, but in reality if we think in geological time (millions of years) this movement is very fast. But back to our earthquakes: we said that these arise because at some point the rock breaks along a fault, in depth, and releases all the energy that had accumulated to resist movement. This energy is dispersed in the soil from hypocenter to all directions in the form of waves (in part also in the form of heat) that may be: volume waves, involving a volume and thus in this case the land itself, and surface waves that propagate only on the surface of the Earth. The volume waves can be further divided in P-wave that is primary (also called longitudinal), those who arrive first, and then those who travel within the earth with the speed higher (of the order of 6 km per second) and are also different in the way we travel in the ground. In addition to being the fastest these alternately compress and release the soil in their propagation direction just like the sound waves and in fact, when this kind of waves arrive at the surface undergo a refraction in the air and can be transmitted to the atmosphere in the form of waves sound. Then we have the S-wave that is secondary (also called transverse) because they are slower (in fact arrive for the second) and they move earth alternately low and high angles to the direction of propagation and by their nature can not travel in liquids. As we said in addition to the volume waves we have those surface, and these are also called long because they travel for long distances and are very similar to those that appear when you cast a stone on the water, this type of waves are those that cause damage to homes and foundations. This energy usually is discharged with a strong main shock, mostly preceded by premonitory small shocks (such foreshocks) and followed by a series of numerous shook said replicas. But sometimes earthquakes can occur directly with the main shock and of course are the most dangerous. The energy release due to forces acting on the lithosphere (the rigid casing that covers the earth) can also occur with continuity, that is, the two blocks on the two sides of the fault can gently slide one next to the other more smoothly and without abrupt accumulation of energy, so without causing major earthquakes. The earthquakes that are associated with ground movement along faults are the most common but there are also earthquakes associated with volcanoes and their activities (movement of magma overpressure of gas etc. ..).

The Seismograph


You'll wonder how you measure the seismic waves of the surface with a tool that rests on the ground, then if the entire surface of the same moves? To overcome this problem we use the seismograph. A seismograph is an instrument consisting of a roll of paper and a “nib” that writes on the paper on the roll. The seismometer can be considered as a simple “pendulum system.” It consists of a mass suspended by a thread free to swing. The trick is that the nib is held suspended by a spring which makes keeping the nib the same position, while during the earthquake the paper roll will go up and down following the movements of the ground. The nib is more or less in the same position since the spring, to which it is attached, it absorbs the movements of the ground and not sending them to this. The trace obtained on the paper is called seismogram. The operation of the seismograph is based on the principle of inertia

The seismograph detects the passage of the seismic waves, they register and produce a graph of the movement of the soil. From reading the graph (seismogram), one can obtain all the characteristics of the earthquake energy, distance to the epicenter, depth of the epicenter, the extension of the fault from which it is generated, and so on.

To understand the strength of an earthquake, seismology adopted a scale of intensity based on the effects, which has been joined by the evaluation of the magnitude that best defines the strength of an earthquake whatever effects with which it occurs.

How to measure earthquakes

The magnitude is a quantity that expresses the force of an earthquake, it is obtained from the records of seismographs and does not take account of the damage caused. The intensity, however, is not based on instrumental data, but it depends on geographical and economic conditions of the area and the degree of urbanization.

The Mercalli scale

Historically, man has constantly sought to classify different types of earthquakes. Almost all classifications were based on the most obvious properties: their intensity. The Mercalli scale, classifies earthquakes depending on intensity. The Mercalli scale is used less and less as it does not provide objective data and does not allow to compare earthquakes that occur in different areas. The Mercalli scale, invented by Giuseppe Mercalli in 1897, it is based only on the extent and amount of damages, that is, when an earthquake occurred was an estimate of the damage and based on this earthquake was assigned to a determined value ranging from 1 (no damage, only instruments warn him) to 10 (total destruction). A subsequent amendment brought then to 12 degrees. This measurement expresses the magnitude, which refers to the maximum size oscillations recorded by seismic instruments in appropriate conditions and an objective measure of the energy released. This measurement determines the data that is macroseismic, this means it evaluates the effects of the earthquake on people and things, restricted to the area where the earthquake is perceived With the study macroseismic is assigned to each place a degree of intensity which of course will be up in the epicentral area and gradually decreasing. The isosisme are lines that separate areas of the surface where an earthquake has occurred with varying intensity, where the innermost represents the area of ​​the epicenter of the earthquake while the outer ones indicate the areas in which the effects were minimal. These are important because they provide information on the morphological structure of the areas considered. The magnitude represents the strength of an earthquake in comparison with a standard earthquake taken as reference.

The Richter scale

The Mercalli scale is now superseded by one that is based on more objective values: the Richter scale (invented by Charles Richter in 1935). The Richter scale ranks earthquakes according to magnitude. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of October 2012), and it was the largest Japanese earthquake since records began. The Richter scale determines the zero level for producing vibrations of seismograph, located 100 km from the epicenter. The increase of 1 degree of magnitude corresponds to a 10-fold increase of the maximum amplitude recorded. The Richter magnitude scale is expressed as a logarithm to the base 10 of the ratio between the amplitude of the earthquake and the maximum amplitude that would be produced by the earthquake epicentral distance to the same standard. AM = log10A (A max is the amplitude of the trace, expressed in millimeters, recorded by a standard tool).

The pattern of propagation of surface waves

With these tools, and the same earthquakes, experts can study the interior of the planet and see what's beneath the crust on which we live (in fact you may not know anything directly since no one has ever gone in the center of earth and also the most modern techniques of oil drilling does not allow you to go deeper than 10-15 km, and it has also been able to divide the interior of the earth in various parts like the crust, the mantle and the core.

Internal structure of the earth

Through the study of wave propagation has been possible to trace the internal structure of the earth: It is formed by concentric layers separated by seismic discontinuity; surface that separates two different materials for the physical characteristics that affect the propagation of elastic waves, in these areas the waves undergo a change in speed which corresponds to a change of direction of their trajectory.


  • Crust extends from the surface to a depth of 10-35 km (even if it is solid refraction phenomena are observed in the propagation of the waves, which showed the presence of partially molten material, such as under the ocean basins)
  • Follows the Moho discontinuity.
  • Cape, extends from the Moho up to 2,900 km, it is solid except in the area of ​​the asthenosphere (between 70 and 250 km depth) where the shear waves undergo a sharp decrease and then return to grow in depth.
  • Between the mantle and the outer core is the Gutenberg discontinuity.
  • Outer core 2,900 to 5,170 Km is fluid because the P-wave velocity decreases while S waves can not pass through it.
  • Inner core ranges from 5,170 to the center of the earth is solid and under the pressure of the overlying strata and is rich in iron and nickel (heavy).

Crust and the mantle together form the lithosphere which unlike the asthenosphere is solid.

From a morphological point of view and geology we can distinguish four main types of active seismic zones:

  • Along the axis of the mid-ocean ridges, in correspondence of which the earthquakes are shallow (less than 70 Km)
  • Along the large fractures of the earth's crust (the San Andreas fault and the fault of Anatolia) characterised by the absence of shallow earthquakes and volcanic activity.

^ At the ocean trenches and island arcs systems (those that border the western Pacific).

  • In these areas the earthquakes can be surface (up to 70 km) intermediate (70 to 300) and deep (300 to 700). The oceanic lithosphere descends into the mantle at the pits along an inclined plane called plane Benioff, highly active from the seismic point of view.
  • Along the continental zone associated with high mountain ranges (Himalayas), which extends across Asia, from Burma to the Mediterranean.

How earthquakes propagate

The energy released from hypocenter propagates in all directions by means of seismic waves.

Seismic waves are of three types:

  • P waves, or longitudinal;
  • S waves or transverse;
  • L waves, or superficial.


P waves, also known as the first, are longitudinal. They propagate in all media, but are deflected when they pass from solid medium to a liquid one.

S waves, also known as second, are transversal. They do not propagate in fluids and are diverted when changing the density of the medium. The waves L finally propagate only on the terrestrial surface.

The geographical distribution of earthquakes

The geographical areas where they are more frequent are called seismic areas. Their distribution is not random, but the epicenters of earthquakes are mainly located along the margins of contact between the lithospheric plates. Every shot of the volcanic activity is always preceded by such volcanic earthquakes, but these earthquakes have nothing to do with tectonic earthquakes.

The tidal waves and tsunamis

An earthquake, if the epicenter (the point on the surface located on the vertical hypocenter) is in the middle of the sea then it will result in a tidal wave (also called tsunami). Many of these are caused by a sudden vertical movement of the bottom of the sea and the waves are formed on the surface (like when you throw a stone) very large which can travel at speeds of 500 to 1000 km per hour. When the waves of the typically arrive near the coast rise (because it decreases the depth of the sea) up to heights of 40 meters and over. The tidal waves are long waves, with periods between 5 and 60 minutes (on average 15-20 min.), Generated impulsively for the displacement of the mass of water and which, approaching the coast, can reach very high heights. The international scientific community has unanimously adopted the term tsunami from the Japanese “tsu” = harbor and “nami” wave = (wave port) to indicate the phenomenon of tsunamis. Tsunamis are caused in most cases by underwater earthquakes or near the coast and, less frequently, by air or submarine landslides, volcanic eruptions and, rarely, by the impact of meteorites in the water. Not all earthquakes submarines are able to generate tsunamis. For this to occur it is necessary that the earthquake has a depth of not too high, a significant magnitude and, especially, has a mechanism which causes a vertical displacement of the seafloor able to put in motion the mass overlying water. Even submarine landslides, with sliding of sediments (often triggered by earthquakes), can change the balance of the mass of water and produce a tsunami, as well as the fall in the water of large blocks of rock or sediment in the case of landslides routes. Sometimes violent submarine volcanic eruptions can create an impulsive force that moves the water column and generates the tsunami. In addition tsunamis of volcanic origin may be due to slipping into the sea of ​​masses of incandescent lava material along the steep flanks of the volcano. From a physical point of view tsunami waves are characterised by wavelength (distance between two crests) very high, of the order of tens or hundreds of kilometers, so very large compared to the depth of water in which they travel, also in the open ocean. This feature means that the tsunami waves behave like “waves in shallow water.” These waves traveling at high speed in the open sea, even reaching 700-800 km / hour, and are able to propagate for thousands of kilometers retaining virtually unaltered their energy and being therefore able to crash with exceptional violence on coasts also very away from the point of origin. The tsunami waves, which in the open sea often go unnoticed for their lack of height, when they approach the coast undergo a transformation: their speed is reduced (being directly proportional to the depth of the water) and consequently the height of wave increases, until you get to reach some tens of meters when it hits the coast. The height and the impact of the waves on the coast is a function of many parameters. In fact, over the water depth of the topography of the seabed and the characteristics of the coast, such as the presence of inlets, bays, straits, and estuaries that can produce amplification effects, play a crucial role. Sometimes the tsunami is manifested by a phenomenon of early withdrawal of water (regression) that leaves dry ports and vessels for a short time. In fact this is the arrival of the wave trough and is therefore a factor that heralds the arrival of the next ridge and the subsequent flooding (ingression). The tsunami reaches the coast that may appear similar to a tide that rises and falls rapidly, raising the general level of water even several meters, or can present as a train of waves, of which the first is not necessarily the greatest; or looks like a real wall of water and, in these cases, the impact of the tsunami waves on the coast is very often devastating. After the flood, when a tsunami wave withdraws tends to drag with him everything he has encountered in its path on the beach and leave on the ground water and debris that form deposits that are important to reconstruct the ingression maximum altitude reached by the wave. Tsunamis are a very important phenomenon and often underrated, can produce significant damage and loss of many lives. Fortunately, catastrophic tsunamis are rare events, however minor and major tsunami hit often in the world. In particular, the Pacific region is one in which these phenomena are more frequent and disastrous, with waves able to cross the entire Pacific Ocean in less than 24 hours. The region of Japan-Taiwan is the most active area, where it generates about 30% of the total number of tsunamis in the Pacific, although not all are destructive. The tsunami generated by the earthquake in Chile in 1960, as well as destroying all the villages along 800 km of coastline, traveled 17,000 km of Pacific Ocean and arrived in Japan after about 22 hours and caused considerable damage. The Sendai and Tōhoku earthquake in North Japan occurred on 11 March 2011 at a depth of 30 kilometers. The quake, with a magnitude of 9.0, with its epicenter in the sea and subsequent tsunami, is still the most powerful ever recorded in Japan and the seventh globally.

So to conclude we can consider the earthquakes as the tool with which the earth changes shape and evolves, the mountains grow and rise, the valleys open, in short, the earthquakes show that our planet is “alive” and constantly changing.

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