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  Before you begin the study of earthquakes, let¡¯s take a look at what exactly are these terrifying activities that are constantly threatening our lives beneath our feet. On an elementary level, an earthquake can be briefly explained as ¡°a sudden and violent movement of the Earth¡¯s surface and crust caused by the deformation and rupture of the inner layers of the Earth¡±.
  Rocks within the Earth are subjected to various types of forces and other factors that cause these rocks to deform in a number of ways, such as bending, twisting and fracture. The deformation of the rocks is called strain. Those forces that account for the strains are called stress. Before we move on here, it is necessary to explore the relationship and mechanism of strain and stress.
  There are generally two types of stress. The first type is called ¡°confining stress¡±, which forces are uniformly distributed over an object. The other type is called ¡°differential stress¡±, where forces are not uniformly distributed. There are three types of differential stress; they are tensional stress, compressional stress and shear stress. All rocks within the Earth experience some type of stress as the result of gravitational force and plate tectonics force.

  The deformation process of the rocks can be divided into three distinct and successive stages.

  • Elastic deformation: where the strain is reversible.
  • Ductile deformation: where the strain is irreversible.
  • Fracture: irreversible strain which causes the rock to fracture.

  .: Illustration of various types of strains

  To fully understand the different stages of deformation, one crucial concept you have to know is that all materials are elastic at some level, includes the rocks within the Earth¡¯s inner crust. To help yourself understand the concept, imagine a piece of spring. As you apply more force while you are attempting to stretch it, it becomes longer as a way it deforms, and because the elastic nature of the spring, it will resume to its original shape and length once you stop the stretching. However, you do not expect it to be stretched infinitely as you continue to apply more force. Moreover, you would realize that once the force you applied to the spring exceeded a certain amount, it loses its elasticity and becomes unable to resume its original shape permanently. That limit of its deformation is called the elastic limit. Once the force an object receives exceeds the elastic limit, it will become deformed permanently. If you continue stretching the spring, you will eventually be able to break it. The same concept applies to a rock.

  The physics behind this is Hook¡¯s Law, which is a law regarding the elasticity of matter claimed by the British scientist Robert Hooke (1635-1703). The law states that ¡°the deformation of an elastic object is proportional to the force applied to it¡±.

   The law can then be represented mathematically as F ¡Ø x, which F is the force applied and x is the degree of deformation, and x is proportional to the force applied.

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   The next concept introduced is the fault. A fault is the offset of the Earth¡¯s crust caused by the fracture of crustal rocks. Again, since this tutorial is intended for all kinds of readers, a basic yet important concept has to be illustrated here before we take a look at faulting in greater detail in order to accommodate readers with little knowledge of geology and geological features.

  Since we are going to examine the various types of faults, we have to understand the three-dimensional orientation of a fault. The two terminologies that are going to be mentioned here are strike and dip. For an inclined plane, the strike is the direction parallel to any horizontal line on the plane, and the dip is the angle between the horizontal plane and the inclined plane. fault

The several types of faults are:

  Faults that have offset of crusts along the direction of the dip. Note that here for simplicity; we call the part of the crust above the offset hanging wall block and the part below footwall block. The several types of dip-slip faults are descried below:

  • Normal faults: faults caused by a horizontal stress upon the crust. The crust fractures and eventually becomes pulled apart by the stress. The rock mass moves downward along the dip relative to the footwall block.
  • Reverse faults: this type of fault occurs when the crust experiences horizontal compressional stress that compresses the two rock masses. The hanging-wall block for this fault moves upward along the dip relative to the footwall block, which is the opposite direction in the normal fault. One special type of reverse fault is called the thrust fault, which the angle of the dip is very low (usually around 15¡ã). This makes one block of rock mass thrust over the other one.
  • Horsts & Grabens: a fault which consists a series of normal faults occur adjacent to each other that moves in alternating directions. In this case the downward blocks are called grabens and the upward blocks are called horsts. Sometimes the grabens can form rift valleys and the horsts can form mountain ranges.
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  Now you have understood the different types of faults and the elastic nature of rocks, you are then ready to take a look at the mechanisms of the formation of earthquakes.

  The elastic nature of the crustal rock and the various types of stresses it experiences combined together accounts for the primary driven force for the formation of earthquakes. Earthquakes form due to the sudden release of elastic potential energy stored in the strained crustal rocks within the Earth. As the crustal rocks are becoming more and more strained, they will eventually exceed their elastic limit and become deformed and fractured. The rock fractures in response to the strain along the fault, releases its elastic potential energy and produces a dislocation of the crust; this mechanism is called ¡°elastic rebound¡±. According to the elastic rebound theory, if the elastically strained rocks are unable to resume their original shape along a fault, they accumulate more potential energy as they are being ¡°confined¡±, and when they eventually have adequate space to move (such as slippage), the enormous amount of potential energy accumulated will be suddenly released. The released energy sends out different kinds of seismic waves that travel through the Earth to cause vibrations.

   As the name implies, body waves travel through the Earth towards all directions from the focus. There are basically two types of body waves.

P-wave illustration   Primary wave: also called ¡°P-wave¡±, can travel through both solid and liquid. It is the fastest wave among all the waves emanated from the earthquake. Therefore it is the first indication of the occurrence of an earthquake. P-waves are longitudinal waves, which cause the crustal rocks to vibrate back and forth along the direction of the wave. As it travels through a median, for example, the Earth¡¯s curst, it alternately pushes (compresses) and dilates the rocks. In general, P- waves travel on average of about 6km/sec through the Earth¡¯s crust and 8km/sec through oceanic crust.
S-wave illustration   Secondary wave: also called ¡°S-wave¡±, only travels approximately half the velocity of the P-wave. It is a transverse wave, which cannot travel through liquid. They cause vibrations of objects perpendicular to the direction of its movement.

   The second class of seismic wave is called surface waves. They do not travel through the Earth like the body waves, but instead along the path parallel above the Earth. They are transverse waves, which cause vibrations of objects perpendicular to the direction of its movement just like the S-waves. The two types of surface waves are described below.

Love wave illustration   Love wave: does significant damage to the foundations of most man-made structures. It vibrates the ground and other objects in its path perpendicular to the path of propagation.
  Rayleigh wave: is the shortest seismic wave of all. Rayleigh wave vibrates objects in its path in a vertical plane with an elliptical movement as the wave pass by. Much like the oceanic waves.

   One intriguing piece of fact about seismic waves is that as they travel through different mediums, they are constantly being bent and refracted. Therefore when the reach the surface of Earth, much of their energy is reflected to the ground, therefore significantly increases the vibrations that they cause.     

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