Seismic waves

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Seismic waves

  1. 1. أحمد صبح إبراهيم خيال Sec. 8 B.N. 14 SEISMIC WAVES
  2. 2. <ul><li>Seismic waves are waves of energy that travel through the core of the earth or other elastic bodies generated from earthquake, explosion, or some other process that imparts low-frequency acoustic energy. </li></ul><ul><li>The propagation velocity of the waves depends on the density and elasticity of the medium. </li></ul><ul><li>Velocity tends to increase with depth, and ranges from approximately 2 to 8 km/s in the Earth's crust up to 13 km/s in the deep mantle. </li></ul><ul><li>Various types travel at different velocities. </li></ul><ul><li>Refraction or reflection of seismic waves is used for research of the Earth's interior, and artificial vibrations to investigate subsurface structures. </li></ul>Seismic Waves
  3. 3. <ul><li>Seismic waves are predicted during the 19 th century. </li></ul><ul><li>It is similar to sound waves except that the periods of oscillations are far longer. </li></ul><ul><li>The frequency range of these waves is large from as high as the audible range to as low as the free oscillations of the entire Earth </li></ul><ul><li>There are two types of seismic waves: body and surface waves </li></ul>Seismic Waves
  4. 4. <ul><li>travel through the interior of the Earth </li></ul><ul><li>follow ray paths refracted by the varying density and modulus (stiffness) of the Earth's interior </li></ul><ul><li>(density and modulus, in turn, vary according to temperature, composition, and phase similar to the refraction of light waves) </li></ul><ul><li>the two types are P-waves and S-waves </li></ul>Body Waves
  5. 5. <ul><li>P waves, also called primary or pressure waves, are longitudinal or compressional in nature. </li></ul><ul><li>These waves are composed of alternating compressions and rarefactions. </li></ul><ul><li>P waves can travel through any medium. </li></ul><ul><li>In solids, these waves generally travel almost twice as fast as S waves. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. </li></ul><ul><li>These waves travel at ~6 km/s near the surface to ~10.4 km/s near the Earth’s core about 2900km below the surface. </li></ul><ul><li>As the waves enter the core, the velocity drops to ~8 km/s increasing to ~11 km/s near the center. </li></ul><ul><li>These results from increased hydrostatic pressure as well as from changes in rock composition and phase. </li></ul>P waves
  6. 6. <ul><li>The transmitting rocks are alternately compressed and expanded giving the rock particles a back-and-forth motion along the path of propagation. </li></ul>P waves
  7. 7. <ul><li>Almost all the information available on the structure of the Earth's deep interior is derived from observations of the travel times, reflections, refractions and phase transitions of seismic body waves, or normal modes. Body waves travel through the fluid layers of the Earth's interior, but P-waves are refracted slightly when they pass through the transition between the semisolid mantle and the liquid outer core. As a result, there is a P-wave &quot;shadow zone&quot; between 104° and 140° from the earthquake's focus, where the initial P-waves are not registered on seismometers. </li></ul>P wave shadow zone
  8. 8. P wave shadow zone
  9. 9. <ul><li>S waves, also called secondary or shear waves, are transverse in nature </li></ul><ul><li>Unlike P waves, S waves can only travel through solids. </li></ul><ul><li>These waves travel from ~3.4 km/s near the surface to ~7.2 km/s near the boundary of the liquid core (Gutenberg discontinuity). </li></ul><ul><li>Also, these waves travel at a slower rate but with greater amplitude. </li></ul><ul><li>S waves travel transversely to the direction of propagation and involves the shearing of the transmitting rock causing the rock particles to move back and forth perpendicular to the direction of propagation. </li></ul>S waves
  10. 10. <ul><li>As the waves pass, the rock is distorted first in one direction and then in another. </li></ul>S waves
  11. 11. <ul><li>Unlike the P-wave, the S-wave cannot travel through the molten outer core of the Earth, and this causes a shadow zone for S-waves opposite to where they originate. They can still appear in the solid inner core: when a P-wave strikes the boundary of molten and solid cores, called the Lehmann discontinuity, S-waves will then propagate in the solid medium. And when the S-waves hit the boundary again they will in turn create P-waves. This property allows seismologists to determine the nature of the inner core. </li></ul>S wave shadow zone
  12. 12. <ul><li>S-waves don't penetrate the outer core, so they're shadowed everywhere more than 104° away from the epicenter. </li></ul>S wave shadow zone
  13. 13. <ul><li>Surface waves travel only on the surface of the Earth. </li></ul><ul><li>These waves are guided by the free surface of the Earth. </li></ul><ul><li>Surface waves are analogous to water waves. </li></ul><ul><li>Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave. </li></ul><ul><li>They follow along after the P and S waves have passed through the body of the planet. </li></ul><ul><li>S waves disperse into long wave trains and at substantial distance from the source, they cause much of the shaking felt during earthquakes. </li></ul>Surface Waves
  14. 14. <ul><li>There are two types of surface waves: Rayleigh waves and Love waves. </li></ul>Surface Waves
  15. 15. <ul><li>Love Waves are named after A. E. H. Love who predicted their existence in 1911. </li></ul><ul><li>Love waves travel with a slower velocity than P- or S- waves, but faster than Rayleigh waves. </li></ul><ul><li>These waves are propagated in a surface layer that overlies a solid layer with different elastic properties. </li></ul><ul><li>The displacement of the rock particles is entirely perpendicular to the direction of propagation and has no vertical or longitudinal components. </li></ul><ul><li>The energy created by these waves spreads from the source in two directions rather than in three. </li></ul>Love Waves
  16. 16. <ul><li>These produces a strong record at seismic equations even when originated from distant earthquakes. </li></ul>Love Waves
  17. 17. <ul><li>Rayleigh waves are named after Lord Rayleigh (John William Strutt, 3rd Baron Rayleigh, OM) who predicted their existence in 1885. </li></ul><ul><li>The motion in this kind of wave is a combination of longitudinal and vertical vibration that give elliptical motion to the rock particles. </li></ul><ul><li>These waves have the strongest effect on seismographs. </li></ul><ul><li>Rayleigh waves are generated by the interaction of P- and S- waves at the surface of the earth, and travel with a velocity that is lower than the P-, S-, and Love wave velocities. </li></ul>Rayleigh Waves
  18. 18. <ul><li>The intensity of Rayleigh wave shaking at a particular location is dependent on several factors: </li></ul><ul><li>The size of the earthquake. </li></ul><ul><li>The distance to the earthquake. </li></ul><ul><li>The depth of the earthquake. </li></ul><ul><li>The geologic structure of the crust. </li></ul><ul><li>The focal mechanism of the earthquake. </li></ul><ul><li>The rupture directivity of the earthquake. </li></ul>Rayleigh Waves

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