The average density of the layers gets on increasing from crust to core and it

The average density of the layers gets on increasing

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The average density of the layers gets on increasing from crust to core and it is nearly 14.5g/cm3 at the very centre. Seismic Waves Seismic waves are waves of energy that travel through the Earth's layers, and are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions that give out low-frequency acoustic energy. Many other natural and anthropogenic sources create low- amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer. The propagation velocity of the waves depends on density and elasticity of the medium. 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. Earthquakes create distinct types of waves with different velocities; when reaching seismic
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observatories, their different travel times help scientists to locate the source of the hypocenter (focus). In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth's interior, and man-made vibrations are often generated to investigate shallow, subsurface structures. Among the many types of seismic waves, one can make a broad distinction between body waves , which travel through the Earth, and surface waves , which travel at the Earth's surface. Body waves They are generated due to the release of energy at the focus and moves in all directions travelling through the body of the earth. Hence, the name – body waves. They travel only through the interior of the earth. Body waves are faster than surface waves and hence they are the first to be detected on a seismograph. There are two types of body waves as primary waves and secondary waves.
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Primary waves (p-waves): Primary waves are the fastest body waves (twice the speed of s-waves) and are the first to reach during an earthquake. They are similar to sound waves, i.e, they are longitudinal waves, in which particle movement is in the same direction of wave propagation. They travel through solid, liquid and gaseous materials. They create density differences in the earth material leading to stretching and squeezing. Secondary waves (s-waves): They arrive at the surface with some time-lag after primary waves. They are slower than primary waves and can pass only through solid materials. This property of s-waves led seismologists to conclude that the earth’s outer core is in a liquid state. (the entire zone beyond 105 o from the epicentre does not receive S-waves) They are transverse waves in which directions of particle movement and wave propagation are perpendicular to each other.
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Surface Waves When the body waves interact with surface rocks, a new set of waves is generated called as surface waves.
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