SEISMIC WAVES: Seismic waves are waves of energy that travel through the Earths layers, and are a result of an earthquake, explosion, or a volcano that imparts 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 Earths crust up to 13 km/s in the deep mantle.[1] Earthquakes create various types of waves with different velocities; when reaching seismic observatories, their different travel time help scientists to locate the source of the earthquake hypocenter. In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earths interior, and man made vibrations are often generated to investigate shallow, subsurface structures. Types of seismic waves: Among the many types of seismic waves one can make a broad distinction between body waves and surface waves. Body waves travel through the interior of the Earth, surface waves travel across the surface Surface waves decay more slowly with distance than do body waves, which travel in three dimensions Particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage Other modes of wave propagation exist than those described in this article; though of comparatively minor importance for earth-borne waves, they are important in the case of asteroseismology. 1) Body waves: Body waves travel through the interior of the Earth. They create raypaths refracted by the varying density and modulus (stiffness) of the Earths interior. The density and modulus, in turn, vary according to temperature, composition, and phase. This effect resembles the refraction of light waves. a) Primary waves: Primary waves (P-waves) are compressional waves that are longitudinal in nature. P waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name Primary. These waves can travel through any type of material, including fluids, and can travel at nearly twice the speed of S waves. In air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. b) Secondary waves: Secondary waves (S-waves) are shear waves that are transverse in nature. Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. 2) Surface waves: Seismic surface waves travel along the Earths surface. They are called surface waves because they diminish as they get further from the surface. Their velocity is lower than the velocity of the seismic body waves (P and S). Because of the long duration and large amplitude of the surface waves, they can be the most destructive type of seismic wave. Surface waves travel more slowly than the other type of seismic waves. a) Rayleigh waves: Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water (note, however, that the associated particle motion at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves, roughly 90% of the velocity of S waves for typical homogeneous elastic media. In the layered medium (like the crust and upper mantle) the velocity of the Rayleigh waves depends on their frequency and wavelength. See also Lamb waves b) Love waves: Love waves are horizontally polarized shear waves (SH waves), existing only in the presence of a semi-infinite medium overlain by an upper layer of finite thickness.[2] They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest amplitude. Stoneley waves: A Stoneley wave is a type of large amplitude Rayleigh wave that propagates along a solid-fluid boundary or under specific conditions also along solid-solid boundary. They can be generated along the walls of a fluid-filled borehole, being an important source of coherent noise in VSPs and making up the low frequency component of the source in sonic logging.[3] The equation for Stoneley waves was first given by Dr. Robert Stoneley (1894 - 1976), Emeritus Professor of Seismology, Cambridge. P and S waves in Earths mantle and core: An earthquake occurs, seismographs near the epicenter are able to record both P and S waves, but those at a greater distance no longer detect the high frequencies of the first S wave. Since shear waves cannot pass through liquids, this phenomenon was original evidence for the now well-established observation that the Earth has a liquid outer core, as demonstrated by Richard Dixon Oldham. This kind of observation has also been used to argue, by seismic testing, that the Moon has a solid core, although recent geodetic studies suggest the core is still molten
Posted on: Sun, 02 Feb 2014 05:59:52 +0000
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