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November 26, 2008 10:39AM
P-waves are type of elastic wave, also called seismic waves, that can travel through gasses (such as sounds), elastic solids and liquids, including the Earth. P-waves can be produced by earthquakes and recorded by seismometers. The name P-wave stands for primary wave, as the P-wave is the fastest among the elastic waves, compared to the S-waves. In isotropic and homogeneous solids, the polarization of a P-wave is always longitudinal; thus, the particles in the solid have vibrations along or parallel to the travel direction of the wave energy. In isotropic and homogeneous solids, the polarization of P-waves is always longitudinal. This means that the particles in the body have vibrations along or parallel to the direction of travel of the wave energy. Earthquake advance warning is possible by detecting the non-destructive primary waves that travel more quickly through the earth's crust than do the destructive secondary and Rayleigh waves. The amount of advance warning depends on the delay between the arrival of the P-wave and other destructive waves, generally on the order of seconds up to about a minute maximum for deep, distant, large quakes. The effectiveness of advance warning depends on accurate detection of the P-waves and compensation for ground vibrations caused by local activity (such as trucks or construction work). 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 "shadow zone" between 104° and 140°, where the initial P-waves are not registered on seismometers. In contrast, S-waves do not travel through liquids, rather, they are attenuated.A type of seismic wave, the S-wave, secondary wave, or shear wave (sometimes called an elastic S-wave) is one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves. The S-wave move as a shear or transverse wave, so motion is perpendicular to the direction of wave propagation: S-waves, like waves in a rope, as opposed to waves moving through a slinky, the P-wave. The wave moves through elastic media, and the main restoring force comes from shear effects. These waves are divergenceless and obey the continuity equation for incompressible media: Its name, S for secondary, comes from the fact that it is the second direct arrival on an earthquake seismogram, after the compressional primary wave, or P-wave. 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. In fact, this property allows seismologists to determine the nature of the inner core. The velocity of an S-wave in an isotropic medium can be described by the shear modulus μ and density ρ. As transverse waves, S-waves exhibit properties, such as polarization and birefringence, much like other transverse waves. S-waves polarized in the horizontal plane are classified as SH-waves. If polarized in the vertical plane, they are classified as SV-waves. When an S- or P-wave strikes an interface at an angle other than 90 degrees, a phenomenon known as mode conversion occurs. As described above, if the interface is between a solid and liquid, S becomes P or vice versa. However, even if the interface is between two solid media, mode conversion results. If a P-wave strikes an interface, four propagation modes may result: reflected and transmitted P and reflected and transmitted SV. Similarly, if an SV-wave strikes an interface, the same four modes occur in different proportions. The exact amplitudes of all these waves are described by the Zoeppritz equations, which in turn are solutions to the wave equation. In elastodynamics, Love waves are essentially horizontally polarized shear waves (SH waves) guided by an elastic layer, which is "welded" to an elastic half space on one side while bordering a vacuum on the other side. In seismology, Love waves (also named Q waves (Quer: German for lateral)) are surface seismic waves that cause horizontal shifting of the earth during an earthquake. A.E.H. Love predicted the existence of Love waves mathematically in 1911; the name comes from him (Chapter 11 from Love's book "Some problems of geodynamics", first published in 1911). They form a distinct class, different from other types of seismic waves, such as P-waves and S-waves (both body waves), or Rayleigh waves (another type of surface wave). Love waves travel with a slower velocity than P- or S- waves, but faster than Rayleigh waves. The particle motion of a Love wave forms a horizontal circle or ellipse moving in the direction of propagation. Moving deeper into the material, motion decreases to a "node" and then alternately increases and decreases as one examines deeper layers of particles. The amplitude, or maximum particle motion, decreases rapidly as one examines deeper layers of particles. Since Love waves travel on the Earth's surface, the strength (or amplitude) of the waves decrease exponentially with the depth of an earthquake. However, given their confinement to the surface, their amplitude decays only as, where r represents the distance the wave has traveled from the earthquake. Surface waves therefore decay more slowly with distance than do body waves, which travel in three dimensions. Large earthquakes may generate Love waves that travel around the Earth several times before dissipating. Love waves take a long time to dissipate due to the huge amount of energy that they contain. For this reason, they are most destructive within the immediate area of the focus or epicentre of an earthquake. They are what most people feel directly during an earthquake. In the past, it was often thought that animals like cats and dogs could predict an earthquake before it happened. However, they are simply more sensitive to ground vibrations than humans and able to detect the subtler waves that precede Love waves, like the P-waves and the S-waves.