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P-waves are a type of body wave, called seismic waves in seismology, that travel through a continuum and are the first waves from an earthquake to arrive at a seismograph. The continuum is made up of gases (as sound waves), liquids, or solids, including the Earth. P-waves can be produced by earthquakes and recorded by seismographs. The name P-wave can stand for either pressure wave as it is formed from alternating compressions and rarefactions or primary wave, as it has the highest velocity and is therefore the first wave to be recorded. In isotropic and homogeneous solids, the mode of propagation of a P-wave is always longitudinal; thus, the particles in the solid vibrate along the axis of propagation (the direction of motion) of the wave energy. ==Velocity== The velocity of P-waves in a homogeneous isotropic medium is given by : where ''K'' is the bulk modulus (the modulus of incompressibility), '''' is the shear modulus (modulus of rigidity, sometimes denoted as ''G'' and also called the second Lamé parameter), '''' is the density of the material through which the wave propagates, and '''' is the first Lamé parameter. Of these, density shows the least variation, so the velocity is mostly ''controlled'' by ''K'' and ''μ''. The elastic moduli P-wave modulus, , is defined so that and thereby : Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s. The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13 km/s through the core. Geologist Francis Birch discovered a relationship between the velocity of P waves and the density of the material the waves are traveling in: Which later became known as Birch's law. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「P-wave」の詳細全文を読む スポンサード リンク
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