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P wave and S wave from seismograph Velocity of seismic waves in Earth versus depth. [1] The negligible S-wave velocity in the outer core occurs because it is liquid, while in the solid inner core the S-wave velocity is non-zero. A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body.
Seismology (/ s aɪ z ˈ m ɒ l ə dʒ i, s aɪ s-/; from Ancient Greek σεισμός (seismós) meaning "earthquake" and -λογία (-logía) meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic waves through planetary bodies.
A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any affected location or at a seismograph. P waves may be transmitted through gases, liquids, or solids.
Seismic waves travelling at a critical angle (blue line) will be refracted critically with an angle of refraction equal to 90°. An illustration of seismic reflection and refraction. Seismic refraction usually requires a wide incident angle so that the refracted seismic wave can travel critically (angle of refraction equals 90°).
Seismic waves are mechanical perturbations that travel in the Earth at a speed governed by the acoustic impedance of the medium in which they are travelling. The acoustic (or seismic) impedance, Z, is defined by the equation: = , where v is the seismic wave velocity and ρ (Greek rho) is the density of the rock.
Taking the divergence of seismic wave equation in homogeneous media, instead of the curl, yields a wave equation describing propagation of the quantity , which is the material's compression strain. The solutions of this equation, the P waves, travel at the faster speed α = ( λ + 2 μ ) / ρ {\textstyle \alpha ={\sqrt {(\lambda +2\mu )/\rho }}} .
The P wave is the first wave that is bigger than the other waves (the microseisms). Because P waves are the fastest seismic waves, they will usually be the first ones that the seismograph records. The next set of seismic waves on the seismogram will be the S waves. These are usually bigger than the P waves, and have higher frequency.
It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity. It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves. [2]