Seismic wave phase

Related to the internal structure of most of the Earth is to examine the body wave travel time (travel time) on that. Seismic wave propagation path can be used wave line (ray) to express. Different wave line has a different name, called the wave-phase (phase). 

P wave and S-wave speed and medium-density and flexible nature, the Earth's interior P-wave and S wave velocity usually increases with depth, but in some depth as a result of material or phase changes in particular have changed , such as the outer core part of that is because it is the relationship between the liquid and can not transmit S waves and the sharp drop in P-wave velocity. 

Affected by the P wave S-phase studies, and the travel time curve (travel time curve) by the performance characteristics of the Earth found that the presence of non-continuous surface wave velocity, that is, the passage of seismic waves, the speed of drastic change. The most famous one for discontinuous surface (Moho discontinuity) for the crust and upper mantle boundary; Second, discontinuous surface Gothenburg (Guttenburg discontinuity) for the mantle and the core line. 

Because the speed of the existence of discontinuous surface, the local shock waves through occurs when the phenomenon of reflection and refraction, and therefore can also be seen on seismic records spread P-wave S-wave reflection and refraction, such as wave-wave phase. Through different parts of the wave of the Earth have different names. Starting from the source of seismic waves through the Earth's interior and then back to the Earth's surface, then the letter in the P wave or S wave lines still express P or S; outer nuclear ball line in P to K said that without the existence of S-wave; in kernel P wave and S ball line to I and J, respectively, express; ─ letter in the core of the reflex points on the border in order to express c; ─ in the core outer core boundary on the reflection point to i express. 

We can wave line is divided into five categories; 1) wave lines in letter only to P, S, PP, SS, PS, PPP, ..., such as express. 2) wave-line through the mantle in the earth's core mantle boundary reflection ─ after the return to the surface through the mantle to PcP, PcS, ScS ... such as express. 3) wave-line through the mantle and outer core to return to the surface, to PKP, PKS, SKS ... and so on express. 4) wave-line through the mantle, outer core, then core outer core boundary ─ reflex, and then through the outer core, mantle and return to the surface, to PkiKP, PKiKS, SkiKS ... such as express. 5) wave-line through the mantle, outer core and kernel to PKIKP, PKIKS, PKJKP, SKJKS ... such as express. 


When the source is not surface when waves can customize the source down the spread of the spread can also be upward. Upward spread of seismic reflection in the table, and then down the spread of this from this source to the surface wave line, to p, s express, so there may be wave of pP, sS, pPKP ... and so on. 


Because the Earth's internal structure is very complex, all floors of the wave propagation speed is different and thus can cause refraction, affect the transmission of shock waves. To P-wave as an example, P-wave in the internal dissemination of the Earth, the distance from the earthquake epicenter and to 1 degrees, 2 degrees ... 103 degrees can still be clearly recorded in the direct P wave, but from 103 onwards, P-wave because after refraction arising from the earth's core, because the relationship between the angle of incidence, refraction of the P wave will appear in addition to 143 degrees. 
So epicenter distance of 103 degrees to 143 degrees, while still observing the P-wave phase, but in this area of the P wave for the diffraction wave, energy is very weak compared with other places, in this interval that is with the formation of the shadow (shadow zone)