The structure of Earth"s deep interior cannot be studied directly. However geologists useseismic (earthquake) waves to determine the depths of layers of molten andsemi-molten material within Earth. Because different species of earthquake waves behavedifferently when they encounter material in various states (for example,molten, semi-molten, solid), seismic train station established roughly Earth detect and recordthe toughness of the different varieties of waves and also the directions from whichthey came. Geologists usage these records to create the structure of Earth"s interior.

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The 2 principal species of seismic waves space P-waves (pressure; goes with liquid andsolid) and S-waves (shear or secondary; goes just through hard - not with liquid). The take trip velocity of these 2 wave species is no the very same (P-waves room fasterthan S-waves). Thus, if over there is an earthquake somewhere, the very first waves thatarrive room P-waves. In essence, the void in P-wave and also S-wave arrival provides a firstestimate that the street to the earthquake.

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Above image shows some common seismograms with arrival the P- and S-waves marked.

As we understand from physics, every waves change direction when they pass through layers ofdifferent thickness (refraction). That is what provides light collection in a magnifyingglass, and also that is also what renders seismic waves travel in curved routes through the Earth(because of the raising pressure, materials are much more dense in the direction of the core, travelvelocity that seismic tide increases). Refraction that seismic waves causes them tocurve away from a direct path. Reflection causes them come glance off specific surfaces(e.g. Main point mantle boundary) when they hit the at as well shallow of one angle. The resultof this behavior, in mix with the truth that S-waves can not travel throughliquids, is the illustration of seismic shadows, the contrary of the actual earthquakesite.

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The geometric distribution and also extent of these shadows together measured for a givenearthquake (many receiver stations - seismographs, space needed everywhere the civilization to dothat) permits us to calculation the position of major boundaries in the Earth"s interior, aswell as offering us information around the solid vs fluid character of the miscellaneous layers,and even around some of your physical properties.

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The greatest discontinuity in the Earth"s internal is the core/mantle boundary, becausethere we have a strong density contrast in between the iron main point (density in between 10-11 g/cm3)and the silicate mantle (density indigenous 3.3-5.5 g/cm3, increases with depth).