A guide to practical seismology / Руководство по практической сейсмологии

The Earth is approximately spherical, with a mean radius R = 6370km, a very small flattening (+7/ − 15km), mass  6 × 1024kg, and an average density 5.5g/cm3; the law of gravitational attraction is F = GmMr/r3, where F is the force directed along the separation distance r between two point bodies with mass m and M; and G = 6.67 × 10−8cm3/g · s2 is the gravitation constant.
Little is known about Earth’s interior. The drilling down into the earth reaches at most 10 − 15km. It is accepted that the Earth consists of several shells. First, at the surface, there is a solid crust, extending down to approximately 70km, on average; locally it may have 5km thickness. Down to approximately 3000km an extremely viscous mantle exists. The next 2000km down to the centre are occupied by a liquid outer core. Finally, a solid inner core exists at the centre. The viscosity of the mantle is 1022 − 1025g/cm · s; for comparison, water has 10−2g/cm · s. The physical properties are discontinuous at the boundaries of these layers, like the "Moho" discontinuity (named after Mohorovicic) between crust and mantle, or the Gutenberg discontinuity between mantle and the liquid core, or the Bullen (or Lehmann) discontinuity between the inner core and the outer core. Changes in propagation of the elastic (seismic) waves have been measured and have indicated such discontinuities. The main chemical elements in the Earth’s shells are Fe, O, Si, Mg, S, Ni, Co, Al. The crust is made mainly of silicon dioxide and aluminium oxide. The crust density is 3g/cm3; the inner core has probably the density 13g/cm3. <...>