a seismic wave that travels along the earth surface
Seismic wave Among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, and surface waves, which travel at the Earth's surface.[2]:48–50[3]:56–57
Mantle (geology) The top of the mantle is defined by a sudden increase in seismic velocity, which was first noted by Andrija Mohorovičić in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho".[24][27] The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, an irregular layer with a maximum thickness of perhaps 200 km (120 mi). Below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the seismic shear velocity is reduced; this so-called low-velocity zone (LVZ) extends down to a depth of several hundred km. Inge Lehmann discovered a seismic discontinuity at about 220 km (140 mi) depth;[28] although this discontinuity has been found in other studies, it is not known whether the discontinuity is ubiquitous. The transition zone is an area of great complexity; it physically separates the upper and lower mantle.[26] Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous. The D" layer at the core–mantle boundary separates the mantle from the core.[15][24] In 2015, research using gravitational data from GRACE satellites and the long wavelength nonhydrostatic geoid indicated viscosity[29] increases by a factor of ten to 150 about 1,000 kilometres (620 mi) below earth's surface; separate research also indicates sinking tectonic plates stall at this depth, leading Robert van der Hilst to speculate "In term's of structure and dynamics, 1,000 kilometers could be more important" (than the currently accepted 660 km depth upper—lower division).[30] The lower mantle also contains some discontinueous zones, called "thermochemical piles" which have been interpreted as either thermally differentiated, upwellings bringing warmer material towards the surface, or as chemically differentiated material.[31] A principal source of the heat that drives plate tectonics is the radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle.[32]