Seismology is the study of the propagation of seismic, or sound, waves through the Earth, including analysis of earthquake sources, mechanisms, and the determination of the structure of the Earth through variations in the properties of seismic waves. The analysis is quantitative and typically requires high-powered computers.
The structure of the deep parts of the Earth can be mapped by seismology. Seismographs are stationed all over the world. studying the propagation of seismic waves from natural and artificial sources, such as earthquakes, nuclear explosions, and other seismic events, allows for the calculation of changes in the properties of the Earth in different places. If the Earth had a uniform composition, seismic wave velocity would increase smoothly with depth because increased density is equated with higher seismic velocities. By plotting the observed arrival time of seismic waves, however, seismologists have found that the velocity does not increase steadily with depth, but that several dramatic changes occur at discrete boundaries and in transition zones deep within the Earth.
Seismologists calculate the positions and changes across these zones by noting several different properties of seismic waves. Some are reflected off interfaces, just as light is reflected off surfaces, and other waves are refracted, changing the velocity and path of the rays. These reflection and refraction events happen at specific sites in the Earth, and the positions of the boundaries are calculated by using wave velocities. The core-mantle boundary at 1,802 miles (2,900 km) depth in the Earth strongly influences both P- and S-waves. It refracts P-waves, causing a P-wave shadow and, because liquids cannot transmit S-waves, none gets through, causing a huge S-wave shadow. These contrasting properties of P- and S-waves can be used to map accurately the position of the core-mantle boundary.
Variation in the propagation velocity and direction of seismic waves illustrates several other main properties of the deep Earth. Velocity gradually increases with depth, to about 62 miles (100 km), where the velocity drops slightly between 62-124 miles (100-200 km) depth, in the low velocity zone. The reason for this drop in velocity is thought to be small amounts of partial melt in the rock, corresponding to the asthenosphere, the weak sphere on which the plates move, lubricated by partial melts.
Another seismic discontinuity exists at 248.5 miles (400 km) depth, where velocity increases sharply due to a rearrangement of the atoms within olivine in a polymorphic transition into spinel structure, corresponding to an approximate 10 percent increase in density.
A major seismic discontinuity at 416 miles (670 km) could be either another polymorphic transition or a compositional change. This is the topic of many current investigations. Some models suggest that this boundary separates two fundamentally different types of mantle, circulating in different convection cells, whereas other models suggest that there is more interaction between rocks above and below this discontinuity.
The core-mantle boundary is one of the most fundamental on the planet, with a huge density contrast from 5.5 g/cm3 above, to 10-11 g/cm3 below, a contrast greater than that between rocks and air on the surface of the Earth. The outer core consists mainly of molten iron. An additional discontinuity occurs inside the core at the boundary between the liquid outer core and the solid, iron-nickel inner core.
Seismic waves can also be used to understand the structure of the Earth's crust. Andrija Mohorovicic, a Yugoslav seismologist, from Volosko in Croatia, noticed slow and fast arrivals from nearby earthquake source events. He proposed that some seismic waves traveled through the crust, some along the surface, and others were reflected off a deep seismic discontinuity between seismically slow and fast material at about 18.6 miles (30 km) depth. Geologists now recognize this boundary, called the Mohorovicic (or Moho) boundary, to be the base of the crust and use its seismically determined position to measure the thickness of the crust, typically between 6.2-43.5 miles (10-70 km).
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