What is seismic tomography?
It is the modeling through 3-D mapping of changes in seismic wavespeed. We have a good idea of what the average structure of the Earth is like, we know what the main layers are, how fast seismic waves travel through them on average, and how seismic wavespeed change with depth, but lateral variations in seismic wave velocity are superimposed to this average model. Seismic tomography is a direct way of detecting these variations. It is a technique similar to those used in medical imaging (e.g. CAT scan) except that the energy source comes from earthquakes instead of a controlled source such as x-rays in the case of CAT scans.
Why do we care?
Most earthquakes, volcanoes, mountains, tsunamis, etc occur due to the relative motion of the tectonic plates that pave Earth’s surface. These motions at the surface are driven by convection currents inside Earth’s mantle, as initially postulated by Arthur Holmes. Convection is a mode of heat transport in which material lighter than the surroundings rises whereas denser material sinks. It is driven by buoyancy forces, which are controlled by the density difference between a material and its surroundings. Determing 3-D variations in seismic wavespeed is important because it can give us some clues as to how temperature and chemical composition change throughout the Earth, which themselves determine 3-D changes in density. Mantle tomography is thus an important component of better understanding mantle convection, plate tectonics, and planetary evolution.
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