Before illustrating how Seismic tomography works, let's get started with the analogy of CT Scans. Doctors use CT scans to look at organs and bones inside the body without surgery. CT scan machines shoot X-rays through a patient’s body to obatin images that show the patient’s internal structures from different directions. Computers combine these images into a three-dimensional picture of the body 2.
In a technique similar to CT scans, Seismologists use seismic waves to make images of Earth’s interior. The energy is released from the focus of an earthquake as seismic waves that spread outwards in all direction. These seismic waves travel through the Earth's interior until reach the surface. Upon arrival Earth's surface, these waves shake the ground, and the digital earthquake records have been received by hundreds stations distributed on the earth surface all over the world. Each station has seismometers, which are the instruments that record the up-and-down and side-to-side motions of the ground in the form of squiggly lines known as Seismogram.
By recording seismograms of many earthquakes at each station, It has been able to seismologists to create high-resolution images of Earth’s interior using seismic tomography. Using the travel distance of the wave from the focus of the earthquake to the seismometer at the recording station, seismologists can calculate the average speed of the seismic waves. They then make slices at different depths that show the areas where the seismic waves traveled slower or faster than average. The speed of the seismic waves depends on the type of rock materials, temperature of the rocks (hot molten rocks or cold rocks) and stability of the area ( tectoniically active area or areas located within the plate). Generally speaking, seismic waves travel slower through less-stiff rock materials, hot molten rocks and tectonically active area while they move faster through rigid rock materials, cold rocks and stable areas that located withinplate. 2.
By using few number of stations, few received waves cut across the unknown structure. However, good distribution of stations and great number of the stations on the ground allow seismologists to get much data that cut across the structure. To obtain good resolution tomography, the area of interest should be covered by great number of stations to obtain much data that will lead, in turn, to detect the shape and the size of unknown structure inside the earth from the tomographic results. 2
- Incorporated Research Institutions for Seismology, from http://www.iris.edu/hq/inclass/animation/seismic_tomography_ct_scan_as_analogy#
- Incorporated Research Institutions for Seismology, from http://www.iris.edu/hq/inclass/fact-sheet/seismic_tomography