Regional & Whole-Earth Structure
Earth structure taken from 'Earthquakes' by Kaye Shedlock and Louis C. Pakiser.
Seismology is the study of earthquakes and the Earth using seismic waves. From recordings of earthquake-generated waves, information about the earthquake source may be derived, including its magnitude, location, time of occurrence, depth, and its orientation and movement on the fault. Teleseismic waves (waves coming from distant earthquakes) provide information about the entire Earth structure (crust, mantle and core). Seismic waves generated by a controlled source are used to image the Earth's crustal structure. These images reveal the depth and area of basins, fault networks, and the physical properties of rocks. Seismic imaging techniques have analogs in medical science. Waves transmitted (refracted) directly through the earth produce a "catscan"-type image, while waves reflected back to the surface from layer boundaries or faults produce a "sonogram"-type image. Traveltime observations and variations in amplitude, frequency, and waveform are combined to produce a model of the geologic structure. The model must be consistent with other geophysical and geological data, such as measurements of the strength of gravity, measurements of the strength of the magnetic and electric fields, and laboratory measurements of the speed of seismic waves in rock samples.The related field of strong motion seismology uses waves from large earthquakes to study the earthquake source in detail, predict the strength of future shaking, establish safer building codes and improve seismic engineering design.