Earthquakes and Earth’s Structure

Most of what we know about the structure of the Earth has been learned from the observation and interpretation of the propagation of seismic waves, In geophysics, we distinguish between seismology and exploration seismics: While in seismology information on the structure of the Earth is derived from earthquakes and the natural oscillations of the Earth excited by strong earthquakes, exploration seismics uses man-made vibrations (e.g., from explosions, vibrators, air guns, dropping weights, hammers) and natural microtremors to explore the upper crust for structures characteristic of deposits and reservoirs, especially of hydrocarbons. While exploration seismics has historically evolved from seismology and even today the two fields are strongly interconnected, we will not discuss them further here. Interested readers are referred to the numerous specialized texts that deal with the exploration methods of applied geophysics.