Geophysics

Geophysics is the study of the physics of the Earth and its environment in space. One emphasis is exploration of the Earth’s interior using physical properties measured at or above the Earth’s surface, along with mathematical models to predict those properties. Sub-disciplines include seismology, the study of earthquake waves. geomagnetism, the study of the magnetic field; and geodesy, the study of gravity fields and the topography of the Earth’s surface. Seismology provides key evidence for the large-scale structure of the Earth and the behavior of earthquakes. Minerals, and therefore other physical properties at density and depth, are derived from experiments and mathematical modeling in mineral physics. The mathematical models that underlie geophysics also predict large-scale movements within the Earth (geodynamics).

Earthquake Seismology
Natural sources of seismic waves – earthquakes – are of great practical and scientific importance, and their seismological fingerprints provide valuable insights into the causes, mechanisms and hazards of earthquakes. Earthquakes typically occur by slip on faults, and seismic signatures allow constraining possible fault directions. The need to understand why earthquakes start – and stop – suddenly is provided by explaining the behavior of friction on fault surfaces. Surface consequences of earthquakes, especially ground motions and tsunamis, are direct hazards. Within the upper crust, other effects, such as changes in fluid pressure in porous rocks, are important because they can affect future earthquakes and also stimulate mineral deposits. Volcanic behavior often involves earthquakes. Slow earthquakes have recently been recognized as periodic, earthquake-like events that release energy over a period of hours to months rather than the seconds to minutes characteristic of a typical earthquake.

Seismology and Earth Structure
Earthquakes create rapid motions in the Earth’s interior, which create waves (oscillations of material points), which can traverse the entire planet with sufficient amplitude. The theory of elasticity is used to model such waves, which, like other types of waves, are reflected and refracted by boundaries and follow curved ray paths in heterogeneous media. When such waves reach the surface, ground motions can be recorded and used to constrain structures along the path. Different types of oscillations create waves that travel at different speeds. Primary (P) waves involve oscillations parallel to the direction of travel and travel faster than secondary (S) waves, which involve oscillations parallel to the direction of travel. S waves cannot travel through liquids and provide evidence of Earth’s liquid outer core. Refraction at a depth of ~40 km beneath the continents provides evidence of a sharp increase in density at this depth (the Mohorovičić discontinuity, or ‘Moho’), related to a change in the large-scale composition of the rock at the base of the crust. Seismic waves are just two well-known examples of how they provide information about Earth’s structure.