Geophysics: Seismology, Gravity Anomalies, and Isostasy Explained

Geophysics is the study of the Earth using quantitative physical methods. The discipline is divided into several branches, including solid‑Earth geophysics, atmospheric and hydrospheric geophysics, and solar‑terrestrial physics. Key subdisciplines comprise seismology, tectonics, geomagnetics, gravity studies, atmospheric science, and ocean physics. Geophysics also encompasses the description and investigation of the origin and evolution of the Earth’s major systems: the core, mantle, and crust of both continents and oceans.

Geophysical methods fall into three main categories. The first uses reflection and refraction seismology to measure the passage of sound waves through the Earth, thereby determining physical properties of the subsurface. The second employs electromagnetic sensors to quantify the electrical and magnetic properties of rocks. The third comprises potential field methods, which map regional variations in gravity and magnetism.

Seismology. Seismology is the study of seismic (sound) wave propagation through the Earth. It includes the analysis of earthquake sources, mechanisms, and the determination of Earth’s internal structure from variations in wave properties. The analysis is highly quantitative and typically requires powerful computers.

The deep structure of the Earth is mapped using seismology. Seismographs stationed worldwide record waves from natural and artificial sources—earthquakes, nuclear explosions, and other seismic events. By comparing arrival times, scientists calculate changes in Earth properties at different locations. If the Earth had a uniform composition, seismic wave velocity would increase smoothly with depth because higher density generally yields higher velocities. However, observed arrival times show that velocity does not increase steadily; instead, several dramatic changes occur at discrete boundaries and transition zones.

Seismologists locate these boundaries by noting how waves behave. Some waves are reflected off interfaces (like light reflecting off a mirror), while others are refracted, changing both velocity and path. The core‑mantle boundary, located at 1,802 miles (2,900 km) depth, strongly affects both P‑waves and S‑waves. It refracts P‑waves, creating a P‑wave shadow zone. Because liquids cannot transmit S‑waves, none pass through, producing a large S‑wave shadow zone. These contrasting properties allow accurate mapping of the core‑mantle boundary.

Velocity variations also reveal other deep‑Earth features. Velocity gradually increases to about 62 miles (100 km) depth, then drops slightly between 62 and 124 miles (100–200 km) in the low velocity zone. This drop is attributed to small amounts of partial melt in the rock, defining the asthenosphere—the weak, partially lubricated layer on which tectonic plates move.

A seismic discontinuity at 248.5 miles (400 km) depth shows a sharp velocity increase. This results from a polymorphic transition in olivine, whose atoms rearrange into a spinel structure, causing an approximate 10% density increase. Another major discontinuity at 416 miles (670 km) may represent either another polymorphic transition or a compositional change. This remains a topic of active research. Some models propose that this boundary separates two fundamentally different mantle layers circulating in separate convection cells; others argue for more interaction between rocks above and below.

The core‑mantle boundary is one of Earth’s most fundamental interfaces, with a huge density contrast: 5.5 g/cm³ above versus 10–11 g/cm³ below—a greater contrast than between surface rocks and air. The outer core consists mainly of molten iron. An additional discontinuity exists inside the core at the boundary between the liquid outer core and the solid, iron‑nickel inner core.

Seismic waves also illuminate crustal structure. Andrija Mohorovičić, a Yugoslav seismologist from Volosko (Croatia), observed slow and fast arrivals from nearby earthquakes. He proposed that some waves traveled through the crust, some along the surface, and others reflected off a deep discontinuity between seismically slow and fast material at about 18.6 miles (30 km) depth. Geologists now recognize this Mohorovičić discontinuity (or Moho) as the base of the crust, using its seismically determined position to measure crustal thickness, which typically ranges from 6.2 to 43.5 miles (10–70 km).

Gravity Anomalies (Potential Field Studies). Gravity anomalies are the difference between the observed gravity value at a point and the theoretically calculated value based on a simple gravity model. The measured gravity reflects the distribution of mass and rock units at depth, as well as topography. Average gravitational attraction at the surface is 32 feet per second squared (9.8 m/s²), with one gravity unit (g.u.) equal to one ten‑millionth of this value. An older unit, the milligal, equals 10 g.u. The range in gravity at sea level is about 50,000 g.u. (9.78–9.83 m/s²), meaning an adult human weighs slightly more at the poles than at the equator because Earth’s equatorial radius is larger.

Geologically significant gravity variations are typically only a few tenths of a g.u., so instruments must be extremely sensitive. Some surveys use closely or widely spaced gravimeters on the surface, while others rely on satellite orbit perturbations.

Determining gravity anomalies requires removing the overall gravity field of the Earth (the geoid) to leave an elevation‑dependent measurement. The free‑air gravity anomaly corrects measured gravity using only the point’s elevation, Earth’s radius, and mass. The Bouguer gravity anomaly additionally accounts for the shape and density of rock masses at depth. Sometimes a third correction, the isostatic correction, is applied when a load (e.g., a mountain or sedimentary basin) is supported by mass deficiencies at depth—like an iceberg floating lower in water. However, several isostatic compensation mechanisms exist, and it is often difficult to know which applies at different scales, so this correction is frequently omitted.

Different geological bodies produce characteristic gravity anomalies. Ophiolites (belts of oceanic crust thrust onto continents) represent unusually dense material and generate positive anomalies up to several thousand g.u. Massive sulfide metallic ore bodies are also dense and yield positive anomalies. Conversely, salt domes, oceanic trenches, and mountain ranges increase the amount of low‑density material in the crustal column, producing negative anomalies. The highest mountains on Earth—the Himalayan chain—exhibit negative values as large as 6,000 g.u.

Geophysics and Isostasy. Isostasy is the principle of hydrostatic equilibrium applied to the Earth. It describes the lithosphere floating on the asthenosphere, similar to how low‑density ice floats at a specific level on water depending on relative densities. Isostatic forces play a major role in controlling Earth’s surface topography.

Several isostatic models exist. The simplest treat crustal blocks as isolated blocks floating in a fluid substrate (the asthenosphere), free to move vertically without interacting. Two main variations of these simple models are the Pratt model and the Airy model. In the Pratt model, crustal blocks of different density extend to a constant depth of compensation; topography height varies inversely with block density. Thus, high‑density oceanic crust resides lower than lower‑density continental crust. In the Airy model, the compensation depth varies for each block, but crustal density is assumed constant. Thick blocks have high topography and a thick root to compensate, whereas thin blocks have subdued topography.

Both models are simplistic, having been developed in the 1700s before plate tectonics was understood. The Airy model is generally more applicable than the Pratt model, but it does not accommodate known crustal density variations (e.g., between continents and oceans). Isostatic anomalies are deviations in measured gravity from values expected using a chosen isostatic model and compensation depth. Such anomalies indicate that either the model or the assumed compensation depth requires adjustment.

Further Reading: Keary, P., Keith Klepeis, and Fredick J. Vine. Global Tectonics. Oxford: Blackwell, 2008.
Shearer, Peter M. Introduction to Seismology. Cambridge: Cambridge University Press, 2009.
Sheriff, Robert E. Encyclopedic Dictionary of Applied Geophysics. 4th ed. Tulsa, Okla.: Society of Exploration Geophysicists, 2002.
Turcotte, Donald L., and Gerald Schubert. Geodynamics. 2nd ed. Cambridge: Cambridge University Press, 2002.
Vanicek, Petr, and Nikolaos T. Christou. Geoid and Its Geophysical Interpretations. New York: CRC Press, 1994.