Seismic waves are vibrations that travel through the Earth, generated by sudden movements like earthquakes or by controlled artificial sources. The study of how these waves travel and change is the foundation of seismic tomography, which allows scientists to create three-dimensional images of the Earth’s interior. This process functions much like a medical CT scan, using the paths and speeds of waves to infer the composition, temperature, and physical state of deep materials. By recording the arrival times of compressional waves (P-waves) and shear waves (S-waves) at global seismograph stations, researchers map structures from the crust down to the core.
Mapping the Planet’s Deepest Layers
Seismic waves are the primary tool used to define the fundamental, concentric layers of the Earth, separated by distinct boundaries called seismic discontinuities. The most prominent is the Mohorovičić discontinuity, or Moho, which marks the transition between the less dense crust and the denser, underlying mantle material. P-waves increase their velocity as they cross the Moho and enter the mantle, where rocks are stronger and more compressed due to greater pressure.
Deeper still, seismic waves delineate the structure of the mantle, which is divided into the upper mantle, the transition zone, and the lower mantle. Within the upper mantle, a low-velocity zone exists where both P-waves and S-waves slow down, suggesting a region close to its melting point that is less rigid. The transition zone, between 410 and 660 kilometers deep, is defined by abrupt increases in wave speed caused by pressure-induced phase changes in mantle minerals.
The Core-Mantle Boundary (CMB) is a key visualized feature, where the mantle meets the liquid outer core at about 2,900 kilometers. This boundary is identified because S-waves, which cannot travel through liquids, are completely blocked, creating a clear S-wave shadow zone on the opposite side of the planet. P-waves pass through the liquid outer core but slow down significantly and refract, producing a separate P-wave shadow zone. The final major boundary is the Lehmann discontinuity, separating the liquid outer core from the solid inner core, where P-waves speed up due to the extreme pressure solidifying the iron-nickel material.
Identifying Tectonic Features and Processes
Seismic tomography visualizes the dynamic processes that drive plate tectonics and heat transfer within the mantle. Variations in seismic wave speed from the expected average are interpreted as thermal or compositional anomalies. Areas where waves travel faster than average indicate colder, denser rock, while slower speeds suggest hotter, more buoyant material.
Subducting slabs, which are cold sections of oceanic lithosphere sinking into the mantle, appear in tomographic images as distinct high-velocity anomalies. These regions show velocity increases of up to 1% to 3% compared to the surrounding mantle, indicating material that is approximately 500°C to 1,000°C cooler. Conversely, mantle plumes, which are columns of hot, rising material often originating near the CMB, are visualized as low-velocity anomalies, reflecting their higher temperature and lower density.
Near the surface, seismic imaging reveals the geometry of magma chambers beneath active volcanoes, where molten rock causes a significant slowdown in wave speeds. Detailed seismic analysis maps the complex architecture of deep fault systems and plate boundaries, providing insight into structures responsible for generating major earthquakes. These visualizations help scientists understand the deep processes that connect the core’s heat to surface geology.
Locating Subsurface Fluids and Geological Hazards
Seismic wave analysis is applied to shallower depths for near-surface engineering and resource exploration. Exploration seismology uses controlled energy sources to generate waves that reflect off geological layers, including those that might trap hydrocarbons like oil and natural gas. Geophysicists analyze these reflection patterns to identify specific subsurface structures, such as anticlines or fault traps, likely to hold commercial reservoirs.
Seismic methods are effective for mapping water resources, as wave speed is sensitive to the type of fluid filling rock pores. Faster velocities indicate solid bedrock or pores saturated with groundwater, allowing scientists to map the location and extent of aquifers. Slower velocities suggest the presence of air-filled pores or highly weathered rock near the surface, which is useful for understanding water flow pathways.
For infrastructure planning, engineering seismology uses these waves to assess ground stability and potential hazards before construction. This involves determining the depth to solid bedrock and mapping shallow, localized faults that could pose a risk to buildings or transportation networks. Seismic imaging provides a clear picture of the near-surface geological structure, helping ensure the safety and longevity of human development.