Seismic Tomography and Earth Structure

By inverting the travel times and waveforms of seismic waves, seismologists build three-dimensional images of the Earth's interior, revealing the crust, mantle, and core and the velocity anomalies that map convection and tectonics.

Definition

Seismic tomography is the inverse-problem methodology that reconstructs three-dimensional images of seismic velocity within the Earth from large sets of travel-time and waveform measurements, and together with one-dimensional reference models it defines our knowledge of the Earth's internal structure.

Scope

This topic covers the determination of the Earth's internal structure from seismic observations: the discovery of the major boundaries (Moho, core-mantle boundary, inner core), reference Earth models, and the tomographic inversion of travel-time and waveform data for three-dimensional velocity variations. It treats body-wave and surface-wave tomography, resolution and the regularization of ill-posed inverse problems, and the interpretation of velocity anomalies in terms of temperature, composition, and flow. The emphasis is on imaging the interior rather than on the source.

Core questions

• How were the Earth's principal internal boundaries discovered seismically?
• How are travel times and waveforms inverted for three-dimensional velocity structure?
• What limits the resolution of tomographic images, and how is the inversion stabilized?
• How are velocity anomalies interpreted in terms of temperature, composition, and flow?

Key concepts

• Reference Earth models and radial velocity structure
• Mohorovicic discontinuity, core-mantle boundary, and inner core
• Body-wave and surface-wave tomography
• Inverse problems, regularization, and resolution
• Velocity anomalies as proxies for temperature and composition

Key theories

Layered reference Earth model

Decades of travel-time data were synthesized into spherically symmetric reference models such as PREM that specify velocity, density, and attenuation as functions of depth, providing the baseline against which three-dimensional anomalies are measured.

Tomographic inversion

Seismic tomography treats imaging as a linearized inverse problem, relating perturbations in observed travel times or waveforms to perturbations in velocity along ray paths or sensitivity kernels, and solving the regularized system to recover three-dimensional structure.

Mechanisms

Seismic velocity depends on the elastic moduli and density of rock, which vary with temperature, composition, phase, and the presence of melt or fluids; cold, dense material is generally fast and hot or partially molten material slow, so a velocity image inverted from many crossing ray paths becomes a proxy map of the thermal and compositional state of the interior.

Clinical relevance

Tomographic images of the mantle constrain the location of subducted slabs and rising plumes, link the interior to plate motions and hotspot volcanism, and at crustal scales support hazard and resource studies; reference models anchor earthquake location worldwide.

History

Mohorovicic identified the crust-mantle boundary in 1909, Gutenberg located the core-mantle boundary, and Lehmann discovered the inner core in 1936; the 1981 Preliminary Reference Earth Model and the rise of computational tomography in the 1980s opened the era of three-dimensional imaging of the deep Earth.

Key figures

• Inge Lehmann
• Andrija Mohorovicic
• Adam Dziewonski
• Don Anderson

Seminal works

• dziewonski1981
• nolet2008
• lehmann1936

Frequently asked questions

How is seismic tomography like a medical CT scan?

Both reconstruct an internal image from many measurements made along paths that cross the body in different directions; in seismic tomography the rays are seismic waves and the quantity imaged is wave speed, which varies with the rock's temperature and composition.

How do we know the Earth has a solid inner core?

Inge Lehmann found in 1936 that certain seismic waves reaching the far side of the Earth could only be explained if the liquid outer core surrounds a distinct solid inner core that refracts the waves differently, a structure since confirmed by many other observations.