How seismologists use earthquake waves to create 3D maps of the Earth's interior

Nature · Published 3 October 2026 Swipe more like this →

In short: Seismic tomography acts like a medical CT scan for the planet. By tracking how seismic waves from earthquakes or explosions speed up, slow down, or bounce off deep rock layers, scientists reconstruct the hidden, complex architecture of the Earth's crust, mantle, and core in three dimensions.

Seismic tomography is an inverse problem where researchers compare observed seismic data against a theoretical Earth model. Because Earth is not uniform, seismic waves reflect and refract as they encounter variations in temperature, composition, and density. By iteratively adjusting the model until it matches the recorded travel times and waveforms of P, S, and surface waves, geoscientists can map the subsurface. While the technique shares principles with medical CT scans, it faces unique challenges, such as the curved paths of seismic rays and the inherent uncertainty in locating the exact hypocenter of an earthquake.

The field matured in the 1970s, fueled by the expansion of global seismic networks and the rise of computing power. Early milestones included 2D velocity maps of the whole Earth in 1977 and the first iterative 3D model in 1984. Today, methods range from ray theory—which simplifies waves into 1D lines—to full waveform tomography, which models the complete complexity of wave propagation. Although ray theory is computationally cheaper, full waveform methods provide significantly higher accuracy by accounting for the influence of the surrounding medium on seismic records.

Tomography has transformed our understanding of deep-Earth phenomena, such as mantle plumes and subducting plates. Images reveal cold, fast-moving slabs sinking into the mantle and large low-velocity zones near the core-mantle boundary that likely feed volcanic hotspots. Despite these successes, the field remains constrained by uneven seismometer coverage, particularly in ocean basins, and the non-unique nature of tomographic solutions. Because different initial models can produce different results, researchers must carefully interpret the color-coded anomalies in these 3D models, balancing the limitations of wavelength resolution against the physical reality of the Earth's interior.

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