D2 Seismic Tomography

Topic

Seismic tomography is a technique that produces three-dimensional images showing how seismic velocity varies within the Earth's interior, based on large datasets of travel times or full waveforms. Much like medical tomography, seismic tomography combines data from numerous rays traversing the Earth's interior in various intersecting directions to reconstruct the velocity distribution through which they passed; this velocity distribution, in turn, reflects variations in temperature, composition, and state (solid or molten) within the mantle and crust.

Travel-time tomography divides the Earth's interior into a grid of cells and adjusts the seismic velocity of each cell to minimize the difference between observed travel times and those predicted for the same path by a standard one-dimensional reference model. When the observed velocity exceeds that of the reference model, the resulting positive anomaly indicates cooler, denser material—such as a subducted oceanic crustal slab; conversely, when the observed velocity is lower than that of the reference model, the negative anomaly indicates hotter, upwelling material—such as a mantle plume.

Unlike travel-time tomography, surface-wave tomography is far more sensitive to the structure of the crust and upper lithosphere, precisely because long-period surface waves do not penetrate as deeply into the Earth's interior as body waves do.

Full-waveform inversion represents the most advanced form of seismic tomography available today: rather than relying solely on the arrival time of each wave, this technique utilizes the entire recorded seismic waveform, simultaneously incorporating the amplitudes and phases of multiple wave types. By leveraging all that additional information, full-waveform inversion allows for the resolution of much finer internal structures than earlier techniques and reduces the ambiguity that often affects seismic inversion based solely on arrival times.

Finally, ambient noise tomography does not require an earthquake as a seismic wave source; instead, it correlates the ambient microseismic noise continuously recorded by two seismographic stations. From the correlation between these two stations, it extracts the same information that would be obtained if one station had acted as the source of an actual seismic wave and the other had recorded it—thereby completing, alongside travel-time, surface-wave, and full-waveform tomography, the suite of techniques used in seismic tomography to reconstruct the Earth's interior, as mentioned at the beginning of this description.

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