Harvard Seismology: 3-D Earth Structure

The first major global tomographic study was made by Professor Dziewonski in early 70's. The idea of this study was that the travel time anomalies observed for many ray paths, criss-crossing the Earth between various points near the Earth's surface and reaching different depths in its interior, could be resolved formally into a three-dimensional (3-D) model. This is now called `seismic tomography', as it conceptually resembles the medical CAT-scan. The early results were reported orally in 1974 and 1975 by Dziewonski and a full report was published in January 1977 (Dziewonski et al., 1977). A more detailed description can be found a statement written by Professor Dziewonski. The motivation for studying 3-D structure of the Earth's interior is that it may offer the best information on the dynamic processes in the deep interior of the Earth. As the seismic wave speeds change with temperature, it is plausible to obtain 3-D snapshots of the convection pattern in the Earth. By performing waveform and travel time inversions using long period seismic records, we have obtained global models for the long wavelength 3-D velocity structure of the Earth.

Active participants in this project are Adam M. Dziewonski, Göran Ekström, Lapo Boschi, Yu Gu, Wei-jia Su, and Xian-feng Liu.

• An anisotropic S-velocity model of the upper mantle, S20A , showing the unique character of the anisotropy of the Pacific upper mantle.
• Xian-Feng Liu's Ph.D thesis: The Three-Dimensional Shear Wave Velocity Structure of the Earth's Lower-Most Mantle (Compressed postscript file, about 200 pages, 11 MB) and a few pages best viewed in color: Pages 44, 65-66, 70-78, 83, 85-90, 106, 172, 175.
• Liu & Dziewonski (1997) have derived a degree 16 model S16U6L8 by including the travel times of ScS-S, SS-S, S-SKS, SKKS-SKS other absolute travel time data and waveform data in the inversion. The model is parameterized separately for the upper mantle and the lower mantle using B-spline basis functions. The paper has been submitted to CMB monograph. You can download a hardcopy and Figure 24 (color) here .
• MK12WM13 : Simultaneous Inversion for 3-D Variations in Shear and Bulk Velocity in the Mantle (Su & Dziewonski, 1997).
• Liu & Dziewonski (1994) have derived a global model SKS12_WM13 (in a format which is self-explained) by including the travel times of ScS-S, SS-S, S-SKS, other absolute travel time data and waveform data in the inversion.
• Su, Woodward & Dziewonski (1994) have derived a global model S12_WM13 by including the travel times of ScS-S, SS-S and other absolute travel time data and waveform data in the inversion.
• Global travel time inversion using cubic B-splines parameterization, which makes an excellent compromise between the natural smoothness of spherical harmonic and the reduction of free parameters available by using cell-parameterized models for regional inversions.
• Topography of the 660 km discontinuity using long period SS precursors (Gu, Dziewonski & Agee, 1997) .
• Topography of the 410 km discontinuity using long period SS precursors (Gu, Dziewonski & Agee, 1997) .
• Joint inversion of waveforms and travel times using both body waves and surface waves.