Tomographic Imaging of the Western Pacific

Tomographic imaging of the Western Pacific Region was performed based on the world-wide seismic data composed by travel-time and path of seismic rays. The purpose of this study was to estimate high-resolution structure of the Earth's mantle and to map subducting lithosphere at the plate boundaries. This information can improve our understanding about physical properties of the Earth's interior and tectonic fabric of the Western Pacific.

Principal Investigator

Alexei Gorbatov



Facilities Used



RFCD Codes


Significant Achievements, Anticipated Outcomes and Future Work

A new inversion technique was developed to perform tomographic imaging of the Western Pacific. Two main innovations were introduced: a three-dimensional ray-tracing technique to determine the seismic ray path in the heterogeneous Earth mantle, and a combined inversion scheme that permits separation of bulk-sound and shear moduli in two images based on the P and S wave speeds. This newly developed tomographic scheme can not only detect seismic wave heterogeneities in the Earth mantle but can also give an idea about physical properties of those anomalies.

High-resolution tomographic image obtained from joint P and S tomographic inversion reveals penetration of the subducted slabs into the lower mantle below Philippine, Mariana, and Tonga-Kermadec subduction zones. These results are new evidence for deep subduction.

Relationships between bulk-sound and shear moduli point to the clear difference in the physical properties of subducted slabs near the age of 100 millions years. This peculiar feature suggests that physical properties of oceanic plates vary with time and age boundary near 100My is an important step in this evolution.

Further analysis of obtained results could reveal new tectonic features of the Western Pacific Region and, probably, the reason for the differences between bulk-sound and shear moduli anomalies with age and regions. Improvement of the inversion technique can refine our results. We plan to introduce new regularization approaches in order to increase resolution of tomographic images. Additionally, more data will be processed and incorporated into the inversion.


Computational Techniques Used

The pseudo-bending method (Koketsu & Sekine, GJI, 1998) was used for three-dimensional ray-tracing algorithm.

LSQR (Page and Saunders, ATMS, 1982) and Inversion for multiple parameter classes (B.L.N. Kennett & M. Sambridge, GJI, 1998) were used to solve large and sparse system of linear equations.

The computational power of the APAC National Facility allows us to perform all calculations in the relatively short duration of this project, which cannot be done on conventional workstations.

Graphical program GeoPlot, developed by Ajay Limaye (VIZLAB), was used to visualize all tomographic results.


Publications, Awards and External Funding

B.L.N. Kennett, A. Gorbatov, Why is there a Strong Shear Signature for Deep Slabs?, EOS Am Trans Act., S41-B, 2001.

A. Gorbatov, B. L. N. Kennett, Western Pacific Subduction - New Results from Regional Joint P and S Travel-Time Tomography., EOS Am Trans Act., T22D-08, 2001.