An Updated One-Degree Seismic Tomographic Model Based on a Sensitivity Kernel Database

Seismic tomography is instrumental in mapping 3D velocity structures of the Earth’s interior based on travel-time measurements and waveform differences. Although both ray theory and other asymptotic methods have been successfully employed in global tomography, they are less accurate for long-period waves or steep velocity gradients. They also lack the ability to predict “non-geometrical” effects such as those for the core diffracted phases (P_diff , S_diff) which are crucial for mapping heterogeneities in the lowermost mantle (D'' layer). On the other hand, sensitivity kernels can be accurately calculated with no approximations by the interaction of forward and adjoint wavefields, both numerically simulated by spectral element methods.

We have previously shown that by taking advantage of the symmetry of 1D reference models, we can efficiently and speedily construct sensitivity kernels of both P and S wavespeeds based on the simulation and storage of forward and adjoint strain fields for select source and receiver geometries. We have also shown the effect on global models inverted based on ray theory versus finite-frequency kernels in a parallel LSQR inversion.

The CRUST1.0 model provides an updated version of the global sediment thickness on a finer 1° grid, based on active source seismic studies as well as receiver function studies. Crustal corrections for CRUST1.0 for the mantle may vary significantly from the ray-theoretical values, depending on crustal thickness, crustal velocities and wave period. We improve upon our previous model by incorporating the CRUST1.0 sediments and crustal corrections. This provides better response in the upper mantle and crustal regions. We produce this updated model for the S phases, and intend to extend the results to both P and S phases in the future.