2.4 Tomography inversion
An optimal initial velocity model is often necessary for the success of tomographic inversions. In this study, we invoke the global 3-D lithospheric model LITHO1.0 (Pasyanos et al., 2014) as the starting model. Our tomography region spans the area from 20°N to 30°N in latitude, 93°E to 111°E in longitude, and from the surface down to 155 km in depth. We parameterize our model by a 3-D grid with grid spacing of 0.1° horizontally and 2.5 km vertically, which enables us to capture small-scale anomalies in regions where the ray paths are dense.
In this study, we adopt the inversion scheme FMM of Rawlinson and Urvoy (2006) which uses the multi-stage fast marching algorithm (de Kool et al., 2006) to compute the P-wave travel times in 3-D models and the subspace technique (Rawlinson et al., 2006) to iteratively solve the inverse problem of finding the model parameters that minimize the residual between the observed and model-predicted travel times with appropriate regularization parameters. We have conducted extensive inversion tests to find the optimal regularization parameters, and the final values of 0.1 and 5.0 are determined for the damping and smoothing constraints, respectively, leading to results with sufficient small-scale structural features but without unwarranted strong localized anomalies and sharp contrasts in velocities.