Adjoint‐State Teleseismic Traveltime Tomography: Method and Application to Thailand in Indochina Peninsula

We propose a novel framework for teleseismic traveltime tomography that requires no ray tracing. The tomographic inverse problem is formulated as an Eikonal equation‐constrained optimization problem, aiming at the determination of a slowness model that minimizes the difference between observational and predicted differential traveltimes. Two improvements have been made over previous ray‐based methods. First, the traveltimes from the source outside the study region to any positions within the study region are computed using a hybrid approach. This involves solving a 2D Eikonal equation to obtain the traveltimes from the source to the boundary of the study region and solving a 3D Eikonal equation to compute the traveltimes from the boundary to any positions within the study region. Second, we compute the sensitivity kernel using the adjoint‐state method. This method avoids the computation of ray paths and makes the computational cost nearly independent of the number of receivers. We apply our new method in Thailand and adjacent regions. The final velocity model reveals a thick lithosphere beneath the Khorat Plateau and two mantle upwelling branches beneath its southern and western margins. The mantle upwelling may result from the mantle convection triggered by surrounding subduction systems and/or a slab window of the Indian Plate. The presence of the mantle upwelling corresponds to the source zone of the erupted Cenozoic basalts in the Khorat Plateau, indicating lithospheric modification beneath the plateau. The insightful tomographic result verifies our method and provides new perspectives on the structural heterogeneities and dynamics of the Indochina Block. Plain Language Summary We propose a novel teleseismic traveltime tomography method based on the adjoint‐state method. The critical procedure of computing sensitivity kernels is realized by solving the Eikonal and adjoint equations. This approach eliminates the need for calculating ray paths, which is required in conventional ray‐based tomography methods. As a result, our approach avoids the potential failures of conventional shooting and bending methods for ray tracing and makes the computational cost independent of the number of receivers. We test our new method in Thailand, using the new data from the Thai Seismic Array. Our model reveals a thick lithosphere beneath the Khorat Plateau as a high‐velocity anomaly. In addition, two mantle upwelling branches are imaged beneath the southern and western m