Three-Dimensional Crustal Shear Wave Velocity Structure and Azimuthal Anisotropy Across Northern and Central Thailand From Direct Joint Inversion of Rayleigh Wave Phase Velocities

This study presents a three-dimensional shear wave velocity (Vs) model and azimuthal anisotropy of the crust beneath northern and central Thailand derived from ambient noise tomography. Continuous seismic data from 99 broadband stations across Southeast Asia were processed to extract Rayleigh wave phase velocity dispersion curves. These curves were then used in a direct joint inversion for both isotropic Vs perturbations and azimuthal anisotropy at depths of 15-35 km. The inversion was initialized with a local 1D velocity model specifically derived for northern Thailand. Results reveal significant lateral heterogeneities in crustal structure, with Vs ranging from 3.615 to 3.675 km/s at 15-20 km depth to 3.650 to 3.720 km/s at 30-35 km depth across the study area. A prominent high-velocity anomaly (Vs > 3.675 km/s at 15-20 km depth, Vs > 3.745 km/s at 20-30 km depth, and Vs > 3.720 km/s at 30-35 km depth) is observed in the northwestern sector (18.5 degrees-19.5 degrees N, 98 degrees-101 degrees E), corresponding to the Shan-Thai terrane. The velocity contrast between the Shan-Thai and Indo-China terranes is delineated by a lateral velocity gradient, with the Shan-Thai Terrane consistently exhibiting higher velocities throughout the investigated depth range. Azimuthal anisotropy patterns show complex variations, with a prominent NW-SE trend in fast polarization directions in northern Thailand, consistent across all depth ranges. The strongest anisotropy is observed in the northern Thailand, coinciding with high-velocity zones. The spatial correspondence between present-day surface kinematics, as recorded by GPS measurements, and crustal anisotropic fabric suggests a coherent deformation pattern that extends through multiple crustal levels, with pronounced expressions along terrane boundaries.