Love Wave group Velocity tomography and Seismic b-Value Analysis of Northeast India

Northeast India is among the most seismically vulnerable regions of the world due to its complex tectonic framework, high population density, and the presence of thick sedimentary layers that can strongly amplify seismic waves. As a result, even moderate earthquakes may cause substantial damage. Previous surface wave investigations in this region have been largely restricted to Rayleigh wave tomography. In this study, we present the first high resolution Love wave tomography of Northeast India, providing new insights into the shear wave velocity structure of the crust and upper mantle. Love wave dispersion data are used to derive one–dimensional (1D) shear wave velocity models, which are subsequently combined to construct a three dimensional (3D) shear wave velocity model of the region. The reliability of the imaged structures is evaluated through resolution analysis to ensure that observed velocity variations represent realistic subsurface features. Because Rayleigh and Love waves are sensitive to different components of shear wave velocity, discrepancies between their velocity structures are used to investigate radial anisotropy within the lithosphere. The combined analysis of Rayleigh and Love wave results provides important constraints on the anisotropic properties of the crust and upper mantle and their tectonic significance.

In addition to structural imaging, we assess the seismic hazard of Northeast India through estimation of the Gutenberg–Richter b–value, which reflects the relative occurrence of small and large earthquakes. The calculated b–values indicate generally high tectonic stress and an elevated potential for large earthquakes across the region. Within the Indo–Burma Ranges (IBR), the southern, central, and northern segments all exhibit b–values below 1.0, suggesting significant seismic hazard. The Mikir Hills show notably low b–values, lower than those reported in earlier studies, whereas the Shillong Plateau records b–values consistent with previous estimates. A depth dependent analysis reveals pronounced changes in b–values at specific depths, which appear to correlate with the Moho discontinuity. In the IBR, a marked transition occurs at approximately 50 km depth, while similar changes are observed at ~30 km beneath the Mikir Hills and ~35 km beneath the Shillong Plateau. These observations suggest a strong relationship between seismicity patterns and the crust–mantle boundary, highlighting the role of lithospheric structure in controlling earthquake generation.

Overall, this study integrates high resolution Love wave tomography with seismicity analysis to advance our understanding of the subsurface structure, radial anisotropy, and seismic hazard of Northeast India, emphasizing the need for continuous monitoring and improved earthquake preparedness in this tectonically active region.