Crustal and Uppermost Mantle Structure beneath the Indian Ocean Geoid Low using Surface Wave Tomography

The Indian Ocean Geoid Low (IOGL) is a significant negative geoid anomaly (-106 m), located south of the Indian subcontinent. Several studies have been carried out to investigate the causes responsible for IOGL, results indicate that its origin could be low velocity/density anomalies in the depth range of mid-to-upper mantle and/or high velocity/density anomalies in the depth range of lower mantle. However, a concrete model to explain the origin of IOGL and especially the effect of the shallow structure on IOGL is still enigmatic. In the present study, we investigate the high-resolution 3D shear velocity structure beneath the Indian Ocean region down to a depth of 300 km using surface wave tomography. For this analysis, collated extensive data from more than 700 broadband seismological stations of various data centres (IRIS: Incorporated Research Institutions for Seismology, FDSN: International Federation of Digital Seismograph Networks, IN: Indian Stations) to obtain a good azimuthal and spatial coverage. This dataset has been pre-processed and utilized to measure the fundamental mode of Rayleigh wave group velocities sampling the IOGL region. Further, visually checked the quality of the dispersion curves and considered only good-quality ones, which resulted in ~19,300 good-quality dispersion curves in a frequency range of 10-120 s and then applied the regionalization to extract the geographical distribution of local group velocities in different periods. Later, inverted the regionalized dispersion data using the trans-dimensional inversion approach. Regionalized shear wave velocity maps are in good agreement with surface tectonics such as low-velocity anomalies along the large-scale ridges and previous tomography studies. The western Indian Ocean shows very fast velocity anomalies at short-period (~20 s) and low-velocity anomalies in long periods (>100 s) which could be attributed to the magmatic underplating originating from the various hotspots and the presence of channelled flows asthenosphere towards eastward to spreading ridges. Further, the results rules-out that the contribution of the lithosphere could be negligible in explaining the IOGL as there are no low-velocity anomalies beneath the IOGL region. In addition, the obtained high-resolution regional surface wave tomography from this study enables the research community to obtain/measure the precise IOGL anomaly and understand the detailed tectonics and crustal/lithospheric deformation beneath the study region.