[S16-1] Large low shear velocity provinces and deep mantle structure

[S16-1-01] Shear Wave Velocity Structure and Anisotropy atop the Core Mantle Boundary Beneath the Indian Ocean Geoid Low

^○Padma Rao Bommoju¹, Ravi Kumar Mangalampally^{2, 3} (1.ESSO-National Centre for Earth Science Studies, Thiruvananthapuram, India., 2.CSIR-National Geophysical Research Institute, Hyderabad, India., 3.Institute of Seismological Research, Gandhinagar, India.)

The Indian Ocean Geoid Low (IOGL) that spans a vast areal extent south of the Indian subcontinent is a spectacular feature on the Earth, whose origin is still debated. In this study, we investigate the shear velocity structure and anisotropy of the lower mantle beneath this geoid low and the adjoining geoid high region utilizing the differential travel times, amplitude residuals and shear wave splitting of high-quality S and ScS phases. Results reveal large variations in ScS travel time residuals indicating that the lowermost mantle beneath the IOGL is heterogeneous. The ScS-S differential travel times are ~3s slower than those predicted by IASP91 model, primarily due to velocity increase in lowermost mantle beneath the IOGL and ~2s higher than IASP91 beneath the geoid high region, due to velocity decrease in lowermost mantle. The largest negative residuals (~7.7s) are concentrated below the IOGL. Modeling of differential travel time residuals reveal that the maximum positive and negative residuals can be explained in terms of a reduction in shear velocity of 0.9% and an increase of 1.6% respectively in a 1000km thick layer above the Core Mantle Boundary (CMB). Also, the ScS/S amplitude residuals beneath the IOGL are positive, implying high impedance contrast at the CMB, owing to the presence of high velocity material. The observed high velocities are attributed to the presence of dehydrated high density slab graveyards atop the CMB beneath the Indian Ocean. Further, we investigate the lowermost mantle anisotropy by analyzing high-quality ScS phases corrected for source and receiver side upper mantle anisotropy. The splitting results reveal significant anisotropy (~1.01%) in D'' layer. The observed fast axis polarization azimuths in ray-coordinate system indicate a TTI style of anisotropy. Lattice Preferred Orientation deformation of palaeo-subducted slabs experiencing high shear strain is a plausible explanation for observed D'' layer anisotropy beneath the IOGL.