We investigate the upper mantle seismic discontinuities at 410 and 660 km depth beneath the Indian Ocean Geoid Low (IOGL). To map the discontinuities' topography, we use differential travel times of PP and SS waves and their precursors. Our final data set consists of 37 events with Mw ≥ 5.8, which densely cover our investigation area, also with crossing ray paths. We use array methods to detect the low‐amplitude precursor signals. The best quality data show a deepened 410 km discontinuity in the center of the IOGL as well as a mostly elevated 660 km discontinuity beneath the northern Indian Ocean, which we interpret as a hot anomaly currently residing in the mantle transition zone. We conclude that the largest negative geoid anomaly might be caused by a combined effect of hot material in the midmantle below the innermost IOGL and cold material below 660 km farther south.

MgO makes up about 20% of the Earth's lower mantle; hence, its rheological behaviour is important for the dynamics and evolution of the Earth. Here, we investigate the strength of twin boundaries from 0 to 120 GPa using DFT calculations together with structure prediction methods. As expected, we find that the energy barrier and critical stress for shear-coupled migration of the 310/[001] interface vary strongly with pressure. However, what is surprising is that the twin boundary also exhibits sudden strong discontinuities in strength which can both weaken and strengthen the boundary with increasing pressure. Since twin boundary migration is a proposed mechanism for both deformation and seismic attenuation in MgO, these results may suggest that MgO can undergo sudden changes in rheology due to transitions in grain boundary structure. The multiplicity of interfaces, however, necessitates the need for further studies to examine the role that phase changes in grain boundary structure play in mediating polycrystalline plasticity in the Earth.