As a Professor at U.C. Santa Cruz and Director of the Centre for the Study of Imaging and Dynamics of the Earth, Thorne Lay's primary research interests involve analysis of seismic waves to interrogate the deep structure of the Earth's interior and to study the physics of earthquake faulting. In the realm of deep Earth structure, the focus is on imaging structures associated with internal dynamics of the mantle, particularly the core-mantle boundary region and the vicinity of subducting lithosphere. Studies of the core-mantle boundary have revealed a complex seismic structure believed to involve a thermal and chemical boundary layer at this, the largest internal compositional contrast within the planet. One of the major questions about subducting lithosphere that is being addressed is the depth extent to which the subducting material penetrates, which is closely linked to the debate about layered mantle versus whole mantle convection. Other Earth structure interests include the lateral variations of lithospheric structure, which are studied using body waves and surface waves, and the nature of regional waves propagating in the crust.