Gradient maps: A tool in the interpretation of tomographic images

Gradient maps are introduced as a tool for the interpretation of tomographic images. The interpretation of tomographic images can be strongly dependent on the choice of reference models or a particular colour scale, but the gradient maps provide an image that is independent of the reference model. Incorporating new data in central and eastern Australia to update a surface wave tomography model, the horizontal gradient is calculated using a finite difference approximation. Results at 175 km depth clearly show that the gradient maps give information that may be missed in the simple interpretation of the change from fast to slow velocities highlighted in the tomographic images. It appears that there are a number of steps in the thickness of the lithosphere within eastern Australia.