Characterising Earth structure from seismic data

Our understanding of the 3-D structure of the Earth has been built around reference earth models such as PREM (1981) and ak135 (1995). These models have specific characteristics associated with the way they were constructed that influence inferences made about the Earth. Even the presentation of 3-D results from seismic tomographic models can be strongly influenced by the reference models used, particular when relative perturbations are plotted. Observational advances have indicated a few places where radial models need to be updated, notably at the top of the outer core and in the inner core. An updated model ek137 has recently been developed that improves the rendering of core structure whilst retains the good fit to the properties of the full range of seismological phases. Spin transitions in iron bearing lower mantle minerals have long been regarded as seismologically invisible, but the relative behaviour of the shear and bulk modulus in the body-wave models suggests that a weak signature survives 3-D averaging.