Internal structure of the Earth inferred from free oscillations

Abstract

A new set of weighted means of observed periods of free oscillations are used to estimate an earth model by least-squares inversion. All observed modes are considered, but only the fundamentals plus 1S0 and 2S0 are used explicitly in the inversion. The method uses partial derivatives of period with respect to P velocity, S velocity, and density. Derivatives of period with respect to the size of the core affect the periods of low-order spheroidal modes by as much as 0.8 sec/km of change. The model is developed in two stages. First, geophysical arguments that are generally independent of inferences from free oscillations are used to develop an approximate earth model, HT11GS1. Secondly, inversion is used to derive from it a new model, DI-11, which fits the observed periods with an overall rms deviation of only 0.61 second. DI-11 is characterized by a rapid increase in density at depths 500 to 800km, an Adams-Williamson density below 1100km, a density jump of 2 g/cm3 at the base of the outer core, and a shear velocity of 2.18 km/sec in the inner core. The existence of a solid inner core gets some support from the observations of free oscillations.