Towards exact free oscillation spectra through generalised normal mode coupling

Long period free oscillation spectra provide one of the main constraints on large-scale lateral structures within the Earth’s mantle. These observations are particularly noteworthy for their direct sensitivity to density variations, which gives them the potential to resolve long-standing questions relating to the nature of the two Large Low Shear Velocity Provinces. However, due to both computational expediency and incomplete theory, there are inaccuracies within existing codes for forward modelling of free oscillation spectra. This has limited the ability of previous studies to reliably infer Earth structure using such observations.

This poster outlines work on a new open-source code for modelling free oscillation spectra within laterally heterogeneous Earth models. We apply a generalised normal mode coupling method that overcomes various limitations with the traditional mode coupling approach. We account fully for the non-linear dependence of the matrix elements on density and boundary topography, and exactly solve the equations of motion. Computational costs have been minimised by using high-performance libraries, and efficient numerical linear algebra, in addition to parallelisation. Our code is also suitable for calculation of sensitivity kernels using the adjoint method. Benchmarks against current codes as well as performance benchmarks are shown to demonstrate the accuracy and efficiency of our new method.