AxiSEM: broadband 3-D seismic wavefields in axisymmetric media

We present a methodology to compute 3-D global seismic wavefields for realistic earthquake sources in viscoelastic anisotropic media, covering applications across the observable seismic frequency band with moderate computational resources. This is accommodated by mandating axisymmetric background models that allow for a multipole expansion such that only a 2-D computational domain is needed, whereas the azimuthal third dimension is computed analytically on the fly. This dimensional collapse opens doors for storing space-time wavefields on disk that can be used to compute Frechet sensitivity kernels for waveform tomography. We use the corresponding publicly available AxiSEM (www. axisem. info) open-source spectral-element code, demonstrate its excellent scalability on supercomputers, a diverse range of applications ranging from normal modes to small-scale lowermost mantle structures, tomographic models, and comparison with observed data, and discuss further avenues to pursue with this methodology.