SPHY3D: A hybrid seismic computational framework for box-tomography of spherical Earth
Authors: Karakostas Foivos , Morelli Andrea , Molinari Irene , VanderBeek Brandon , Faccenda Manuele 
Published: February 2023

Computational seismology encountered a dramatic advance during the past decades with the development of SEM codes that use the simultaneous increase of the available computational power. Meanwhile, the use of teleseismic events for regional seismic tomography is suggested with the application of the box-tomography methodology (Masson and Romanowicz, 2017). In this work we use these advances in order to suggest a package for box-tomography, using AxiSEM for 1-D global wavefield simulations (Nissen-Meyer et al., 2014) and SPECFEM3D for 3-D regional seismic simulations (Komatitsch and Tromp, 1999). These codes have been previously used and validated for such hybrid simulations (Monteiller et al., 2021), however with the limitation on the dimensions of the examined region, where 3-D full waveform topography is applied, due to the Cartesian setting that does not honour the curvature of the Earth. Although recent advances solved this limitation for SPECFEM3D Global, by permitting the use of a small Earth chunk, the Cartesian description of the regional model allows computing the injection of the 1-D computed wavefield from the global model to the regional box. Therefore, we developed and present comparative results of a package that transforms the geometry of the Cartesian simulation in a "spherical Earth" setting and allows the performance of hybrid simulations for box tomography in regions larger than a couple of degrees. The code changes the shape of a Cartesian rectangular mesh into a curved one and through a series of interpolations adjusts the geometry of any given structure model, the topography of the surface and the interfaces, and the position of the receivers. The simulations are tested against real data, as we perform our computations on a dynamically interesting area, with the presence of a subduction slab in the central Mediterranean. We test the methodology on seismological inverse models for the local structure (Rappisi et al., 2021).References:Komatitsch, D. and Tromp, J., 1999. Introduction to the spectral element method for three-dimensional seismic wave propagation. Geophysical journal international, 139(3), pp.806-822.Masson, Y. and Romanowicz, B., 2017. Box tomography: localized imaging of remote targets buried in an unknown medium, a step forward for understanding key structures in the deep Earth. Geophysical Journal International, 211(1), pp.141-163.Monteiller, V., Beller, S., Plazolles, B. and Chevrot, S., 2021. On the validity of the planar wave approximation to compute synthetic seismograms of teleseismic body waves in a 3-D regional model. Geophysical Journal International, 224(3), pp.2060-2076.Nissen-Meyer, T., van Driel, M., St&#228;hler, S.C., Hosseini, K., Hempel, S., Auer, L., Colombi, A. and Fournier, A., 2014. AxiSEM: broadband 3-D seismic wavefields in axisymmetric media. Solid Earth, 5(1), pp.425-445.Rappisi, F., VanderBeek, B.P., Faccenda, M., Morelli, A. and Molinari, I., 2022. Slab geometry and upper mantle flow patterns in the Central Mediterranean from 3D anisotropic P-wave tomography. Journal of Geophysical Research: Solid Earth, p.e2021JB023488.