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Short and long-term Seismic deformations in volcanic areas: a fully 3D approach for wave propagation and Postseismic deformations

Final Activity Report Summary - SP3D (Short and long-term Seismic deformations in volcanic areas: a fully 3D approach for wave propagation and Postseismic deformations)

Wave propagation phenomena can be studied at the present time thanks to powerful numerical techniques stemming from finite differences to finite and Spectral elements method (SEM). Spurred by the computational power made available using parallel computers, these techniques embraced the area of three-dimensional seismic wave propagation. In particular, the SEM is capable of handling a wide spectrum of simulations ranging from geophysical fields, both at global and regional scales, to the seismic engineering field, while it combines the flexibility of the Finite-element method (FEM) with the accuracy of the pseudospectral method. Thanks to these features, the adoption of SEM is a natural choice for simulations of seismic wave propagation in complex geological structure as the volcanic regions.

We therefore performed an extensive simulation of the seismic wave propagation in Campi Flegrei Caldera, an Italian volcanic region endangering a highly-populated region. The results highlighted the impact of the, often underestimated, Campi Flegrei topography on the propagation field. In some cases, the diffraction due to the topography could change the travel time and the wave amplitude by 30 %.

This successful simulation still required a first and fundamental step, namely the decomposition of the computational domain into a family of non-overlapping elements. Often underestimated, the spatial discretisation (mesh) was an essential step for an accurate simulation, but, unfortunately, the creation of a realistic three-dimensional mesh of complex geological model represented a challenge, requiring months of obscure work. This was particularly true for spectral element methods, which were only able to provide a superior numerical accuracy and performance with respect to a finite element simulation using hexahedral elements.

Even though three-dimensional unstructured tetrahedral meshes could be quite easily achieved with either commercial or non-commercial algorithms, the automatic creation of three-dimensional non-structured hexahedral meshes was still recognised as a challenging and unresolved problem, after more than 20 years of active research. The mesh created for Campi Flegrei was a first application of an automatic parallel hexahedral unstructured meshing tool developed by means of CUBIT, a powerful meshing tool released by the Sandia National Laboratory (please see online), and a python code, developed during the SP3D project specifically for geophysical problems. It decreased the amount of human time required for the meshing process by 80 %, in terms of weeks or months. The meshing tool was designed for general geological models and could be adopted for stratified problems or alluvial basins.

The same meshing tool was used for finite element simulations of the post-seismic deformation associated to the 2004 Sumatra Earthquake and the 2005 Nias Earthquake. A simulation of the deformations produced by magma intrusions at Stromboli was also in progress by the time of the project completion.