Modeling the European crust for seismic wave propagation

Abstract

Looking into the structure, composition and behaviour of the Earth is one of the main goals of the seismic studies. Many geophysical problems — such as surface wave, group velocity and full waveform tomography , determination of mantle flows, gravity studies, source inversion — need plausible models as starting point for such studies. Crustal structure varies greatly over small scale length and has a strong effects on seismic waves. A priori models of the crust are thus often used to model seismic wave propagation at large distance and to account for shallow structure when imaging upper mantle structure. Focusing on forward earthquakes simulations, plausible crustal and mantle models are the first step to obtain realistic seismograms and results. Recent development in computer facilities and numerical methods — Spectral Element Method, ADER-DG method, Finite Difference method — enable to solve the wave equation in 3D complex media with high accuracy. These methods require a discrete representation of the investigation domain (mesh) through which we propagate wave. To model seismic wave propagation at the scale of a continent — i.e. signals travelling to stations a few hundred or thousand kilometers from the earthquake source — we have a problem connected to the detail and reliability of current models, that are sufficiently accurate when we look at the global scale, but often miss significant features at the scale of sedimentary basins and mountain ranges, that become very important as we zoom closer. Reliable and detailed information on these structures exist, for instance deriving from active-source studies, but are often not integrated in wide-area compilations such as desirable. At the European scale, it becomes clear that current crustal models are not adequate for modeling regional datasets with enough detail. The global model CRUST2.0 is frequently used for crustal correction and wave propagation, but its resolution is too low for continental-scale studies. Many other detailed information are available, but at different scales, with different information contents, and following different formats: this information needs to be merged into a larger-scale, coherent representation. The other important issue is that connected to the faithful implementation of a known structure in computational meshes used in forward simulations of wave propagation. The shallow crustal discontinuities indeed are difficult to represent, because of the small size of the shallower elements of the mesh that lead to a very short time step. In this study, I am mostly interested in addressing these two fundamental issues, i.e. how to retrieve a ’good’ crustal model for Europe, on the basis of existing knowledge, and how to best represent it for efficient, but accurate, numerical simulation of seismic wave propagation. In the first part (Chapter 1), we analyse the surface wave sensitivity to the crustal structure presenting an exercise, based on surface wave dispersion matching, to reparameterize CRUST2.0 global model in a simpler grid that can be considered equivalent to CRUST2.0 in modeling surface waves. The models is tested from a wave propagating point of view with SPECFEM-3D code. We collect all the informations available on the this region and we create a new comprehensive reference crustal model for the European plate (Chapter 2) that describes the complex structure of the Europe with higher resolution and more plausibility than previous models. However, we can improve the resolution of such large scale compilation: we collect tens of seismic lines in the East Alps region (Chapter 3) building up, applying a geostatistics technique, a complete regional crustal model of that area that was included in EPcrust. This would be an example in which new local models could be developed and integrated in the continental one. The results are available on www.bo.ingv.it/eurorem/EPcrust. Since new models are available, before starting a 3D implementation of the models in numerical methods, in Chapter 4 we quantitatively analyse in 2D the influence of the representation and uncertainties in the knowledge of crustal parameters on simulated wave field. We evaluate different synthetic test cases respect to the reference, analysing the frequency and source-receiver-distance dependence of our approximations. For the simulations, we use an high order ADER-DG scheme implemented in the SeisSol2D code able to honour the discontinuities in the crust with high fidelity. From a seismological point of view the next step after developing a model would be a validation of the model itself. In chapter 5, we go through a validation process of EPcrust. The main goal is to understand if our new model is able to give a better fit of the real data. We use the Spectral Element Method as implemented in SPECFEM3D-Globe. This choice would be a compromise between accuracy of the representation of the crustal structure and computational cost. The ADER-DG methods, well suited for an accurate representation of the sharp interface within the crust, is at the moment computationally too expensive for 3D simulations at continental scale. At the and of this thesis, we give a brief overview on methods and theory applied to obtain our results.