RegSEM, a flexible regional Spectral Element code: application to continental scale problems

Since the introduction to seismology of the so-called Spectral Element Method (the SEM: a powerful numerical tool which allows accurate simulations of wave propagation in complex elastic media), numerous seismologists have assessed various global tomographic models to test how well the SEM synthetics fit observed waveforms and investigate the parameters which need to be better resolved to improve these. On the other hand, several techniques have been developed to use the SEM in the inverse problem and obtain 3D tomographic images. These techniques rely on the adjoint method. So far, they have been applied only on a local and regional scale. In this work, we first present RegSEM: a very flexible and user-friendly code designed to simulate wave propagation at the regional scale (100 to 10000 km). In particular, it includes ellipticity, attenuation, arbitrary anisotropy, Moho and free surface topography, and Perfectly Matched Layers at the border of the region. We show some validation tests performed in PREM to compare with exact normal mode solutions. In a second step, we use RegSEM to compute both regular and adjoint wavefields. We perform forward modeling simulations of waveforms in different crust and upper mantle models of the North American continent. Comparing these waveforms with the corresponding data, we point out the parameters which need to be better constrained before inverting for mantle structure, such as shear wave velocity and anisotropy of the shallow structures, and Moho topography. Then, based on these results, we set up a 3D tomographic inversion of full waveforms using the adjoint method. Waveform misfit kernels for each parameter are shown in the context of synthetic target models.