Seismological Laboratory
UC Berkeley
Email:
yann@seismo.berkeley.edu
poster/oral: poster
| The D" structure is believed to hold the key to many largely unanswered questions in deep Earth geodynamics and its study can be efficiently addressed by seismological forward modeling approaches. However, present global waveform modeling (based on normal mode perturbation or ray tracing) cannot handle wave propagation in this region due to the association of high velocity contrasts and diffraction, presence of topography on a solid-liquid interface, anisotropy, and also the need of high frequency simulations at the global scale (at least 0.1 Hz). To fill this gap, we have developed an extension of the coupled method of spectral elements and a modal solution (Capdev-ille et al., 2001) allowing us to use a relatively thin layer of spectral elements to rep-resent a 3D D" layer model, "sandwiched" between two modal solutions for the rest of the Earth, assumed spherically symmetric. The Spectral Element Method (SEM) is a high-order variational method that combines the geometrical flexibility of finite element methods together with the precision of pseudo-spectral methods. The SEM has been shown by different authors to provide an accurate solution to most of dif-ficult 3D problems. However, when dealing with Earth models that are not fully 3D, the drawback of using the SEM is an unnecessary increase in both computer memory and CPU time, thus limiting the applications to long-period simulations. The coupled method presented here circumvents this problem by limiting the use of the SEM to only a small part of the Earth allowing higher frequency simulations. The coupling is performed using two Dirichlet to Neumann (DtN) operators, one on each interface. DtN operators are constructed in the frequency-spectral domain using modal solutions in the spherically symmetric sphere and spherical shell. Using this new method, we are able to perform simulations in D" up to 0.1Hz on parallel computers of reason-able size and in time short enough to be suitable for forward modelling of specific S waveform profiles of interest. Results of preliminary forward modelling of real data in realistic 3D D" models will be presented. Capdeville Y. et al (2001). Coupling Spectral Elements and Modal Solution etc. accepted for Geophys. J. Int. 1 |