IUGG 2003
Abstract Information

Primary Author Information

Name Ivo Oprsal
Affiliation Swiss Seismological Service, ETH Zurich, Switzerland, on leave from Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic
Co-Author Information
Co-Author - 1 Name Johana Brokesova
Affiliation Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic
Co-Author - 2 Name Jiri Zahradnik
Affiliation Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic
Co-Author - 3 Name Domenico Giardini
Affiliation Swiss Seismological Service, ETH Zurich, Switzerland

Abstract Information

Abstract Number 3
Abstract Title 3D FINITE-DIFFERENCE SEISMIC MODELING - COMBINED EFFECTS BY HYBRID METHODS
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Complex interactions resulting in the earthquake ground motions can be, in many cases, efficiently modelled by hybrid methods. Our two-step hybrid method combines an arbitrary 3D method evaluating the source and path effects, and the 3D finite differences (FD) to incorporate the site features. In the 1st step we use ray, discrete wavenumber (DWN), or FD method to simulate the source and path effects in a background regional structure. The regional model is considerably simpler than the local structure which is absent in the 1st-step model. The complex local structure is enveloped in a formal box of receivers called 'excitation box' (EB). The wavefield, due to the source and path, is recorded at EB and stored on a disk as so-called excitation. In the 2nd step, the wavefield in a relatively small model containing only the EB and its vicinity is computed. At this step, the local site structure is included inside the EB. The wavefield from the 1st step is injected at the EB into the 2nd-step model. The 2nd step is based on 3D FD performing on irregular rectangular grid. The FD approximation is of a second-order accuracy, the boundary conditions at the material interfaces are approximated via treatment of effective (geometrically averaged) material parameters. The EB is fully permeable in both steps and the result comprises all effects of the source, path, and site. The computational cost and memory requirements are significantly reduced compared to the case of all-in-one FD method. In our examples, the 1st-step crustal structure is 1D and the source is represented by a point double couple. The free surface is non-planar, the example of a local structure is a modified Volvi-Lake site with Vp/Vs ~ 4 and maximum ratio of the velocities Vpmax/Vsmin ~ 10. The maximum frequency of the synthetics is 5Hz and the distance of the site from the source is ~ 10km. The comparison of the results for the ray-FD, DWN-FD, and FD-FD hybrid solutions shows a very good agreement, in spite of the fact that the ray excitation contains less complete wavefield than the DWN and FD excitations. The whole 2nd-step FD modeling needs only 6 hours on P3 1Ghz, and this makes the hybrid method an effective and competitive tool for the earthquake ground motion computations.

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