3D Finite-difference Modeling of Strong Ground Motion in the Upper Rhine Graben - 1356 Basel earthquake

The disastrous Basel earthquake of October 18, 1356 (I0=X, M ≈ 6.9), appeared in, today seismically modest, Basel region (Upper Rhine Graben). The lack of strong ground motion seismic data can be effectively supplied by numerical modeling. We applied the 3D finite differences (FD) to predict ground motions which can be used for microzonation and hazard assessment studies.

The FD method is formulated for topography models on irregular rectangular grids. It is a 3D explicit FD formulation of the hyperbolic partial
differential equation (PDE). Elastodynamic PDE is solved in the time domain. The Hooke’s isotropic inhomogeneous medium contains discontinuities and a topographic free surface.

The 3D elastic FD modeling is applied on a newly established P and S-wave velocities structure model. This complex structure contains main
interfaces and gradients inside some layers. It is adjacent to the earth surface and includes topography (Kind, Faeh and Giardini, 2002, A 3D Reference Model for the Area of Basel, in prep.). The first attempt was done for a double-couple point source and relatively simple source function. Numerical tests are planned for several finite-extent source histories because the 1356 Basel earthquake source features have not been well determined, yet. The presumed finite-extent source is adjacent to the free surface. The results are compared to the macroseismic information of the Basel area.