THREE DIMENSIONAL MODELING OF STRONG GROUND-MOTION SITE EFFECTS IN THE UPPER
RHINE GRABEN DUE TO THE FINITE-EXTENT SOURCE - 1356 BASEL EARTHQUAKE

The Basel earthquake of October 18, 1356, is considered to be one of the most serious earthquakes in Europe of the last centuries (I0=IX, M ~ 6.9). Combined effects of the finite-extent source and complex local structure are modelled to supply the strong motion estimates for today seismically moderate region. To model the ground motion, 3D finite differences (FD) were applied.

The 3D explicit FD method for topography models on irregular rectangular grid is an approximation to 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 boundary conditions are satisfied via the treatment of the effective parameters. A particle-velocity dependent term is added to the PDE to approximate viscoelastic behavior of the medium.

The 3D FD modeling is computed for the recently established P and S-wave velocities structure of the Basel area (Kind, 2002), including the topography. The first attempts for a double-couple point source and relatively simple source function are now upgraded to kinematic finite-extent source simulation. The finite-extent source is adjacent to the free surface, because the fault has been recognized through trenching on the Reinach fault. The 1356 Basel earthquake source features cannot be determined and therefore several rupture histories are tested. The results are compared to the macroseismic information of the Basel area and to recently established 2D simulations.