Numerical Simulations of Earthquake Scenarios in the Lower Rhine Embayment Area

The choice of the Lower Rhine Embayment as study area for strong
ground motion modeling may be puzzling at first glance. This region
in the northwest of the European continent is characterized by
active tectonics on a complex system of fault-zones with relatively
low deformation rates. Consequently, the area has shown low to
moderate seismicity in the time frame covered by observational
seismology. However, historical and geological evidence proves that
the fault systems of the Lower Rhine Embayment have the potential
of producing large earthquakes with magnitudes 6 and above
accompanied by surface rupture. The presence of large sediment
deposits in this region leads to local amplification of ground
motion with large lateral variations. Dense population and an
agglomeration of industry results in an elevated seismic risk.
Assessment of seismic hazard in regions characterized by low recent
seismicity is afflicted with large uncertainties. This is mainly
due to the dearth of observational data on strong ground motions
associated with large earthquakes. Numerical simulations of
earthquake scenarios can account for estimates on peak ground
motion and waveforms and therefore help closing this gap. Naturally
the first step consists in accurate reproduction of the few
observed events. An additional crucial quantity is the range of
variations of the simulation results within the uncertainty margins
associated with input parameters. Knowledge about this behavior
enlarges the significance of numerical simulation results. Four
historical and recent earthquake scenarios are modeled using a
finite difference approach. Results are analyzed with special
emphasis given to their intrinsic variability with model complexity
and simulation settings. The choice of investigated parameters is
adopted to the differing scope of observational data available for
the individual events. In general encouraging similarity between
synthetic and observed ground motions is found, even when a
simplified model is used. However, detailed investigation carried
out for the most recent earthquake scenario - the magnitude 4.9
July 22 2002 Alsdorf event - strongly suggests the significance of
an appropriate source description and the modeling of anelastic
behavior on simulation results. Finally a web-based application for
storage and visualization of synthetic ground motion data is
presented.