A model of composite seismic sources for the Lower Rhine Graben

The Lower Rhine Graben (LRG), straddling the border zone of Belgium, the Netherlands, and Germany, is an active tectonic structure in continental NW Europe. It is characterized by NW-SE oriented normal faults, and moderate but rather continuous seismic activity. Many faults have been mapped in the LRG, but so far a model of fault hierarchy or fault segmentation was lacking. In the frame of a European database of seismogenic sources, we have devised a seismic-source model for the LRG, consisting of so-called composite seismic sources. Each composite seismic source may encompass one or more fault segments, but it is unlikely that a rupture segment would extend across more than one source. We distinguish 15 seismic sources based on major stepovers, bifurcations, gaps, and important changes in strike, dip direction or slip rate. The sources are further subdivided into one or more informal fault sections, each with an associated surface trace. We have compiled all relevant data concerning the seismic-source parameters required for the database, putting lower and upper bounds on strike, dip, rake, slip rate, and depth, and an upper bound on earthquake magnitude. Combination of literature and seismological data indicates that fault dips in the LRG likely range between 50° and 65°. Minimum and maximum strike has been determined for each source based on the one-sigma variation of their mapped surface-trace orientations. We determined the variation in rake by resolving the shear-stress direction on planes with the aforementioned ranges in strike and dip from a published regional stress tensor. The primary data for slip rates are vertical displacements recorded by fluvial terraces intersecting faults in the LRG. We compiled an extensive set of vertical displacement rates, allowing us to assign minimum and maximum rates to each source. These vertical displacement rates range mostly between 0.01 and 0.07 mm/yr, and corresponding slip rates between 0.01 and 0.09 mm/yr. The Peelrand and Erft/Swist faults appear to be the fastest slipping faults. Earthquake hypocenters indicate a maximum depth of ~25 km. The minimum depth is set at 0 km, as all faults display offset of late Quaternary deposits, and paleoseismic studies have shown the occurrence of surface-rupturing earthquakes in the recent geologic past. The length of the Eft/Swist fault is considered to represent the longest credible rupture length, corresponding to a maximum magnitude MW=7.1. Both paleoseismic studies and source-length considerations suggest that Mmax of the different sources ranges between 6.3 and 7.1. This composite source model should provide a new basis for modelling seismic hazard, as well as to guide further paleoseismic studies in the LRG.