Fault-plane solutions and source parameters of the 1992 Roermond, the Netherlands, mainshock and its stronger aftershocks from regional seismic data

Abstract

The earthquake of April 13, 1992 in the border region between the Netherlands and Germany near the town of Roermond, ranges with its local magnitude of ML- = 5.9 among the largest seismic events observed in historical times in the Lower Rhine Embayment. Several hundred buildings in the area around the epicenter suffered light to moderate structural damage (intensity VII effects). The main shock was preceded about 0.2 seconds before by a foreshock of local magnitude ML = 4.8 and followed, up to the middle of May, by more than 200 aftershocks with local magnitudes up to ML = 3.8. The whole earthquake sequence has been well recorded by the dense seismic station network in the Lower Rhine Embayment and its vicinity. Based on these local and regional recordings the hypocenter locations, source parameters and focal mechanisms of the stronger events of the sequence have been determined with high precision by the Department of Earthquake Geology of the University of Cologne. The source of the main shock was located in the southwestern vicinity of Roermond, about 14 to 18 km below the surface at the depth continuation of the Peel Boundary Fault. This fault zone forms the eastern border of the Roer Valley Graben. The focal mechanism was of the extensional dip-slip type (normal fault) with the active fault plane trending in NW-SE direction (124°) and dipping steeply (at 68° to the SW). The western block (Roer Valley Graben) moved down with respect to the eastern block (Peel Horst). A diameter of the source area of 4 to 5 km has been determined from the spectral analysis of P- and SH-wave signals, using the Brune source model. The average displacement along the fault plane was approximately 21 cm, the associated static stress drop 4.4 MPa and the seismic moment 1017 Nm. The spatial distribution of the aftershocks points to a source zone which trends in the same general direction as the Peel Boundary Fault and has a total length of about 10 km. The main conclusions about the Roermond earthquake sequence fit well into the tectonic setting of the Lower Rhine Embayment and the general pattern of seismotectonic dislocations in the western and central European area.