The April 13, 1992 earthquake of Roermond (The Netherlands); first interpretation of the NARS seismograms

Abstract

The Roermond earthquake of April 13th,1992,was recorded by stations of the seismological NARS network located in The Netherlands, Germany, and Belgium. The data of this network allowed an accurate determination of epicenter, focal depth, origin time and focal mechanism of the event. By an arrival time inversion of P-wave onsets, the epicenter and focal depth were located at 51°10.2' N 5°58.3' E and 21 km, respectively. The relatively large focal depth of about 20 km is confirmed by travel time modelling of additional, later arriving, P-wave phases. The average crustal structure beneath the network was implicitly obtained by matching the travel time variations to the different stations. The earthquake mechanism, i.e. the direction of movement along the fault plane, is inferred by modeling the polarities and amplitudes of the various phases. The NARS data are best fitted by a dip-slip movement along a fault plane with a strike of 124° and a southwesterly dip of 70° or by dip-slip movement along the perpendicular plane with the same strike. The good agreement of the attitude of the first fault plane with that of the Peel Boundary Fault, combined with the fact that the hypocentral location plots on the downward extension of that fault, indicates that a downward movement of the Roer Valley Graben has taken place along the Peel Boundary Fault.