Geophys. J. Inr. zyxwvutsrqponm (1994) 119, 31-43 zyxwvutsrqp Modelling the Roermond earthquake of 1992 April 13 by stochastic simulation of its high-frequency strong ground motion Frank Scherbaum Institut fur Allgemeine und Angewandte Geophysik, Ludwig Maximilians Universitat Miinchen, Theresienstrasse 4 1, 80333 Munchen, Germany Accepted 1994 February 18. Received 1994 February 17; in original form 1993 June 14 SUMMARY The Roermond earthquake of 1992 April 13 is one of the most important seismic events for assessing potentially destructive strong ground-motion parameters in central Europe. Owing to the lack of unclipped near-source records, however, the source depth as well as the high-frequency signal contents are only known to within large uncertainties. Using a stochastic simulation technique, site-dependent synthe- tic acceleration seismograms and simulated Wood-Anderson records for the Roermond Earthquake were calculated for the epicentral region and a site in 60 km distance. For the distant site, three different damping models were modelled while for the epicentral region, a tertiary and quaternary coverage of 1500 m was assumed. Both constant- and frequency-dependent Q were considered. The resulting simula- tions were tested for their consistency with (a) epicentral acceleration between 1.0 and 2.0 m secC2 as indicated by the low observed epicentral intensities of slightly above VII, (b) the observed peak ground accelerations at a distance of 50-60 km which are reported to be 0.3 zyxwv f 0.1 m secC2, and (c) the observed Wood-Anderson magnitude of MWA zyxwvu = 5.9 f 0.2. While the stochastic simulation does not result in a single unique model, it allows the comparison of certain models (including their uncertainties) in terms of their consistency with the observations. The most pronounced result shows that of the three different source depths (13,21 and 25 km) which were investigated, the shallow source is by a wide margin.the least consistent with the observations, independent of the remaining model assumptions. On the other hand, peak accelerations of 0.3 f 0.1 m secC2 in 50-60 km distance can be explained by reasonable combinations of seismic moments and corner frequencies for all the site-damping models. While under the constant-Q assumption, the most consistent zyxwvutsr M,/f, combinations show a clear trade-off between zyx fc and M,, for Mo = (5-10) zyxwvu X 10l6 Nm, this trade-off is strongly reduced if Q is assumed frequency dependent as Q = Qo Xf". For an average seismic moment of M,, = 7.5 x loL6 Nm, a corner frequency of 1 .O-1.25 Hz provides the maximum joint consistency scores independent of the other model assumptions. Key words: Roermond earthquake, source parameters, strong ground-motion modelling. 1 INTRODUCTION The Roermond earthquake (Mw,=5.9) of 1992 April 13 was the strongest earthquake ever recorded in the Lower Rhine Embayment (Fig. 1; Ahorner 1992). It caused considerable macroseismic effects such as ground liquefac- tion and was felt over a distance range of several hundreds of kilometres. It was recorded globally on numerous seismic stations and its source parameters have been analysed by various authors (Ahorner 1992; Paulssen, Dost & van Eck 1992; Braunmiller, Dahm & Bonjer 1994; Pelzing 1994). Nevertheless, there are considerable uncertainties as to the focal depth, as well as to the near-source strong ground-motion parameters. Estimations of peak ground acceleration for the epicentral region range from 0.1 to 0.2 g from engineering considerations (Noe 1993) and up to 0.35 g (at site Haelen NE of the epicentre) from an empirical Green's function approach (Gariel ef al. 1994). Berger (1994) obtains values for the epicentral area between 0.135 and 0.18 g based on fitting an empirical formula by Joyner & Boore (1981) to acceleration values observed at a distance range of 50 to 400km. Similar values are obtained from 31