SOURCE PARAMETERS OF THE 1992 ROERMOND EARTHQUAKE, THE NETHERLANDS, AND SOME OF ITS AFTERSHOCKS RECORDED AT THE STATIONS OF THE GEOLOGICAL SURVEY OF NORTHRHINE-WESTPHALIA

The Roermond earthquake of April 13, 1992, was recorded unclipped at f our of the six stations of the Geological Survey of Northrhine- Westph alia (epicentral distances between 54 and 103 km). The local magnitude values determined from these recordings are 5.9 for stations GSH, PLH and OLF, and 6.1 for station WBS. The main shock hypocenter was locat ed at latitude 51 degrees 10.1' N and longitude 5 degrees 55.9' E at a depth of 17.6 km. The focal mechanism determined from P and SH-wave p olarities and amplitude ratios is 120 degrees for strike, 70 degrees f or dip, and - 100 degrees for rake, which corresponds to an almost pur e dip-slip movement along a normal fault. The mean seismic moment, cal culated from several spectra, is 5.4 x 10(16) Nm, the mean dislocation is 35 cm. Until the end of May about 80 aftershocks from the Roermond area were recorded at the stations of the Geological Survey. The main shock also triggered a series of earthquakes at the southern border o f the Roer Valley Graben between Geilenkirchen and Eschweiler, about 3 0-45 km SSE of the main shock epicenter. The largest of these events h ad a local magnitude of M(L) = 3.7.