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

Abstract

The Roermond earthquake of April 13,1992, was recorded unclipped at four of the six stations of the Geological Survey of Northrhine-Westphalia (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 located at latitude 51° 10.1’ N and longitude 5° 55.9’ E at a depth of 17.6km.The focal mechanism determined from P and SH-wave polarities and amplitude ratios is 120° for strike, 70° for dip, and - l00° for rake, which corresponds to an almost pure dip-slip movement along a normal fault. The mean seismic moment, calculated from several spectra, is 5.4 x 1016 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 of the Roer Valley Graben between Geilenkirchen and Eschweiler, about 30-45 km SSE of the main shock epicenter. The largest of these events had a local magnitude of ML = 1.7.