SOURCE MECHANISM OF THE 1992 ROERMOND EARTHQUAKE FROM SURFACE-WAVE INVERSION OF REGIONAL DATA

The Dutch-German border region near the city of Roermond, The Netherla nds was hit by a rather strong crustal earthquake at 1:20 UT, 1992 Apr il 13. The epicentre is located within the Roer Valley Graben, a regio n currently undergoing extension. The centroidal source mechanism of t his event has been retrieved by moment tensor inversion of broad-band long-period surface waves recorded at regional distances (100-1500 km) . The double-couple contribution of the moment tensor corresponds to a lmost pure normal faulting (rake: 262-degrees) on a steeply south-west ward dipping fault (dip: 58-degrees) with a NW-SE trend (strike: 138-d egrees). This result is consistent with local tectonics but violates s ome first-motion P-wave polarity data. The deduced seismic moment is 9 .2 x 10(16) N m corresponding to a moment magnitude of 5.3. A centroid al depth of 18 km fits slightly better than 13 km; however, the differ ences are small and other methods are necessary to constrain the depth further. It is now possible to access a number of broad-band three-co mponent seismic stations in Europe via phone line (e.g. German Regiona l Seismic Network) and the ORFEUS data centre (i.e. Global Digital Sei smic Network). This allows retrieval of waveform data immediately afte r an earthquake. In this paper we present a potentially fast and relia ble procedure for extracting the moment tensor from low-frequency surf ace waves using the Roermond earthquake as an example. Detailed knowle dge of the velocity-depth structure along the travel paths seems unnec essary with our procedure. Further testing is required on all future m oderate to strong earthquakes in Europe using rapidly accessible stati ons to investigate the procedure's usefulness and possible limitations as a tool for rapid moment tensor estimation.