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 Netherlands was hit by a rather strong crustal earthquake at 1:20 UT, 1992 April 13. The epicentre is located within the Roer Valley Graben, a region currently undergoing extension. The centroidal source mechanism of this 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 almost pure normal faulting (rake: 262-degrees) on a steeply south-westward dipping fault (dip: 58-degrees) with a NW-SE trend (strike: 138-degrees). This result is consistent with local tectonics but violates some 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 centroidal depth of 18 km fits slightly better than 13 km; however, the differences are small and other methods are necessary to constrain the depth further. It is now possible to access a number of broad-band three-component seismic stations in Europe via phone line (e.g. German Regional Seismic Network) and the ORFEUS data centre (i.e. Global Digital Seismic Network). This allows retrieval of waveform data immediately after an earthquake. In this paper we present a potentially fast and reliable procedure for extracting the moment tensor from low-frequency surface waves using the Roermond earthquake as an example. Detailed knowledge of the velocity-depth structure along the travel paths seems unnecessary with our procedure. Further testing is required on all future moderate to strong earthquakes in Europe using rapidly accessible stations to investigate the procedure's usefulness and possible limitations as a tool for rapid moment tensor estimation.