Calculs de magnitudes et mécanismes au foyer pour le séisme de Saint-Béat du 4 octobre 1999

Abstract

This paper exposes the various magnitude and focal mechanism computations done for the Saint-Béat (Pyrenees) earthquake of 4 October 1999 at 18:14 UT. The focal mechanisms have been obtained either by regional moment tensor inversion, for which the procedure is briefly described, or from first motion polarities. Although there is a disagreement about the direction of dip of the preferred fault plane, this one is almost sub-vertical and seems to exhibit a global east–west orientation, with a T axis oriented north to NNE and a P axis oriented SSE to south-east. We also deduced from the inversion a seismic moment of 4.3·10 15 N·m, corresponding to a Mw magnitude of 4.4. This one equals the m b magnitude reported by the US Geological Survey and is between the extreme magnitudes Msz 3.7 and M L 4.8 reported by the RéNaSS.