Investigations ofSnandPnphases in the area of the upper Rhine Graben and northern Switzerland

Abstract

SUMMARY 22 regional earthquakes in SW Germany and northern Switzerland, recorded with the Southern Black Forest Digital Seismic Network, are studied for Pn and Sn phases. In contrast to the results of some seismic-refraction studies a large number of these earthquake recordings show clearly correlatable Sn phases. So far there are two remarkable trends concerning the appearance or disappearance of Sn: 1 earthquakes with large focal depths (z= 20–30 km) preferentially show Sn phases. 2 Earthquakes from the SW with rays crossing the southern Rhine Graben show no Sn energy, regardless of focal depth. To investigate the amplitude behaviour of the Sn phase, synthetic seismograms for varying focal parameters, as well as for different structural parameters (submoho velocity gradient and attenuation of S waves), are calculated using the reflectivity method. The synthetics show that the Sn amplitude is sensitive, especially to the fault-plane orientation and the submoho velocity gradient. After correction for the radiation pattern, the missing of the Sn phase from earthquakes in the south-western part of the studied area seems to be associated with structural effects in the uppermost mantle. A small submoho S-wave gradient or a high upper mantle attenuation are the most likely explanations. Another very interesting observation relates to the Pn velocities. The azimuthal variation of the velocity, as well as the high maximum Pn velocity (v= 8.6–8.8 km s−1) itself, point to an anisotropic uppermost mantle. The velocity variation of 0.6km s−1 and the directions of minimum and maximum velocities are in accordance with earlier results from seismic-refraction experiments and their petrological interpretation. The derived Sn velocities have a mean value of 4.71 km s−1. Up to now the quality and number of the Sn recordings do not allow us to resolve a possible S-wave anisotropy of the uppermost mantle.