INVESTIGATIONS OF S-N AND P-N PHASES IN THE AREA OF THE UPPER RHINE GRABEN AND NORTHERN SWITZERLAND

22 regional earthquakes in SW Germany and northern Switzerland, record ed with the Southern Black Forest Digital Seismic Network, are studied for P-n and S-n phases. In contrast to the results of some seismic-re fraction studies a large number of these earthquake recordings show cl early correlatable S-n phases. So far there are two remarkable trends concerning the appearance or disappearance of S-n: (1) earthquakes wit h large focal depths (z = 20-30 km) preferentially show S-n phases. (2 ) Earthquakes from the SW with rays crossing the southern Rhine Graben show no S-n energy, regardless of focal depth. To investigate the amp litude behaviour of the S-n phase, synthetic seismograms for varying f ocal parameters, as well as for different structural parameters (submo ho velocity gradient and attenuation of S waves), are calculated using the reflectivity method. The synthetics show that the S-n amplitude i s sensitive, especially to the fault-plane orientation and the submoho velocity gradient. After correction for the radiation pattern, the mi ssing of the S-n phase from earthquakes in the south-western part of t he studied area seems to be associated with structural effects in the uppermost mantle. A small submoho S-wave gradient or a high upper mant le attenuation are the most likely explanations. Another very interest ing observation relates to the P-n velocities. The azimuthal variation of the velocity, as well as the high maximum P-n velocity (v = 8.6-8. 8 km s(-1)) itself, point to an anisotropic uppermost mantle. The velo city variation of 0.6 km s(-1) and the directions of minimum and maxim um velocities are in accordance with earlier results from seismic-refr action experiments and their petrological interpretation. The derived S-n velocities have a mean value of 4.71 km s(-1). Up to now the quali ty and number of the S-n recordings do not allow us to resolve a possi ble S-wave anisotropy of the uppermost mantle.