SEISMICITY AND STRUCTURAL COMPLEXITY OF THE DINKELBERG BLOCK, SOUTHERN RHINE GRABEN

A detailed analysis of the Dinkelberg seismicity is presented. The sei smic activity of the central Dinkelberg block starts at about 10 km an d increases with depth to a maximum between 18 and 20 km. Joint hypoce ntre determinations clearly prove that no hypocentres are located with in the lower crust which starts at a depth of about 20 km according to results from recent reflection seismic surveys. The seismic activity is concentrated just above the top of the lower crust. Fault-plane sol utions of the Dinkelberg events show that the strike of the nodal plan es is parallel to the directions of near-surface tectonic features. A correlation also exists between the main fault directions and the spat ial distribution of the hypocentres in the Pratteln earthquake cluster at the southern end of the Dinkelberg block. The occurrence of negati ve P residuals from local earthquakes for special propagation directio ns of waves crossing the Dinkelberg block is modelled by a westward-di pping layer. This postulated upper crustal structure together with obs ervations of preferred S-wave polarizations are discussed in regard to recent tectonic models and geological findings.