Seismicity pattern and style of seismic faulting at the eastern borderfault of the southern Rhine Graben

Abstract

Seismicity recorded on high-gain, digital networks in the southern Rhine Graben reveals that the area around the eastern borderfault system shows the highest level of seismic activity. Across this fault system remarkable differences of maximum focal depths exist: 12–13 km are characteristic within the graben, and 18–20 km beneath the western foothills of the southern Black Forest, located approximately between the eastern borderfault of the graben and the river Rhine (inner borderfault). Farther to the east, in the southern Black Forest proper, maximum depths around 20–22 km are found. At particular sites a few earthquakes with depths of up to 26 km, close to the Moho, were detected. The deep cut-off of seismic activity in the southern Black Forest and its western foothills is persistent for about 100 km distance between the Freiburg focal area in the north and the Swiss Jura in the south. Seismic reflection profiling indicates that the deep foci lie well within a reflective lower crust (southern Black Forst, Swiss Jura) or just above a highly reflective top of the lower crust (western foothills of the southern Black Forest, Dinkelberg block). Within the graben proper, seismic activity is restricted to the upper crust and ceases about 5 km above the top of the lower crust.Low seismic activity is found in areas of low P-wave velocities as in the upper crust of the central Black Forest, north of the Freiburg focal area, and at midcrustal levels in the southern Black Forest (at depths of about ±10 km).Well constrained focal mechanisms are presented for 47 events. Most of them occurred adjacent to the eastern borderfault. Nearly all mechanisms show an oblique component of slip. Strike-slip and normal-faulting mechanisms prevail. Only a few events are of thrust type. Dominance of a certain faulting style with depth was not found. Focal mechanisms of events which occur within 10 km of the borderfault show variations of faulting style, indicating local deviations from the average regional stress tensor and/or from the frictional parameters in the proximity of the particular rupture planes. Although the nodal planes correlate with the strike directions of surface trends of major faults in general, it is difficult to recognize their link to individual faults.