P- and SH-wave reflection seismics of the reactivated intraplate Osning Thrust in northern Germany

Neotectonic movements can cause severe hazards and are scientifically and socially relevant, e.g. for seismic hazard assessment, and utilisation of the subsurface. In northern Germany, a presumed aseismic region, little is known about these processes and the associated structures, despite proven neotectonic activity, because many faults are hidden beneath sediments. To improve the knowledge of neotectonic activity, investigations of recently-active fault zones, like the Osning Lineament (OL) in North Rhine-Westphalia, are required.

To better understand the neotectonic evolution of the OL, we use near-surface geophysics, which have not been used at the OL so far. We used a combined approach using high-resolution 2D P- and SH-wave reflection seismics. P-wave seismic alone can often not properly image near-surface impressions of faults due to poor shallow resolution, but this gap can be closed using SH-wave reflection seismics, which offers very high resolution, even at shallow depth. Three P-wave profiles were measured with a hydraulically-driven vibrator vehicle (sweep frequency: 20 to 200 Hz) with a source point spacing of 10 m and plugged vertical geophones at 5 m intervals. Additionally, four SH-wave profiles were surveyed using an electro-dynamic micro-vibrator (sweep frequency: 20 to 160 Hz) with a source point spacing of 2 or 4 m and a landstreamer with horizontal geophones at 1 m intervals.

The seismic profiles show good results with respect to mapping the fault inventory. In the migrated depth sections of the P-wave profiles, several northward-dipping faults in the Cretaceous formations are recognizable, which are interpreted hitherto unknown extensions of the OL. The Quaternary, with a maximum thickness of 20 to 30 m, is only poorly imaged by the P-wave profiles, but there are nevertheless hints that the faults also extend into the Quaternary. The SH-wave profiles support this assumption, due to their higher resolution close to the surface, because of very-low wave velocities between 150 and 500 m/s. In the Quaternary sediments, further faulting and deformation features are recognizable, enabling a more comprehensive interpretation and understanding of the local fault geometry.

In the course of the project, we also carry out a full waveform inversion of the P- and S-wave data to improve the fault imaging. This will be accompanied by testing of different migration methods and seismic attribute analysis.