A revised seismotectonic model for the Albstadt Shear Zone, Southwest Germany

The Swabian Jura near the town Albstadt is one of the seismically most active regions of Germany. Concerning tectonics, the region is characterized by a NW-SE striking shallow (<2-3 km) aseismic graben structure, the Hohenzollerngraben (HZG), and the seismically active Albstadt Shear Zone (ASZ), a NNE-SSW striking sinistral strike-slip fault zone at about 1-18 km depth. The ASZ has an extension of at least 50 km, but there is no evidence for surface rupture. Beside the continuous low-magnitude seismic activity, in the 20^th century eight earthquakes with ML>5.0 occurred causing significant damage in the region of the Swabian Jura.

Here, we search for and then analyze very low-magnitude earthquake sequences during 2018 to 2020 in the area of the ASZ to image the seismically active faults. We apply a template matching detection routine, determine relative event locations for the identified earthquake sequences, calculate fault plane solutions based on first motion polarities and finally moment tensor solutions of earthquakes with ML greater than 3.5.

We identified six earthquake sequences and image three types of seismically active faults in the area of the town Albstadt. First, the known ASZ, with NNE-SSW striking sinistral strike-slip faulting at 5-10 km depth. Second, a so far not observed NW-SE striking dextral strike-slip fault at 11-15 km depth, beneath the HZG. A continuation with depth of the HZG surface faults is unlikely, but the co-location of the HZG and the NW-SE striking fault may indicate an inherited zone of weakness below the HZG. And finally, complex faulting in form of NNW-SSE striking sinistral strike-slip and normal faulting in 9-12 km depth indicating a heterogeneous deformation zone at the intersection of the ASZ and the newly discovered NW-SE striking fault zone.

Our results go into a revised seismotectonic model for the area of the ASZ, including two new types of seismically active faults in the area.