Faulting properties and stress field in the northern Alpine foreland: a case study at the Albstadt Shear Zone, Southwest Germany

In the northern Alpine foreland, the region around the town Albstadt on the Swabian Alb is one of the seismically most hazardous areas in Central Europe and characterized by continuous seismic activity. In the last century nine earthquakes with a local magnitude ML $\geq 5.0$ occurred. Nevertheless, no surficial expression of the fault is visible.
The earthquakes rupture a N-S or NNE-SSW striking sinistral strike-slip fault zone, located in the upper crust, called the Albstadt Shear Zone (ASZ). The lateral extension and segmentation of the ASZ is under debate. In the area of the ASZ the Hohenzollerngraben (HZG) is the only tectonic feature visible at the surface. The HZG is described as a 2-3 km deep aseismic graben structure with inverted relief. A seismo-tectonic model of Reinecker and Schneider (2002) relates the HZG and other similar graben structures to the north and south with the movement of the ASZ. The ASZ is partially decoupled from the surface resulting in the opening of the step-like arranged graben structures.
The knowledge about the fault characteristics of the ASZ (e.g. extension, segmentation) is limited, as most of the previous studies focused on strong events and their aftershock sequences. ... mehrFurthermore, the location of the ASZ in the northern Alpine foreland makes it a suitable site to study the stress field and the driving forces of the observed intra-plate seismicity. To analyze the fault characteristics of the ASZ and the related stress field I focus on the continuous seismic activity in the years 2011 to 2020 in the area of the ASZ.
Within the AlpArray seismic project additional seismic stations were installed in the vicinity of the ASZ at the end of 2015. To improve the event locations and the detectability of low-magnitude events I further densified the seismic station network with additional seismic stations in the research area within the StressTransfer seismic network in 2018.
I complemented the earthquake catalog for the years 2011 to 2018 of the state earthquake service of Baden-Württemberg with additional phase arrival times at the AlpArray and StressTransfer seismic stations. With the extended data set I invert for new minimum 1D seismic $v_p$ and $v_s$ velocity models of the upper crust with station delay times. The resulting velocity models ASZmod1 are robust and simple and the seismic velocity increases with depth. The corresponding station delay times can be explained by the lateral depth variation of the crystalline basement.
The relocated continuous seismic activity within 2011 to 2018 aligns north-south slightly east of the 9$^\circ$E meridian. The main focus of the seismicity is between the towns Albstadt and Tübingen. I observe several subclusters indicating a possible segmentation of the ASZ. The majority of the determined fault plane solutions indicates the already known steeply dipping NNE-SSW striking sinistral strike-slip faulting of the ASZ. Nevertheless, I also observe NNW-SSE striking fault planes and minor components of normal and reverse faulting.
To make use of the densified station network since 2018 I apply a template matching detection routine to search for earthquake sequences within 2018 to 2020. In total, I identified six earthquake sequences (with more than 10 events), of which the three biggest sequences (more than 100 events) can be separated in one fore- and aftershock sequence and two earthquake swarms (October 2018 and September 2019). This observation is new, as earthquake swarms were so far not observed in the area of the ASZ. By determining relative event locations together with fault plane solutions of the earthquake sequences the active fault planes could be identified. I observe three different fault types: the already known NNE-SSW striking sinistral strike-slip ASZ at depths of 5-10 km. Beneath the HZG in 11-15 km depth I identify a NW-SE striking dextral strike-slip fault zone. Despite of the same orientation as the HZG, the depth difference between the NW-SE striking fault zone and the shallow HZG boundary faults exclude a direct connection. Though the overlapping location may hint at an existing NW-SE striking weakening zone in the upper crust. At the interception of the ASZ and the NW-SE striking fault zone NNW-SSE striking sinistral strike-slip and normal faulting events are observed and indicate a heterogeneous deformation zone with complex faulting.
The determined direction of the maximum horizontal stress of 140-149$^\circ$ is in good agreement with prior studies, indicating that the stress field in the area of the ASZ is mainly generated by the regional plate driving forces and the Alpine topography. I observe a change of the stress field with depth. Down to about 7-8 km depth the maximum horizontal stress is bigger than the vertical stress. Below this depth, the vertical stress is the main stress component.
The newly derived seismo-tectonic model of the seismicity on the Swabian Alb indicates the activation of weakening zones or pre-existing structures favorably aligned in the current tectonic stress field. This is also observed for other areas of intra-plate seismicity.