Abstract
Abstract. The region around the town Albstadt, SW Germany, was struck by four damaging earthquakes with magnitudes greater than 5 during the last century. These earthquakes occurred along the Albstadt Shear Zone (ASZ), which is characterized by more or less continuous microseismicity. As there are no visible surface ruptures that may be connected to the fault zone, we study its characteristics by its seismicity distribution and faulting pattern. We use the earthquake data of the state earthquake service of Baden-Württemberg from 2011 to 2018 and complement it with additional phase picks beginning in 2016 at the AlpArray and StressTransfer seismic networks in the vicinity of the ASZ. This extended data set is used to determine new minimum 1-D seismic vp and vs velocity models and corresponding station delay times for earthquake relocation. Fault plane solutions are determined for selected events, and the principal stress directions are derived. The minimum 1-D seismic velocity models have a simple and stable layering with increasing velocity with depth in the upper crust. The corresponding station delay times can be explained well by the lateral depth variation of the crystalline basement. The relocated events align about north–south with most of the seismic activity between the towns of Tübingen and Albstadt, east of the 9∘ E meridian. The events can be separated into several subclusters that indicate a segmentation of the ASZ. The majority of the 25 determined fault plane solutions feature an NNE–SSW strike but NNW–SSE-striking fault planes are also observed. The main fault plane associated with the ASZ dips steeply, and the rake indicates mainly sinistral strike-slip, but we also find minor components of normal and reverse faulting. The determined direction of the maximum horizontal stress of 140–149∘ is in good agreement with prior studies. Down to ca. 7–8 km depth SHmax is bigger than SV; below this depth, SV is the main stress component. The direction of SHmax indicates that the stress field in the area of the ASZ is mainly generated by the regional plate driving forces and the Alpine topography.
Highlights
The Swabian Alb near the town of Albstadt (Fig. 1) is one of the most seismically active regions in Central Europe (Grünthal and the GSHAP Region 3 Working Group, 1999)
The selected minimum 1-D seismic velocity model ASZmod1 consists of 5 layers (Fig. 4a and b)
Our layer between 2.50 and 5.60 km depth is in good agreement with the Swabian Jura model, whereas the deeper layer has a higher agreement with the BadenWürttemberg model (Fig. 4a)
Summary
The Swabian Alb near the town of Albstadt (Fig. 1) is one of the most seismically active regions in Central Europe (Grünthal and the GSHAP Region 3 Working Group, 1999). Stange and Brüstle (2005) consider the bottom of the Mesozoic sediments a mechanical decoupling horizon as no earthquakes occur above 2 km depth Another tectonic feature in our research area is the ENE– WSW-striking Swabian Line north of the river Neckar (Fig. 1b). The lateral extent of the fault zone in an N–S direction is still under debate: Reinecker and Schneider (2002) propose an extension from northern Switzerland towards the north of Stuttgart, whereas Stange and Brüstle (2005) do not find this large extension as most of the seismicity happens on the Swabian Alb. As a basis for our study, we use the earthquake catalog of the LED from 2011 to 2018 for earthquakes within the area close to the ASZ (8.5–9.5◦ E, 48–48.8◦ N, Fig. 1b). Our combined data set consists of 575 earthquakes (Fig. 1b) with 4521 direct P-phase and 4567 direct S-phase travel time picks from 69 seismic stations
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