The analysis of slip tendency of major tectonic faults in Germany

. Seismic hazard during subsurface operations is often related to the reactivation of pre-existing tectonic faults. The analysis of the slip tendency, i.e. the ratio of shear to normal stress acting on the fault plane, allows an assessment of the 10 reactivation potential of faults. We use the total stresses that result from a large-scale 3D geomechanical-numerical model of Germany and adjacent areas to calculate the slip tendency for three 3D fault geometry sets with increasing complexity. This allows to draw general conclusions about the influence of the fault geometry on the reactivation potential. In general, the fault reactivation potential is higher in Germany for faults that strike NW-SE and NNE-SSW. Due to the prevailing normal stress regime in the geomechanical-numerical model results, faults dipping at an angle of about 60° generally 15 show higher slip tendencies in comparison to steeper or shallower dipping faults. Faults implemented with a straight geometry show higher slip tendencies than those represented with a more complex, uneven geometry. Pore pressure has been assumed as hydrostatic and has shown to have a major influence on the calculated slip tendencies. Compared to slip tendency values calculated without pore pressure, the consideration of pore pressure leads to an increase of slip tendency of up to 50 %. The qualitative comparison of the slip tendency with the occurrence of seismic events with moment magnitudes M w > 3.5 shows 20 areas with an overall good spatial correlation between elevated slip tendencies and seismic activity but also highlights areas where more detailed and diverse fault sets would be beneficial. an overall good spatial correlation between areas of elevated slip tendencies and seismic activity for one of the investigated fault sets


Introduction
Seismic activity is a crucial aspect for many subsurface constructions and activities such as the production of oil and gas, coal 25 mining, geothermal energy production, the storage of gas or the construction and safve long term operation of a nuclear waste repository. The occurrence of seismic activity is closely linked to the presence of pre-existing tectonic faults and their reactivation (Sibson, 1985). To estimate the potential to trigger seismic events, knowledge about the reactivation potential of tectonic faults is essential (Moeck et al., 2009;Worum et al., 2004). Slip on a fault occurs when the resolved shear stress τ is larger than the frictional resistance τf (Sibson, 1974;Jaeger et al., 2011): 30 where C is the fault cohesion, μ is the coefficient of static friction and σneff the effective normal stress on the fault. The relevant parameters for the assessment of the fault reactivation potential are therefore: 1) The stress tensor to estimate τ and the absolute normal stress σn; 2) The pore pressure required for the calculation of σneff; 3) The fault orientation that influences the magnitudes of σn and τ; 4) The frictional fault properties C and μ that describe the fault's materialbehavior. 35 The stress tensor in previous works has mainly been estimated utilizing stress inversion (McFarland et al., 2012;Yukutake et al., 2015;Ferrill et al., 2020), point-wise stress data from field observations (Neves et al., 2009;Lee and Chang, 2009;Moeck et al., 2009;Morris et al., 2021) or using Monte Carlo Simulation (Healy and Hicks, 2022) for 2D lineaments and in some cases 3D fault geometries. Worum et al. (2004) calculated the 3D stress tensor with an analytical model and used it for the estimation of the fault reactivation potential of 3D faults of the Roer Graben. Stress tensor estimates from 3D geomechanical-40 numerical models have been used to determine fault reactivation potential on regional scales, e.g. for the Upper Rhine Graben (Peters, 2007) or the Val d'Agri (Italy) (Vadacca et al., 2021), but this has not been achieved for entire all of Germany. In this study, we focus on the whole of Germany.
Here, we use the first 3D geomechanical-numerical model of Germany by Ahlers et al. (2021b) that provides an estimate of the 3D stress tensor that is variable with depth and lateral extent (Cornet and Röckel, 2012) due to inhomogeneous density and 45 elastic rock properties. Furthermore, we compile three sets of 3D fault geometries with increasing complexity and use the stress tensor from the Germany model to predict the fault reactivation potential. The fault sets can be used not only to derive a first order estimation of the fault reactivation potential, but also to highlight the effect of fault geometry on the fault reactivation potential. We also investigate the impact of hydrostatic pore pressure as well as assumed overpressure on the reactivation potential estimates and compare our results with the spatial distribution of seismic events with moment magnitudes 50 Seismicity is mainly observed in the Rhine area, the Swabian Jura and Eastern Thuringia as well as Western Saxony (German Research Centre For Geosciences). Induced seismicity has mainly been documented in the context of gas production , geothermal energy production (Bönnemann et al., 2010;Stober and Bucher, 2020) and especially mining 65 activities, which caused induced seismic events with local magnitudes of up to 5.6 (Grünthal and Minkley, 2005). Poro-elastic stress changes should be considered for significant pore pressure changes, as shown for production induced earthquakes . In the case of geothermal sites, fluid injections into the sedimentary rocks have been suggested to not be as seismogenic as injections into crystalline rocks. In general, the presence of faults close to the injection well as fluid pathways increases the risk of seismic events (Evans et al., 2012) 70 2 2.12.2 3D Stress State Stress data are not evenly distributed throughout Germany ( Fig. 1(b)) and vary between different regions of Germany both in terms of orientation and the stress magnitudes, thus the stress regime. For the North German Basin, Röckel and Lempp (2003) describe a normal faulting regime and mostly N-S striking SHmax orientations (SHazi) with an NNW-SSE influence towards the 75 Dutch border and an NNE-SSW influence towards Poland. For the Upper Rhine Graben (URG) area in southwest Germany, Homuth et al. (2014) calculate a transtensional regime with a strong strike-slip influence with SHazi around 135°, while modeling results of Buchmann and Connolly (2007) suggest a present day strike-slip reactivation of the URG. For the Molasse Basin in South Germany, (SHazi) rotates from striking N-S in southeast Germany to NNW-SSE striking in the southwest (Reinecker et al., 2010) and the stress regime most likely varies between normal faulting and strike slip (Drews et al., 2019;80 Seithel et al., 2015) Since these stress data are available only pointwise, we use the stress tensor derived from the 3D geomechanical-numerical model of Germany by Ahlers et al. (2021a) for the assessment of the fault reactivation potential. The model covers Germany and adjacent areas and provides a continuum mechanics based prediction of the stress tensor. The purely elastic finite element (FE) model comprises seven mechanical units, i.e. sediments, four upper crustal units, the lower crust and parts of the 85 lithospheric mantle. The four crustal units represent the crustal framework of Germany as shown in Fig. 1 (a) and the Alps-Carpathian-Pannonia. The lateral grid resolution is 6 x 6 km² and the vertical resolution decreases from 800 m within the sediments to 7500 m at the model base. Each unit is characterized by its respective density, Young's modulus and Poisson's ratio (Ahlers et al., 2021a). The stress tensor used for assessment of the fault reactivation potential is derived from the 3D geomechanical-numerical model of Germany of Ahlers et al. (2021a) that covers Germany and adjacent areas and provides a 90 continuum mechanics based prediction of the stress tensor. The purely elastic finite element (FE) model comprises seven mechanical units (sediments, four upper crustal units, the lower crust and parts of the lithospheric mantle) with a lateral grid resolution of 6 x 6 km² and a vertical resolution decreasing from 800 m within the sediments to 7500 m at the model base.
Each unit is characterized by its respective density, Young's modulus and Poisson's ratio (Ahlers et al., 2021a).
The model is calibrated with stress magnitude data from the magnitude database by  and compared 95 with stress orientations from the World Stress Map database (Heidbach et al. 2016).; both data sets are shown in Fig. 1 (b). The 4 resulting best-fit model provides the 3D absolute stress tensor σij within the model domain (Ahlers et al., 2021a), i.e. for Germany and adjacent areas. In order to consider effective stresses, we assume a hydrostatic pore pressure. Even though overpressure is well documented for the Molasse Basin (Drews et al., 2018;Müller et al., 1988), there is not enough spatial information on pore pressure available to justify the usage of different pore pressure gradients in our analysis. 100 Fig. 1 (c) and (d) show the stress regime in the Germany model and SHazi in 1 km and 8 km depth respectively. In the uppermost km of the model, thrust faulting (TF) and strike-slip (SS) regimes are present. Below 1 km depth, the model is dominated by SS regime with some areas showing normal faulting (NF) regimes. With increasing depth, the NF regime becomes increasingly dominant as can be seen in Fig. 1 (d). In contrast, the stress orientations are almost constant with depth but change noticeably laterally. While SHazi is almost purely N-S in the northeastern part of the model, the orientation switches more towards a NNE-105 SSW orientation in the western part of the model. Additionally, the figure shows fault reactivation stereo plots for five regions in Germany. The plots are based on data provided by the model at the respective locations and illustrate the reactivation potential of faults striking between 0° and 360° and dipping between 0° and 90° represented by their normal vectors. They indicate high reactivation potentials in the upper 1 km of the model in south Germany for shallow to moderately dipping and NNE-SSW to SSE-NNW striking faults. The reactivation potential for faults in north Germany is noticeably lower. In 8 km 110 depth, the reactivation potential is predicted as relatively low for all areas and fault orientations. The highest reactivation potential in this depth is predicted for moderately dipping faults striking roughly in NE-SW direction. A spatially comprehensive collection of 2D fault lineaments in Germany has been compiled by Schulz et al. (2013). 3D fault 130 geometries are available on a regional scale for some regions in Germany, such as the North German Basin (Bundesanstalt für Geowissenschaften und Rohstoffe, 2021), the Molasse Basin (GeoMol Team, 2015) in South Germany or in the model of Saxony (Geißler et al., 2014). However, there are no comprehensive 3D fault geometry compilations available for Germany.
We created a total of three fault sets of increasing complexity. The first fault set is based the 2D fault collection by Schulz et al. (2013) that comprises the 2D lineaments of 900 faults in Germany. The faults used in the second fault set have been chosen 135 according to selection criteria. The selection criteria comprise the length of the fault (≥ 250 km), the horizontal displacement (≥ 10 km), the vertical displacement (≥ 2.5 km) and the seismic activity of the fault (since 800 CE or later). Furthermore, the general special pattern of fault orientations should be reproduced. In areas, where no faults met the criteria, we selected some additional faults to reproduce the general spatial distribution of faults. The application of the selection criteriaThis approach letad to a final compilation of 55 faults.
For these faults the tectonic regime, namely strike-slip, normal faulting or thrust faulting was known from a data collection of 140 (Suchi et al., 2014;Agemar et al., 2016) or respective literature. For the third fault set, we used geological and seismic cross sections in the depth domain to compile data on the 3D geometry of the selected faults. For 23 faults, cross sections with sufficient vertical extent were available. Based on the three described fault sets we generated three different 3D geometry sets of increasing complexity for slip tendency calculation: 1. Vertical fault set: All 900 faults of the fault catalogue (Agemar et al., 2016) were implemented as 90° dipping faults 145 extending to the base of the lower crust. The assumption of a vertical dip is an oversimplification due to the lack of data on most faults and introduces significant errors to the calculated reactivation potentials of faults that dip differently in reality. However, it allows the consideration of a large quantity of faults and therefore a more diverse representation in terms of location and strike than the other two sets with more realistic dips. 2. Andersonian fault set: The 55 selected faults have been implemented depending on their Andersonian fault type as 150 normal faults, thrust faults or strike-slip faults. For normal faults a dip angle of 60° was assigned, for thrust faults of 30° and for strike-slip of 90°. The faults reach the base of the lower crust. The supplementary Table S1 lists the implemented faults with a corresponding ID. 3. Semi-realistic fault set: For 23 faults, a more complex geometry on the basis of seismic and geological cross sections is used. The depth of the faults is not constant as in the Vertical and Andersonian fault sets, but is chosen in accordance 155 with the depths given in the sections used. The vertical cross sections used for the generation of the semi-realistic fault set are compiled in Table 1Table 1. The quantity of available cross sections per fault varied considerably. For many faults, only one cross section was available leading to a uniform geometry over the entire length of the fault. Roer Graben Duin et al., 2006, Geluk et al., 1994 Siegen Thrust Franke et al., 1990 Swabian Lineament Pfiffner, 2017 Teisseyre-Tornquist Zone Narkiewicz et al., 2015 Upper Rhine Graben Brun et al., 1992, GeORG-Projektteam, 2013 Wittenberg Fault Reinhold, 2005 2.4 3D Slip tendency analysis To estimate the fault reactivation potential we use definitions and terms of Morris et al. (1996). Assuming that cohesion can be neglected, they defined the parameter slip tendency as the ratio between and . We use this definition as a first slip tendency type: 165 = (2)

Formatiert: Deutsch (Deutschland)
We further define three additional slip tendency parameters for our analysis. TSeff considers σneff stress that takes the influence of pore pressure on σn (Jaeger et al., 2011) into account.

= (3)
A normalization to μ has been used for example by Peters (2007) and is additionally calculated as and . We 170 choose μ as 0.57 which is in the middle of the range reported by Jaeger et al. (2011). For and slip is likely to occur if they approach values around 1 or larger.
The pore pressure Pp for the calculation of is computed from the depth z [m] (which is the true vertical depth below the 175 topographic surface of the German stress model), gravity g [9.81 m s-²] and the fluid density  [1000 kg m -³]: To estimate the slip tendencies, the fault geometries are discretized as surfaces with triangles with a side length of 800 m. Then the 3D stress tensor components from the geomechanical-numerical model of Ahlers et al. (2021b) are mapped on the corner nodes of the triangles using Tecplot 360 EX v2019 and the AddOn Geostress . The mean stress tensor 180 of the three nodes is multiplied with the normal vector of each triangle to estimate τ and σn. With the TVD and the derived hydrostatic pore pressure the four slip tendency parameters are calculated.

Vertical fault set
The results for the Vertical fault set are shown for all four slip tendency parameters in  190 TSeff is higher than TS and ranges mainly between 0 and 0.7. TSeff is highest in the uppermost fault parts and decreases rapidly with increasing depth as well. NW-SE and especially NNE-SSW striking faults show higher TSeff than faults of other strike.
TSnorm values mainly range between 0 and 0.7 and TSnormeff ranges mostly between 0 and 1. The same trends for depth and fault strike apply as for TS and TSeff. TSnorm and TSnormeff are however higher in the uppermost parts of the faults than TSeff.    Overall, the minimum TSeff values occur consistently at strikes of 75° for all fault types i.e. the reactivation potential is generally the lowest for ENE-WSW striking faults, as could be expected in the context of the stress orientation shown in Fig. 1 (c) and (d). Vertical faults also show a low reactivation potential on NNW-SSE striking segments (corresponding to strikes of 165°).

Semi-Realistic Fault Set
The maximum TSeff occurs for strikes of 5°-25° for all fault types i.e. the reactivation potential is generally highest for N-S to 265 NNE-SSW striking faults; these faults strike at an angle of 25° to SHazi with an orientation between 160° and 175°. The vertical faults also have a high reactivation potential for NW-SE strikes, the Andersonian normal faults for NNW-SSE striking segments. Due to the uniform dip of the Vertical fault set, dip is not a variable of influence for this fault set and only the location in the stress field and the strike of the fault lead to differences in slip tendency.

Influence of Depth and shear stress on slip tendency 275
Four all three fault sets, a strong decrease in the slip tendencies can be observed from the surface to a depth of 5-10 km as is shown in Fig. 7Fig. 6 and Fig. 8Fig. 7 for the Vertical and Andersonian fault set, respectively. In greater depths, slip tendency gradient is low. This is the case for all four slip tendency types. For the Vertical fault set (Fig. 7Fig. 6), slip tendency decreases steadily for all four slip tendency types with the exception of a dent between 32 km and 38 km. However, since only very few fault segments reach this depth, the influence of fault strike strongly superimposes the depth dependency for these depths. For the Andersonian 280 fault set (Fig. 8Fig. 7), the same trends apply in general as for the Vertical fault set. However, for the thrust and normal faults the initial strong decrease in slip tendency occurs within the uppermost 3-4 km. In this depth, the stress regime switches from a strike-slip regime to a normal faulting regime in most parts of the model. The slip tendencies of the strike-slip faults are generally higher than the ones of the thrust and normal faults in the upper 5-10 km but generally lower in greater depths. In contrast to the strike-slip faults, both normal and thrust faults show a slight increase of the mean slip tendency with increasing 285 depth below 5 km depth. The mean slip tendency increase with depth is higher for the thrust faults than for the normal faults.

295
For normal, thrust and strike-slip faults σn increases at a similar rate with increasing depth. On the other hand, τ on strike-slip faults and the faults of the Vertical fault set increases less strongly. Since slip tendency has been defined as τ/σn, low τ leads to low slip tendencies for the strike-slip faults and the faults of the Vertical fault set. Fig. 9Fig. 8 shows τ for the Vertical fault set.
Additionally, σneff, τ and the resulting TSeff of the Landshut-Neuoetting Fault are shown exemplarily. While σneff increases to over 250 MPa, τ only increases to around 20 MPa at a depth of 30 km (note that the range of the color bar of σneff is 10 times 300 the range of the τ). This results in TSeff strongly decreasing with increasing depth for all faults regardless of their strike direction in the Vertical fault set. Influence of fault dip In order to investigate the influence of the 3D fault geometry, we compare the slip tendency histograms of the Vertical (blue), 310 Andersonian (orange) and Semi-Realistic (mint) fault set ( Fig. 10Fig. 9). For all four slip tendency types, the Vertical fault set shows a right skewed bell shape, the Semi-Realistic fault set displays as J-shape and the Andersonian fault set shows a bimodal distribution. The bimodal character of the Andersonian fault set is more distinct for TSeff and TSnormeff. The slip tendency values of the first peak are mainly concentrated on the thrust faults whereas the slip tendency values of the second peak are mainly present on normal faults. 315 As the normal faulting regime is predominant in most parts of the Germany model (especially in depths greater than 4 km) in general σn is lower for normal faults than for thrust faults, which have been implemented with a dip of 60° and 30° respectively in the Andersonian fault set, leading to the bimodal distribution of TS.
The more prominent bimodal distribution of TSeff and TSnormeff in the Andersonian fault set results from the influence of the calculation of the pore pressure as a function of depth. In combination with the normal faulting regime in most parts of the 320 Germany model. This leads to a stronger relative reduction of σneff of normal faults than for thrust faults. The listric geometry of the URG in the Semi-Realistic fault set is based on DEKORP 9N (Brun et al., 1992). The URG shows high TSeff values in the uppermost parts for both the Andersonian and the Semi-Realistic fault set. With increasing depth, the dip of the Semi-Realistic URG faults decreases until it becomes sub-horizontal. This decrease in dip coincides with a significant TSeff decrease. In contrast, TSeff for the Andersonian fault geometries decreases at a significantly lower rate. This results from 330 the fact that while σneff increases at a similar rate for both fault types, τ of the Semi-Realistic URG increases at a much lower rate than it does for the Andersonian URG (also shown in Fig. S5). Results from the Hunsrueck Southern Border Fault, another listric fault, (derived from DEKORP 9N and 1C, Henk, 1993) show a similar behavior.
The overall low slip tendency values of the vertical fault set were to be expected due to the prevailing normal faulting regime in most parts of the model and the uniform 90° dip of the Vertical fault set. The low values do not properly reflect the actual 335 fault reactivation potential of faults with different dips in reality. The reactivation potential for faults with other dips in reality is underestimated in areas with normal and thrust faulting regimes and overestimated in a strike-slip regime.

4.4
Influence of pore pressure The use of a hydrostatic pore pressure is a major simplification since the pore pressure is not hydrostatic everywhere in Germany. Considerable overpressures have been shown for example in the Molasse basin (Drews et al., 2018;Müller et al., 340 1988). Müller et al. (1988) describes pore pressure gradients of up to 24 MPa km -1 in the vicinity of the lineament of the Alpine thrust. Fig. 11Fig. 10 shows TSeff for the Alpine Thrust for pore pressure gradients of (a) 10 MPa km -1 (hydrostatic) (b) 16 MPa km -1 and (c) 22 MPa km -1 . TSeff increases drastically with increasing pore pressure. For the gradient of 16 MPa km -1 TSeff reaches values of up to 0.7 for favorably oriented segments of the fault. For a pore pressure gradient of 22 MPa km -1 TSeff increases to over 0.7 for almost all parts of the fault and reaches values well in excess of 1 over large areas. Even though these pore pressure 345 gradients are unlikely to occur over large areas of the fault, this highlights the crucial impact of the pore pressure on the fault reactivation potential. Comparison between Slip tendency and seismicity In order to evaluate our slip tendency results, we test them qualitatively against the distribution of tectonic earthquakes. The earthquakes are taken from the EMEC seismic event catalogue of Grünthal and Wahlström (2012) that covers the period 355 between 1000 to 2006 CE in the investigation area and provides earthquakes with magnitudes Mw ≥ 3.5. We added events to this compilation with Mw ≥ 3.5 for the years 2007-2021 from the GEOFON data centre at the GFZ German research Centre (Quinteros et al., 2021). For the events with a given hypocentral depth, the majority occur at 8 km (refer to Fig. S6) and the largest moment magnitudes are observed at 8 to 10 km depth. Therefore, we use the slip tendency values at a cross section at 8 km depth for the comparison with seismic events.the seismic event catalogue of Grünthal und Wahlström (2012)

that covers 360
Western Central Europe in the past 1000 years. We selected events with Mw ≥ 3.5 within the investigation area. Out of the selected 1600 seismic events, 1200 do not provide information about the hypocentral depth. For the 400 events with a given depth, the majority occurs at 8 km (refer to Fig. S6). The largest moment magnitudes are observed at 8 to 10 km depth.
Therefore, we assigned a depth of 8 km for the seismic events without depth data. This is not surprising when considering the results of fault detection using photolineations derived from high resolution data 375 of satellite missions such as ERS-1/2. E.g. Franzke and Wetzel (2001) present in their work for southern Germany that there are numerous additional fault networks on smaller scale that could potentially serve as faults for the catalogued seismicity with small magnitudes. However, if we would use only large events with Mw > 6 instead that have according to empirical relations rupture length of > 10 km (Wells and Coppersmith, 1994) these would fit better to our resolution, but in a low strain area these magnitudes do not occur very often and even the largest recorded event in Germany from year 1911 with Mw 5.8 in the Albstadt 380 Shear Zone would not be usable, but only the historical events where the epicenter estimation based on intensity reports is highly uncertain.
A cross section through the Andersonian fault set at a depth of 8 km is shown in Fig. 12 (b), the same color-codes as for Fig.   12 (a) apply. The occurrence of seismic events is in good accordance with the elevated TSeff of the URG, the Roer Graben, the Mariánské Lázne Fault and the Randen-Bonndorf Fault. However, especially in east Germany and in the SE trending 385 elongation of the Roer Graben there are areas without faults despite numerous seismic events. Furthermore, TSeff is rather low along the Albstadt Shear Zone, one of the seismically most active areas in Germany due to the implementation as a 90° dipping strike-slip fault. In contrast, there are also areas with TSeff in the same range as e.g. the URG with no or only very little seismicity, especially in northern Germany. Here, TSeff is either overestimated in comparison to seismically active areas or stress relief is achieved by other processes. 390 Andersonian fault set at a depth of 8 km displaying TSeff. The occurrence of seismic events is in good accordance with the elevated TSeff of the URG, the Roer Graben, the Albstadt Shear Zone, the Mariánské Lázne Fault and the Randen-Bonndorf Fault.
While the faults with the highsome of the seismogenic areas show estelevated TSeff values correlate well with the occurrence of seismic events, the absolute values are rather low, especially in 8 km depth and deeper, where most of the considered seismic 395 events take place. If μ is low enough, seismicity can still occur even with TSeff in the range below 0.4. The range of μ of faults can vary greatly and even reach values below 0.4 for faults with fault gouge (Numelin et al., 2007;Haines et al., 2014) as a compilation by Ferrill et al. (2017) shows. For higher μ, as they have been shown for different locations (Zoback and Healy, 1992;Zoback and Healy, 1985;Brudy et al., 1997) and collected by Peters (2007), TSeff would need to reach higher values in order to explain seismic events. This could be achieved through higher τ or lower σn. In order to achieve these either significant 400 changes regarding the stress tensor from the geomechanical model of Germany or changes in the fault geometry would be required. The changes to the stress tensor required would not be warranted by the calibration data used for the model of Germany. In order to elevate TSeff of the Franconian Line to values of 0.7 and higher, an additional 30 MPa of τ would be required in 8 km depth. The required stress changes however would not fit the data from the the Kontinentale Tiefbohrung (KTB) that has been used for the model calibration. On the other hand, the fault geometries are subject to major insecurities 405 due to the sparse data available on the geometries in greater depths. As shown above, both fault strike and fault dslip drastically impact the resulting slip tendency and the insecurities uncertainties regarding the 3D fault geometries could therefore at least partly explain the overall low values.
A comparison of the faults of the Andersonian fault set with 3 fault plane solutions (GEOFON Data Centre, 1993) ( Fig. 12 (d)) shows that one of the nodal planes' strike is (sub-) parallel to the URG, even though the corresponding dips are steeper at 410 around 75° than they were assumed for the URG in the Andersonian fault set. For the Alpine Thrust, one fault plane solution in the fault's proximity shows a parallel strike of 78° but a much steeper dip of 71° than the 30° dip that had been assumed for  A second major source of uncertainty results from the limited data available regarding the 3D fault geometries of the selected faults with sufficient depth extension (mostly >5 km). Only few seismic sections and geological cross sections could be used for the 3D fault geometry generation. Due to the sparse data available for most faults the 3D geometry has been deduced from only one section. The resulting geometries are therefore unlikely to properly represent the real 3D fault geometries (dip, strike, depth extent) over the entire fault lengths. As shown above, strike and dip have major influences on the resulting slip tendency. 450 Pore pressure data are too sparse to justify the discrimination of areas of distinct pore pressure gradients and thus only a hydrostatic pore pressure could be assumed for the estimation of TSeff and TSnormeff with the above mentioned effects. Since data on the frictional fault properties were not available, we assumed the faults to be cohesionless. If the considered faults have cohesion greater than 0, the resulting slip tendencies would be further reduced.

4.7
Comparison with earlier studies 455 Peters (2007) analyzed the slip tendency of the URG with the help of a numerical model. Using a dip of 60° for faults with a known dip direction and assuming hydrostatic pore pressure calculated slip tendencies reached values up to 0.8 in a depth of 2.5 km and with a normalization to a coefficient of friction of 0.4. Even though the slip tendency in this study has been normalized to a higher coefficient of friction of 0.6 the overall slip tendency values in this study are around 0.2 higher than the 460 results in Peters (2007). However, the URG boundary faults show elevated values for similar segments as the ones in the study by Peters (2007). The study of Worum et al. (2004) calculates TSeff values between 0.2 and 0.4 for the Roer Graben using an analytical model for faults reaching depths of around 2-3 km for comparable Shmin/SHmax-ratios and the strike-slip regime that is present in the Germany model at the before mentioned depths. TSeff of the Roer Graben boundary faults in this study ranges between 0.6 for the southernmost parts of the faults and 0.2 at the northern parts, which is in contrast to the trends displayed 465 in Worum et al. (2004) where the southern parts of the faults show the lowest TSeff values. However, the high TSeff values appear on short segments with ideal orientation for reactivation under the given strike-slip regime with SHazi around 155°.
Slightly less well oriented segments show values that are in better agreement with the results by Worum et al. (2004).

Main outcome & recommendations 470
The slip tendency analysis on basis of the 3D absolute stress tensor from the geomechanical-numerical model of Germany (Ahlers et al., 2021b) allowed the identification of regions with a higher reactivation potential and regions where faults are more stable. Elevated slip tendencies have been found especially for NNE-SSW and NW-SE striking faults such as the URG, the Franconian Line, the Albstadt Shear Zone, the Wittenberg Fault, the Rheinsberg Through, the Landshut-Neuoetting Fault and the Roer Graben. However, a comparison with focal mechanisms points towards reactivation of a more diverse set of faults 475 which should be subject to further studies.For the simple geometries of the Andersonian fault set, a good fit between areas of elevated slip tendencies and seismic activities could be achieved.
The major influence of fault geometry on the calculated slip tendency has been shown by the comparison of three fault sets.
High quality information on fault geometry can be provided for example by interpreted seismic sections for large scale faults.
To improve this kind of analysis, faults should be characterized by multiple seismic cross sections. The analysis also has shown 480 the crucial influence of the pore pressure on slip tendencies for the fault sets considered. However, no spatially comprehensive pore pressure data for the entire area of Germany are available. The same applies for the frictional properties of faults, which are only poorly restrained. Lastly, further and more information on the stress state in Germany is crucial for a more reliable slip tendency analysis. Evaluation and interpretation of the slip tendency results were done by LR with the support of BM, OH, KR, TH, SA and AH. 490 LR wrote this paper with the help of all coauthors. All authors read and approved the final paper.