Spatial Influence of Fault-Related Stress Perturbations in Northern Switzerland

Velagala, Lalit Sai Aditya Reddy; Heidbach, Oliver; Ziegler, Moritz; Reiter, Karsten; Rajabi, Mojtaba; Henk, Andreas; Giger, Silvio B.; Hergert, Tobias

doi:10.5194/egusphere-2025-4559

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https://doi.org/10.5194/egusphere-2025-4559

Preprints

25 Sep 2025

| 25 Sep 2025

Spatial Influence of Fault-Related Stress Perturbations in Northern Switzerland

Lalit Sai Aditya Reddy Velagala, Oliver Heidbach, Moritz Ziegler, Karsten Reiter, Mojtaba Rajabi, Andreas Henk, Silvio B. Giger, and Tobias Hergert

Abstract. The spatial influence of faults on the crustal stress field remains a topic of active debate. While it is well documented that faults often cause perturbations in the stress field at a meter scale, their lateral influence over greater distances, from a few hundred meters to several kilometers, remains poorly understood. This knowledge gap largely results from the lateral resolution limit of stress data. To address this, we use a 3D geomechanical numerical model based on 3D seismic data from northern Switzerland. The model is calibrated with 45 high-quality horizontal stress magnitude data obtained from micro-hydraulic fracturing (MHF) and sleeve re-opening (SR) tests conducted in two boreholes in the Zürich Nordost (ZNO) siting region. The 3D seismic and stress data were collected as a part of site characterization for a potential Deep Geological Repository (DGR) for radioactive waste. This 3D geomechanical numerical model serves as the reference model in our study and includes seven faults, implemented as contact surfaces with Coulomb friction. It is then systematically compared to three fault agnostic models i.e., models without any implemented faults. These fault agnostic models use identical rock properties and model input parameters, are calibrated with the same 45 horizontal stress magnitude dataset and have the same model extent, but differ in their discretization and mechanical properties’ assignment procedure. The results show that at distances of < 1 km from faults, differences in maximum horizontal stress orientation between models range from 3°–6°, and horizontal stress magnitude differences are about 1–2 MPa. Beyond 1 km distance, the differences reduce to < 1.5° and < 0.5 MPa, respectively. These stress differences are far smaller than the uncertainties associated with the horizontal stress magnitude measurements at the ZNO siting region, which average to ±0.7 MPa for the minimum horizontal stress magnitude and ±3.5 MPa for the maximum horizontal stress magnitude. An important implication of this lateral quantification of fault influence on stress state is that explicit representation of faults may not be necessary in geomechanical models predicting the stress state of rock volumes located a kilometer or more from major active faults, an important prerequisite for any DGR campaign. This structural simplification allows for faster model set-up and discretization, leading to a significant reduction in the set-up phase and computational time by more than one order, without compromising the reliability of stress field predictions.

Received: 16 Sep 2025 – Discussion started: 25 Sep 2025

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Lalit Sai Aditya Reddy Velagala, Oliver Heidbach, Moritz Ziegler, Karsten Reiter, Mojtaba Rajabi, Andreas Henk, Silvio B. Giger, and Tobias Hergert

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RC1:
'Comment on egusphere-2025-4559', Anonymous Referee #1, 27 Oct 2025
Dear Dr. Beaudoin,
I have completed my review of “Spatial Influence of Fault-Related Stress Perturbations in Northern Switzerland” by L.S.A.R. Velagala and co-authors. Overall, I found this to be a well-written manuscript on a topic that is highly appropriate for inclusion in Solid Earth. As such, I recommend that it be accepted for publication after minor revision.
Here are several items that I think – once addressed by the authors – will further improve the manuscript:
With regards to the initial conditions of the numerical models, it is unclear if the maximum principal stress is vertical, signifying a “normal faulting stress state” because the text only discusses the magnitudes of the horizontal stresses. Adding the vertical stress curves to Figure 5 would be a simple way to provide clarification. Also, I think it would be helpful if the authors clearly indicated whether the stress magnitudes represent total or effective stresses.

Although the authors provide some text to address their choice to employ a linear elastic constitutive relationship (section 2.2.1), I think this area could benefit from additional discussion. While I agree with their assertion that this modeling assumption helps to simplify the analysis, many geomechanical modeling studies have shown that stress state evolution is impacted by inelastic deformations that occur when elastic limits are reached. As such, I think the authors could strengthen their arguments by including some text to demonstrate that their loading conditions lead to stress conditions that remain in the elastic domain.

The analysis of horizontal stresses (Figures 6-9) could be improved if the authors provided a few enlargements of key figures. For example, enlargements of the top portion of Figure 6 that illustrated the differential stress near the faults. And while the cross-sectional views are useful, I think that one (or more) horizontal (depth) slices would help the readers better appreciate the spatial variability in horizontal stress orientation and magnitude.

While the authors discuss their approach to representing the faults with contact interfaces and provide an analysis of the impact of different friction coefficients on horizontal stress magnitudes and orientations (Figure 10), it is unclear if actual slip occurs on any of the faults, and if so, what is the magnitude of slip. A brief discussion would be helpful for the readers.

Lastly, I think the manuscript’s overall impact on the geomechanical interpretation of subsurface stress states and faulting would be enhanced if the authors could postulate how their results might differ if the scenario involved a different initial stress regime (i.e., strike-slip or thrusting faulting).
Citation: https://doi.org/10.5194/egusphere-2025-4559-RC1
- AC1:
  'Reply on RC1', Lalit Sai Aditya Reddy Velagala, 27 Nov 2025
  We would like to thank the reviewer for their careful evaluation of the manuscript and for the constructive comments provided. In the following text, we present a draft on how we propose to address the comments of the reviewer individually in the revised manuscript.
  Answer to Comment 1:
  In our geomechanical numerical models, the initial stress state is established by applying gravitational loading (producing the vertical stress, S_v) together with the lateral boundary conditions representing the regional tectonic stresses (producing additional horizontal stresses, S_Hmaxand S_hmin). In the initial conditions, we do not make any assumption that S_v is the maximum principal stress (S₁). Instead, the three principal stresses (S₁, S₂, S₃) develop naturally from the applied loading and the lateral boundary conditions.
  To further clarify, we provide a plot (attached below) showing the depth profiles of the S₁, S₂, S₃ together with Sᵥ along the Trüllikon borehole. The left panel displays the magnitudes of S₁, S₂, S₃ and Sᵥ as a function of depth, showing that Sᵥ is not always identical to the S₁. Sᵥ more closely corresponds to one of the principal stresses depending on the local stress regime, plotted as Regime Stress Ratio in the right panel [Simpson, 1997]. While not clearly visible in the plot, there is a very minute difference in between the magnitudes of the principal stresses and the vertical stress. We did not originally include vertical stress curves in Figure 5 because the differences between the model scenarios were small and only noticeable in the shallowest ~100 m. However, we agree that including them will improve clarity, and we will add the vertical stress profiles to the revised figure. We also clarify that all stresses presented in the manuscript refer to total stresses, as pore pressure effects were not considered in this study. We will state this explicitly in the revised manuscript.
  Answer to Comment 2:
  The Mesozoic sedimentary units in the Northern Switzerland can be regarded as tectonically stable with low strain rates and are mechanically intact. This makes linear elastic constitutive relationship an appropriate first-order approximation for modeling the present-day stress state. Several geological factors, specific to the region and the siting area, support this assumption:
  No significant tectonic deformation since Miocene with extremely low strain rates of ~1-3 m/Myr/Km,
  
  Lack of active faulting at repository depths and no evidence of any quaternary faults’ reactivation, and
  
  The differential stresses; S1-S3 of the units range between 0.5 MPa and 13 MPa), far below their measured uniaxial compressive strength limits (33-180 MPa) [Nagra, 2024]. Therefore, the units are far below their peak strength so they are safely within an elastic domain.
  
  Due to these reasons, plastic yielding or damage is not expected under current stresses. Since the primary objective in this paper was to assess the far-field, first-order effects of fault-related stress perturbations at a safe distance (> one km) from faults, the assumption of linear elasticity is appropriate. We briefly mentioned about these region-specific details in the Section 5.4: ‘Limitations of the study’s results and future outlook’. However, the reviewer is correct that we could have explained this better and explicitly, and we will address this in the modified draft.
  Answer to Comment 3:
  We agree that horizontal (depth) slices would offer a complementary perspective on the lateral spatial variability of the stress magnitudes and orientation. Therefore, we will include atleast one or two Figures from each model realization containing horizontal depth slices along the target storage zone, and a stiffer and mechanically contrasting unit above or below it.
  While enlarging the regions near the faults from Figures 6–9 is technically possible, we found that such close-up views do not result in providing additional interpretive value to the reader. As discussed in Section 4.2: “2D results along a cross-section”, the high localized stress concentrations adjacent to the faults are primarily numerical artefacts arising from element resolution limitations in particular near the fault tips. To show this, we provide a zoomed example of the area between Rheinau and D2 faults (below figure) from the top inset of Figure 7, originally showing Δ (S_Hmax-S_hmin) between the REF-NF and REF model. The enlarged view does not reveal information beyond what is already visible in the full image. For this reason, we propose to retain the original cross-section figures while adding the additional horizontal slices.
  Answer to Comment 4:
  The Neuhausen Fault is the only known active fault in our study area and is a tertiary-origin fault. In our numerical models, minor amounts of slip in the order of a few 10s of cm occur on this fault after the application of the lateral displacement boundary conditions (shortening of 4.1 m in the N–S direction and 0.82 m in the E–W direction to achieve the best-fit w.r.t. the stress magnitude data). This slip produces shear stress decrease along the fault that is small compared to the much larger background in situ stresses and the differential stress S1-S3. We will add a brief discussion to clarify that minor slip occurs but does not influence the overall stress field analysis.
  Answer to Comment 5:
  For low strain intraplate regions, we think that our findings are general and independent of the stress regime, but we cannot prove this with our model as it resembles a specific geological setting. Nevertheless, the differential stress S1-S3 is generally increasing with depth in the upper brittle crust (regardless of the stress regime) whereas the slip on the intraplate faults does not. Assuming that the slip-rate and thus, the stress changes along the faults are in the same order for different faulting styles, their impact on the in-situ stress is local and confined to the near-field of the fault. We will add our interpretation to the discussion in more detail.
  In the end, we would like to thank the reviewer once again, as well as all others involved in the discussion phase, for taking the time to provide their valuable comments and suggestions. We are sincerely grateful.
  References:
  Simpson, R. W.: Quantifying Anderson's fault types, Journal of Geophysical Research: Solid Earth, 102, 17909-17919, https://doi.org/10.1029/97JB01274, 1997.
  Nagra: In-Situ Stress Field in the Siting Regions Jura Ost, Nördlich Lägern and Zürich Nordost, NAGRA, Wettingen, Switzerland, NAGRA Arbeitbericht NAB 24-19, 131 pp., 2024.
  Citation of our article:
  Velagala, L. S. A. R., Heidbach, O., Ziegler, M., Reiter, K., Rajabi, M., Henk, A., Giger, S. B., and Hergert, T.: Spatial Influence of Fault-Related Stress Perturbations in Northern Switzerland, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-4559, 2025.
  
  Citation: https://doi.org/10.5194/egusphere-2025-4559-AC1
RC2:
'Comment on egusphere-2025-4559', Christophe Pascal, 02 Dec 2025
Dear authors, dear Nicolas,
Please find below my review of the submitted MS “Spatial Influence of Fault Related Stress Perturbations in Northern Switzerland” by Velagala et al.
The MS presents the results of a 3D numerical modelling study targeting a restricted area north of Zurich, Switzerland. The authors employed a classical FE method with and without contact elements to simulate faults, the aim of the modelling being the simulation of potential stress perturbations, caused by the latter pre-existing discontinuities, and their quantification. It is concluded that the impact of the faults on stress orientations and magnitudes is negligible.

From the formal point of view, the text still contains typos, grammar mistakes and reads very often rather awkward (see some examples listed in “Minor comments” below). It is readable but needs to be seriously improved, which is somewhat puzzling, considering that it is stated in the paper that all the eight authors contributed to the writing. Also, some of the scientific/technical terms of the Ms are improperly used or, at least, ambiguous (e.g. “flexure”, “stiffness”, see comments below). In turn, the figures are informative and of excellent quality. I have, however some minor suggestions to improve them.
From the scientific point of view, the modelling study is a very classical one but it appears robust and its conclusions are sound. I would, however, raise some criticisms that I hope will help the authors to improve the Ms.
First of all, I would moderate the “universality” of the results of the study. For example, stress and structural data gathered in the vicinity of the San Andreas fault (Zoback et al. 1987) demonstrate that stress perturbations can be far more significant than what is advanced in the present Ms. The latter conclusion from Zoback et al. (1987) finds support in e.g. the paleostress reconstructions of Homberg et al. (1987), which evidence stress rotations of up to 20° near the Morez Fault Zone in the French Jura, the numerical modelling of Pascal and Gabrielsen (2001), later on benchmarked by the in-situ stress measurements of Myrvang and Roberts (2004), and the field observations and geomechanical modelling of Maerten et al. (2016). In brief, the results advanced in the present Ms are representative of the modelled area and cannot be safely extrapolated to other regions.

Pascal, C. & Gabrielsen, R.H. 2001: Numerical modelling of Cenozoic stress patterns in the Mid Norwegian Margin and the northern North Sea. Tectonics 20, 585-599.

Roberts, D. & Myrvang, A. 2004: Contemporary stress orientation features and horizontal stress in bedrock, Trøndelag, central Norway. Norges geologiske undersøkelse Bulletin 442, 53-63.

Zoback, M. D., M. Zoback, V. Mount, J. Suppe, J. Eaton, J. Healy, D. Oppenheimer, P. Reasonberg, L. Jones, C. Raleigh, I. Wong, 0. Scotti, C. Wentworth, New evidence on the state of stress of the San Andreas fault system, Science. 238. 1105-1111, 1987.

The origin of the data used to calibrate the models and summarised in Table 1 is enigmatic. Please add the apparently missing references and explain briefly how these data were obtained. Also, important information is missing in the text about the model set-up (e.g. dimensions, type of elements used to mesh the models).

The modelling considers faults as contacts, whereas natural faults are complex zones of deformation, where rock rheology can be drastically different from that of the “intact” host rock and, thus, fault zones may promote pronounced stress perturbations. A thorough discussion on the latter is crucially missing in the paper. Also, the models do not involve fault tips, whereas it is well-known that they host the most dramatic stress perturbations (e.g. Homberg et al. 1997). Once again, the latter point should be addressed in the discussion section.

Taking into account the above comments, I recommend major modifications. I wonder, however, if the topic of the Ms, which is very conventional (i.e. this kind of modelling study is routine work in industry) and brings rather limited scientific added value, suits well the journal but it is up to the editors to decide.

Kind regards
Christophe Pascal

Major comments

L24: “Coulomb friction”. The term does not apply to contact surfaces as Coulomb friction (i.e. internal friction) is a property of the continuous medium that controls rupture (i.e. creation of shear fracture not sliding along a pre-existing one). I recommend to use “Byerlee friction” or “Amontons friction”.

L194: “flexures”. The term “flexure” is traditionally used to indicate a mechanical process and not a geological structure. Please explain what do you mean by “flexure” (fold hinge? Drape fold?). Also, it is unclear to me how the “flexures” impact the stress field (e.g. if they are open folds I do not expect much of it). Please, justify their inclusion into the model.

L199-200: “Neuhausen is the only fault that displays a stratigraphic offset”. The statement is very puzzling, if the other faults do not display any offset then… they are not faults. Please, clarify in text.
L241-242: “The mechanical properties E [GPa], ν [-], and ρ [kg/m³], used in the models are derived from core tests and petrophysical logs obtained from the TRU1-1 and MAR1-1 boreholes.” Please add references and indicate briefly what kind of “core tests” have been conducted.

Table 1: Please indicate also lithologies, formation names are not informative enough for the common reader.

“3.1 Model discretization Strategies”. Please, explain briefly in introduction the rationale of the modelling and why different models are tested. As it is now, the text is rather confusing and does not allow the reader to grasp right away the adopted modelling strategy and its goals.

L340 (and elsewhere in the Ms): “stiffness”. The word “stiffness” has a precise meaning in continuum mechanics and involves typical dimensions of the object under consideration. Obviously, the term is erroneously used in the Ms. I presume that one has to read “Young’s modulus” instead. Please correct.

L397: “resulting in lower differential stresses”. Rigorously speaking: “resulting in lower stress magnitudes” and potentially in lower differential stresses.

L472: “Stress tensor components”. Stress orientation is not considered to be a “stress component” traditionally. Please correct.

“5.2 Impact of varying fault friction coefficient of the implemented faults”. This section reads like a presentation of some of the results and not like a discussion. Please consider moving it to section 4.

L518: “possible values of friction coefficient in Switzerland”. Is there a particular reason to believe that physics operate otherwise in Switzerland compared to the rest of the universe? I presume that friction coefficients are there similar to the ones measured elsewhere on the planet. Please, rephrase.

Minor comments

L98: “Fig. 1D shows”. Should be called after Fig. 1C (has not been called before).

L132: “these three scales”. Please be specific. What scales do you mean?

L133 (and somewhere else in the text): “reduced stress tensor”. Please define, there is no unique form for it.

Fig. 2: please, in order to help the reader indicate the cross section presented in Fig. 3.

L189: “the displacement applied” should read “the applied displacement”.

L213: “two key simplifying assumptions”. I presume one should read “two simplifying key assumptions”.

L261 “Fig. 5”. Please call Fig. 4 before calling Fig. 5.

L271: “vertical displacement is constrained”. I find the expression rather ambiguous (i.e. it could be constrained to e.g. 5 mm/yr or to accelerate progressively or…). I presume you mean something like “zero vertical displacement is prescribed”.

Fig. 4: please add scale.

L300: “faster by approximately an order”. Please be specific and add numbers.

Table 2: what is the reason for (slightly) changing boundary conditions from one model to the other?

L336-337: “vertical red line changing with depth”. Unclear, please rewrite.

Fig. 6: the figure is redundant as Fig. 7 provides already all needed information (same comment concerning Fig. 8, Fig. 9 is sufficient). Please consider removing Figs. 6 and 8.

The text contains other minor defaults but I think it is the privilege of the authors to fix them. The text is 20 pages long and there are 8 authors, thus, each author will have to take care of 2.5 pages. No risk of burnout…
Citation: https://doi.org/10.5194/egusphere-2025-4559-RC2

Lalit Sai Aditya Reddy Velagala, Oliver Heidbach, Moritz Ziegler, Karsten Reiter, Mojtaba Rajabi, Andreas Henk, Silvio B. Giger, and Tobias Hergert

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Short summary

We assess the fault impact on the stress field in northern Switzerland using 3D geomechanical models, calibrated with stress data. We see that faults affect the stresses only locally, with negligible impact beyond 1 km, suggesting that faults may not be necessary in reservoir-scale models predicting stresses of undisturbed rock volumes, such as for a deep geological repository. Omitting them can substantially reduce modelling time and computational cost without compromising prediction accuracy.


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