the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 May 2025
Influence of crustal mechanical layering on the seismic potential of active faults: insights from the southwestern Valencia Trough (W Mediterranean)
Abstract. We present a structural and seismotectonic analysis of active faults in the southwestern Valencia Trough (western Mediterranean) on the basis of subsurface datasets. In our study, we identify and characterise three major active faults: the Cullera Fault, with a long-term slip rates that vary over time between 0.15 ± 0.1 mm/yr and 0.4 ± 0.1 mm/yr; the oblique Albufera Fault, with a long-term slip rate of 0.2 ± 0.1 mm/yr; and the normal Valencia Fault.
The seismogenic character of the southwestern Valencia Trough is controlled by a mechanically weak layer consisting of Triassic evaporites. This weak layer induces partial to complete decoupling between the suprasalt and subsalt successions, leading to two distinct mechanisms driving fault displacement: tectonic activity and salt withdrawal. A quantitative evolutionary analysis of the Cullera Fault reveals that these two mechanisms alternate over time.
The presence of a mechanically weak layer has implications for seismicity. Earthquakes can nucleate within both sub- and suprasalt successions, with total or partial decoupling influencing rupture propagation. We discuss how these two scenarios lead to different earthquakes and thus impact the seismic hazard of a region. Empirical source-scaling relationships, which are commonly used to estimate the seismogenic potential of active faults, generally assume a homogeneous seismogenic crust. To address this limitation, we propose a methodological approach based on the use of the aspect ratio. Our findings highlight the need to incorporate stratigraphic mechanical layering into seismic hazard assessments, particularly in salt-influenced tectonic settings.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1782', Raul Perez-Lopez, 06 Aug 2025
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RC2: 'Comment on egusphere-2025-1782', Anonymous Referee #2, 23 Sep 2025
The authors present a detailed model for stratigraphy, lithology and structure of the southwestern Valencia Trough, based on vintage and modern seismic reflection lines, well data, electric data, gravity data, and taking into account previous studies. The presented reflection lines appear well readable, and the interpretation appears clear or at least reasonable. A crucial feature of the model is a layer of evaporitic Upper Triassic (Keuper) rocks, that behaves mechanically weak and may induce decoupling of the Palaeozoic–Middle Triassic basement from the Upper Triassic –Quaternary units.
The authors identify three major faults in the study area that cut through the salt layer (Albufera, Valencia and Cullera Fault). They appear to be disturbed in their geometry by the Keuper unit in some instances (profiles in Fig. 6a and b), while in other profiles they show a more classic, continuously listric geometry of upper crustal normal faults. The authors estimate 3D fault geometries, slip rates and attempt to distinguish tectonic deformation from salt withdrawal.
So far, this study applies careful analysis and reasoning and is a solid and relevant contribution for understanding a very interesting setting. In my opinion, it is necessary to comment on the seismotectonic implications of the fault geometry. How can we explain the presence and possible interaction of perpendicular normal faults? Could the authors name and compare to other places on Earth where perpendicular normal faults are present? Are there hidden strike slip faults (not easy to identify in sections) necessary to reconcile the geometry (apart from a right-lateral component on the Albufera fault)? Are all faults active at the same time (in particular the Cullera fault is not consistent with available focal mechanisms, and according to throw-depth plots, activity of the Cullera fault has decreased significantly in the Quaternary)?
In contrast, I find the second part of the manuscript (chapters 7 and 8) problematic. Here, the authors shift from the general analysis to the specific focus of mechanical layering and seismic potentials. They distinguish rupture of the entire seismogenic crust or only the suprasalt layers. They hypothesise that the weak layer could limit the width of seismic ruptures, in which case the standard scaling relationships to relate fault length to magnitude no longer apply. This is a valid and interesting question, however in my opinion, some decisions taken by the authors are not justified and major revision is required in this part.
My first concern is the downdip width for suprasalt ruptures. The value of 7.1 km chosen by the authors allows for significant magnitudes to be obtained for suprasalt ruptures with reasonable aspect ratio, however the value should be much smaller according to the profiles shown in Fig. 6. The depth of the Keuper unit at the fault locations is between 1.5 and 3 km, and fault dip is 60 degrees or larger (not 45 degrees, as the authors assume), which rather suggests average downdip width of 3 km or less. Suprasalt ruptures could reach ~15 km^2 and magnitudes slightly above 5, which is interesting, but not relevant to define the seismic potential.
Furthermore, it is not clear if ruptures in the Valencian Trough do actually nucleate in the thin suprasalt layer. The comparison with the mechanical layering in Iran is not helpful here, as the tectonic regime is compressive and the fault geometry is different, the salt layer is significantly deeper, and yet the relevant earthquakes in the Zagros belt do nucleate in the subsalt layer, NOT in the suprasalt layer (i.e., they may be contained below the salt layer, or occasionally involving both, sub and suprasalt units).
Unfortunately, available depth estimates are insufficient to resolve this issue. Those are offshore earthquakes, so we have a lack of short distance observations, and the fact that the catalogue reaches back to 1950 (Fig. 2) does not make it any better… In consequence, the disclaimer in the manuscript “depths assigned to these earthquakes present high uncertainties” should lead us to acknowledge that there is no evidence for significant suprasalt earthquakes in the study area. If we decide to believe the depths in Fig. 2, it even appears as if the suprasalt layer was depleted of earthquakes.
More fundamentally, the presented sections show only limited evidence for mechanical and seismogenic decoupling between subsalt and suprasalt units. Among the shown profiles the Cullera fault in Fig. 6a, b displays a listric profile that may be flattening out in the Keuper, supporting the presence of such a mechanical detachment horizon. However, other faults cross the Keuper with more or less planar geometry, not suggesting any decoupling. In general, the salt horizon is discontinuous under some of the profiles and varies in thickness, and some reasoning or comparison to other settings should be provided to strengthen the mechanical model.
Minor comments:
Introduction: line 75 “these relationships do not consider the influence of potential heterogeneities within the seismogenic crust”: these relationships are empirical, which means they DO consider the real influence of all heterogeneities that were present around the seismic ruptures that were evaluated to build them. The point is rather that the mean regression we use averages out those heterogeneities…
Fig. 1: Could you put labels (dates) on the focal mechanisms?
Fig 2, depth histogram: Looks like depth 0 is excluded (which may mean explosions or fixed depth), please state explicitly. Y-axis annotations are not equidistant and difficult to associate, please check.
I’d suggest rename section 3, “Methods”, to “Data”, because that’s clearly the main focus here.
Fig. 3: Could it make sense to distinguish modern lines and vintage lines in the figure? (from your description in the text, I guess I can, but if you use different line style it would be easier).
On p. 20 you say the Cullera Fault is a planar fault (inside the basement) without significant tilting of the basement-top horizon, however this is in contradiction with the interpretation in figure 6e, where you draw a listric fault that would flatten out near 5 km and shows tilted units in the hanging wall.
In Section 5.3, you claim to “define for the first time the Valencia Fault”, but I wonder if the Valencia Fault could be equivalent to the fault labelled as Central-Western Cabo Cullera Fault in Fig. 1 / Qafi?
In chapters 6 and 7, some explanations are quite long and complicated and some of the information appears repeatedly. Please check writing of this part.
Citation: https://doi.org/10.5194/egusphere-2025-1782-RC2
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- 1
Dear Editor of NHESS and authors,
Here you can find my comments regarding the work entitled:I
Influence of crustal mechanical layering on the seismic potential of active faults: insights from the southwestern Valencia Trough (W Mediterranean)
By: Martin-Rojas, Iván; Ramos, Adriá; De Ruig, Menno; Medina-Cascales, Iván; Santamaría-Pérez, Eva; Alfaro, Pedro
Overall comments:
This manuscript presents an excellent study describing active faulting with seismogenic potential in the Valencia Trough, eastern Spain. The analysis of 2D seismic profiles is particularly valuable, given both the total profile length and the extensive dataset processed. Despite the limitations posed by the vintage seismic profiles, the authors achieve an impeccable interpretation using 2D Move software. In this regard, the work represents a significant step forward in the characterization of potential seismic sources that could affect the densely populated eastern coast of Spain. The application of 2D software for estimating fault areas in active tectonic settings offers a realistic approach to seismic hazard assessment, particularly in offshore regions where constraining the 3D geometry of faults remains challenging.
Some doubts arise regarding the methodology used to estimate the maximum earthquake magnitude based on fault lengths, as well as the consistency of the terminology used throughout the manuscript. I think that authors have to explain better the methodology and the “correction factor” to avoid misinterpretations. Please refer to my specific comments above for details.
Greater doubts are in the comparison between the Valencia Trough and the Zagros region. Please read above in main suggestions.
Main suggestions
S1-I suggest that the authors consider including the phrase “salt-influenced tectonic setting” in the title of the manuscript. This represents the main novel contribution of the work, along with the parameterization of the three active faults in the Valencia Trough.
S2-Section 4. Stratigraphy. I recommend reducing this section, as it does not represent the main objective of the work. Providing such a detailed stratigraphic description may distract the reader from the key focus of the study and could make the narrative less clear. A more concise summary would keep the manuscript streamlined and focused.
S3-Some figures and graphics are difficult to read, and I have included some recommendations into the attached pdf file. For instance, letters with a grey color, small numbers in the isopach lines, and similar colors I plots for the SR long-term estimation.
S4- I have some major concerns regarding the comparison between the Valencia Trough and the Zagros Thrust and Belt based solely on the presence of a “mechanically weak layer.” This comparison overlooks several tectonically significant parameters, starting with the fundamentally different tectonic settings: The Zagros is characterized by crustal shortening and active mountain building, whereas the Valencia Trough corresponds to a uniaxial extensional marine basin. In addition, isostatic processes are not considered, and the rupture mechanisms are inherently different. Reverse faulting in the Zagros involves the mobilization of very large rock volumes in the hanging wall, which is not directly comparable to normal faulting in a sedimentary marine basin. A more cautious discussion is required to avoid oversimplifying the analogies between these two geodynamically contrasting regions. Thermal coupling in different depth are also ignored.
S5- You have not demonstrated this claim (similarities between both tectonic settings, Valencia Trough and Zagros region). What is shown is only a convergence in the geometry of the spatial distribution of seismicity, but this does not constitute a “pattern” based on the underlying mechanical behavior of the faults. A true mechanical pattern would require evidence of consistent fault kinematics, stress distribution, and deformation mechanisms, which are not demonstrated in the current version of the manuscript. Please, consider to remove this part, I have no the relevance in your paper. This only distracts from the methodology that authors propose.
S6- I also recommend removing the estimation of the maximum earthquake magnitude based solely on the total fault length, as the results appear unrealistic (Table 1). This approach tends to overestimate the potential magnitude and is not fully supported by seismotectonic principles. Instead, relying on fault surface area provides a more consistent and physically based estimate, as it is directly related to the seismic energy release (Kanamori and Anderson).
S7-Conclusions have to be revisited. Some relevant information derived from your results are missed.
Technical suggestions and questions
TSQ1-Page 4; Line 85 You refer “mechanical weak layer” to the Triassic evaporitic formation. My question: Are you referring to ductile behavior under a tectonic stress field? If so, how do you estimate the creep movement associated with ductile salt deformation?
TSQ2- Reading the whole manuscript, I still do not fully understand what you mean by the term “mechanically weak layer” regarding the evaporite Traissic layer. Could you please clarify its implication for the seismogenic behavior of the fault system?
Are you suggesting that such a weak layer:
A clearer explanation would help in understanding the role of this layer in the seismic cycle.
TSQ3- The Cullera Fault appears to show a decrease in slip rate from the Pliocene to the Quaternary (Fig. 8). Could the authors clarify the underlying reason for this apparent behavior? Do you have a conceptual model that could explain this trend? Is it a Nubia plate deceleration?
TSQ4- The curves of the Figure 10, Throw depth (T-z) and expansion index plots for the Cullera Fault computed from profiles SGV01-115 and SGV01-117, show a fault throw variation. Could the geometry of these curves be explained by the listric geometry of the Albufera Fault? Variations in fault throw along planes with different dips could potentially generate this type of curve. Could the authors clarify if this interpretation is consistent with their data?
TSQ5- The accommodation space created by the Albufera Fault, and cited in the line 559, page 30, can be explained by salt- tectonics? Please explain your model.
TSQ6- Line 641 Tectonic driving mechanism and differences between tectonic driving and sal-withdrawal driving. I have some doubts regarding this point. The “mechanically weak layer” is described at a depth of 2–5 km, but tectonic earthquakes with significant magnitudes (M6–7) typically require rupture depths greater than this. Moreover, the regional tectonic strain is expected to be distributed below the weak layer as well. This raises the question: does the fault release only the strain accumulated in the supra-salt section, or can it also rupture deeper segments (>10 km) where larger tectonic strain is stored? A discussion of this point would clarify the seismogenic potential of the fault system.
TSQ7- Methodology and the “correction factor” (line 712, page 36). I agree that using the total fault length to estimate earthquake magnitude through common empirical relationships is not the most reliable approach. However, the total fault surface area provides a more reasonable approximation because it is directly related to the seismic energy released during an earthquake (see Kanamori and Anderson). The main concern with introducing a “correction factor” is the potential to generate an artifact in the magnitude estimation, which could lead to biased results if not properly justified or calibrated. On the other hand, why the “aspect radio” factor of the active fault could improve the magnitude estimation? It is not clear in the text.
TSQ8- PAY ATTENTION! revise the fault areas of the Table 2. There are some incongruences with the values obtained from the 2D Move analyses and the values indicated in the Table 2 for the Cullera, Albufera and Valencia faults (pages 20, 24 and 27).
TSQ9- Table 2. I don’t understand how a great fault area triggers an earthquake of lower magnitude as appeared in Albufera and Valencia faults). Please, revise carefully TABLE 2.
Additionally, I have attached the PDF manuscript with some minor detailed comments posted into the PDF file.
OVERALL RECOMMENDATION: MAJOR REVISIONS.
Thank you very much for your work, excellent analysis and results. I hope that authors improve the manuscript.
Thanks also for the editor for the consideration as a reviewer.
Dr Raul Pérez-López
Instituto Geológico y Minero de España – IGME CSIC
Email: r.perez@igme.es