the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jul 2025
Active Piedmont Zone Deformation, a manifestation of activity on the ‘Master Ray Fault’; insight into the seismic hazard analysis of the Tehran metropolitan area
Abstract. The piedmont zone in the frontal active regions of the orogenic belts exhibits various deformation patterns, which helps unravel the seismic sources for cities that flourish in such tectonic settings. A detailed analysis of the active folding, faulting, and related morphological features of the Quaternary alluvial units disclose prominent thrust faults in the Tehran piedmont zone. These faults are kinematically related and play a vital role in a better understanding of the seismic hazard of the Tehran metropolitan area. The five south-dipping thrust faults, the related hanging-wall folding and subsidiary faulting accommodate a considerable amount of north-south shortening during Quaternary. The shortening is observed in the alluviums and the underlying Eocene volcanic bedrock. Interestingly, in western Tehran, the Chitgar area discloses a type locality for active fault-bend folding, backthrusting, oblique-slip normal faulting and fault inversion in the piedmont zone. Our optically stimulated luminescence dating on the Late Pleistocene alluviums in the Chitgar area constrains the slip rate of the primary and secondary faults. According to our analyses, we introduce the ‘Master Ray Fault’ as a crucial seismogenic fault of the Tehran region, manifested as south-dipping thrust faults in the piedmont zone. We estimate the minimum slip rate on the Master Ray Fault to be ca. 0.50 mm a-1. Our study offers a crucial methodological framework for improving the existing understanding on Quaternary thrust fault kinematics and associated morphological features, aiding in the unveiling of potential seismic sources in metropolitan areas located in piedmont zones adjacent to active orogenic belts.
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RC1: 'Comment on egusphere-2025-500', Anonymous Referee #1, 12 Aug 2025
This submitted manuscript proposes a new regional seismotectonic model for the region near the capital city of Tehran, Iran, based on field surveys, remote sensing interpretation, and a catalog of instrumentally recorded earthquakes. Based on observed variations in the strike of some thrust faults, the paper concludes that the "Master Ray Fault" can explain the seismogenic tectonic setting of the Tehran region's magnitude > 7 earthquakes and that tectonic explanations can also explain several historical strong earthquakes. Discussing this regional tectonic model is crucial for understanding the patterns of historical strong earthquakes and the potential for future strong earthquake hazards. While the paper has made some progress, several scientific issues remain that require further discussion and refinement:
1. The collision of the Arabian and Eurasian plates controls the regional tectonic model for northern Iran. The surface deformation generated by the south-to-north thrust in this region is the subject of the paper's field investigations. The current version only discusses the seismogenic capacity of the Tehran Ray region but does not provide an overview of the tectonic setting of the entire tectonic belt. Therefore, the validity of the regional seismotectonic model based solely on fault strike requires additional background information on regional tectonic development (see Allen et al., 2003, JSG in detail);
2. Regarding the coupling of deep and shallow structures, thrust faults, because their strike varies significantly across the cross-section, often appearing as shovel-like faults with a steeper upper portion and a gentler lower portion, whether the surface strike can be used to define the deep strike (and thus confirm the fault extension) is a question worth discussing. This requires support from high-resolution geophysical data. The reviewer emphasized that the paper needs to supplement the paper with deep geophysical data perpendicular to the fault strike in the Tehran region or adjacent areas (including citing other research results) to confirm the rationality of the coupling interpretation of the proposed tectonic model (Fig.13).
3. Since the superficial structural deformation mentioned in Q2 above is only a "result" of focal activity, it is not itself the cause of the earthquake. Therefore, if the model presented in the paper locates the earthquake source at a distance, the micro-epicenter will have deviated from the Tehran tectonic zone. The precise location of the minor earthquake currently presented does not demonstrate the validity of the model presented in this paper. Therefore, when revising the paper, the authors should carefully consider the universality of the regional tectonic model mentioned in Q1 and Q2 for local sections of the fault, and should not infer the overall tectonic characteristics from local sections.
4. Regarding the uncertainty in the magnitude determination of historical strong earthquakes, recent studies of moderate-to-strong earthquakes with instrumental records and comprehensive damage investigations (see studies in Italy, China, Japan, the United States, etc.) have shown that damage near the epicenter of moderate earthquakes can also be severe. Therefore, the magnitudes of historical strong earthquakes calculated based on only a small amount of literature may generally be overstated (Ou et al., 2020, JGR: SE). Therefore, the estimated magnitudes of historical earthquakes cannot be equated with instrumentally recorded magnitudes, leading to discussions of regional substantial earthquake hazard without supporting seismic tectonic context.
5. Other minor issues: a) The paper lacks a summary of the regional tectonic context; this should be supplemented; b) Figure 1a in the paper lacks key place names; c) The field photos in the figure should be positioned on the map; they are currently incomplete; d) The discussion of historical earthquakes in the Discussion section is insufficiently detailed; a figure should be added; e) The rationale for the tectonic model in the Discussion section requires further discussion; f) The paper could supplement the regional tectonic model with a figure (which can be cited); g) some oral sentences should be deleted, for example: line 21: According to our analyses, line 55 According to our findings, line360 As explained under Section 2.
Citation: https://doi.org/10.5194/egusphere-2025-500-RC1 -
RC2: 'Comment on egusphere-2025-500', Anonymous Referee #2, 13 Aug 2025
Review on “Active Piedmont Zone Deformation, a manifestation of activity on the ‘Master Ray Fault’; insight into the seismic hazard analysis of the Tehran metropolitan area” coauthored by Ghassemi and Heydari
This paper studies several south-dipping piedmont faults in Tehran region and relates them to a possible low-angle south-dipping Rey master fault. The authors first describe the geomorphology and evidence of active faulting of the piedmont faults. They notice that the faults have traces in different directions and their surface trace is rather irregular. In the second step, for the first time they date the active faulting in Chitgar fault zone using Luminescence dating of unit B and calculate a shortening rate for the fault zone. In the third step, the authors use locally recorded micro-seismicity and notice that the depth of the earthquakes gradually increases towards south. Using this observation, they conclude that the south-dipping piedmont faults are actually connected to a south-dipping low angle master fault, the Rey fault. They then use their slip rates calculated for the faults in Chitgar fault zone to infer possible slip rates on the reverse master Rey fault. I think the paper should consider the following important points before being publishable in EGUsphere.
- Authors do not try to put their interpretation on a regional scale. For example, they do not state what is the general direction of sigma1 (maximum principal stress) in the region. We know from GPS (Khorrami et al., 2019) (Banimahdi Dehkordi et al., 2024b) and also focal mechanisms recently published by (Banimahdi Dehkordi et al., 2024a) the direction of sigma1 is NE-SW. Such a sigma1 direction cannot result to dominantly reverse mechanisms on Rey and other piedmont fault and one expect to have mostly strike-slip motion on these faults. So there is a need for authors to put their findings into a larger frame and try to discuss how it relates to recent detail seismological and geodetical findings in the region.
- One of the main pieces of evidence for the proposal of the Rey master fault is the observed southward increase in depth of microseismicity. The authors did not consider possible systematic errors in the determination of focal depths. Essentially, to have a reliable focal depth determination, the event should have been recorded by a local network with good azimuthal coverage and also at least recorded by a seismic station at distances not greater than one to 1.5 times of focal depth. Looking into the seismic station of Iranian Seismological Center (irsc.ut.ac.ir), one can quickly realize that the station density is not good enough to record reliably the focal depth of most of the events. The precisely relocated seismicity of Tehran (Banimahdi Dehkordi et al., 2024a) shows that the microseismicity is actually happening to the east of the proposed Master Rey fault. Therefore, I would say that the observed seismicity does not support nor disprove the existence of the master Rey fault.
- It is not clear if the state of stress throughout 137-364 ka ago have been constant. Maybe the active faulting belongs to older times when the direction of sigma1 was different than now and for now the faults are not active at all and that is why they lack any seismicity over thirty year of activity of Tehran seismic network. I should say that even the dense seismic network of Tehran municipality did not detect any noticeable seismicity on these studies faults.
- It is required to show better the geometrical relationship between the thrust fault and the assumed Rey Master fault. To do so please put distance on Figure 13. By the way the Rey fault itself has two main branches, the north and south Rey fault, which one authors relate to the master Ray fault and why?
Minor comments:
- There are ongoing controversy on whether some of the active faults mentioned in this work are indeed an active fault or not! It is essential to review those views and somehow discuss them.
- The authors insist on North Tehran fault (NTF) being as mostly north-dipping thrust fault. This conclusion comes from a recent paper by the first author. Others (Tchalenko, 1975) (Abbassi and Farbod, 2009) have shown that the NTF is mostly en-echelon strike-slip fault. Sigma1 direction and general azimuth of the fault also imply a mostly strike-slip fault. Please consider other views better.
- Please show the Rey and Kahrizak faults on one of your maps.
- On line 87, authors say that “Most transpressional deformation is accommodated by the range-parallel thrust and left-lateral strike-slip structures, such as the NTF, Caspian Fault and Mosha Fault.” This is the most popular view on Alborz but looking into the recent updated seismicity and focal mechanism shows almost very little seismicity on Caspian fault, and pronounced seismicity on North Alborz fault. Additionally, the thrust faults and strike-slip faults are not parallel and also thrust and strike-slip events happened everywhere and thus there is no indication of strain-partitioning (Banimahdi Dehkordi et al., 2024a,b). Due to lack of enough GPS stations, all previous GPS models indicate a large slip on Khazar fault but addition of new GPS stations reveal significant slip on North Alborz fault zone and a much reduced slip rate on Khazar fault.
- On line 92: Now Tehran has several instrumentally recorded earthquakes of larger than magnitude 4 like Mallard 2017/12 earthquake.
- Line 359: I know in this region, there is lots of mine blasts. Usually these mine blasts have magnitudes less than 2 Mn. So please remove all those events less than 2 Mn.
- It is not clear how samples from B units in Chitgar fault can reveal age of active faulting there. Please explain this thoroughly.
References:
Abbassi, M.R., Farbod, Y., 2009. Faulting and folding in quaternary deposits of Tehran’s piedmont (Iran). J. Asian Earth Sci. 34, 522–531. https://doi.org/10.1016/j.jseaes.2008.08.001
Banimahdi Dehkordi, M.J., Ghods, A., Shabanian, E., Mousavi, Z., 2024a. Constraint on the active tectonics of the Alborz using seismology data. Iran. J. Geophys.
Banimahdi Dehkordi, M.J., Mousavi, Z., Shabanian, E., Abbasi, M., Ghods, A., 2024b. Constraint on the active tectonics of the Alborz (Iran) using geodetic data. Iran. J. Geophys. 18, 55–76.
Khorrami, F., Vernant, P., Masson, F., Nilfouroushan, F., Mousavi, Z., Nankali, H., Saadat, S.A., Walpersdorf, A., Hosseini, S., Tavakoli, P., Aghamohammadi, A., Alijanzade, M., 2019. An up-to-date crustal deformation map of Iran using integrated campaign-mode and permanent GPS velocities. Geophys. J. Int. 217, 832–843. https://doi.org/10.1093/gji/ggz045
Tchalenko, J.S., 1975. Seismotectonic framwork of the North Tehran Fault. Tectonophysics 29, 411–420.
Citation: https://doi.org/10.5194/egusphere-2025-500-RC2 -
RC3: 'Comment on egusphere-2025-500', Anonymous Referee #3, 21 Aug 2025
The manuscript addresses active faulting and seismic hazard in the Tehran metropolitan area, focusing on five south-dipping thrust faults and introducing the “Master Ray Fault” as a new potential seismogenic source. The topic is highly relevant for understanding seismic hazards in one of the most densely populated regions of Iran. However, the paper currently suffers from conceptual ambiguities, insufficient methodological rigor, and weak figure presentation. The overall structure and writing style resemble a technical report rather than a concise scientific article, ‘cause except for seismic risk analysis for some active faults, I really do not get the authors’ scientific questions. Significant revisions are required to enhance clarity, rigor, and scientific impact.
Major Issues
- Title and Scope
- The title includes a semicolon (“;”), which is unconventional for scientific articles. It should be reformulated into a concise and informative title.
- The scope of the paper is not well defined: is the main contribution a new fault model (“Master Ray Fault”), a methodological framework, or a regional seismic hazard reassessment?
- Scientific Purpose and Rationale
- The abstract claims that “multi-patterned deformation in the piedmont zone unravels seismic sources.” This correlation between surface deformation and deep seismogenic sources is not convincingly established. A clear methodological or theoretical framework is missing.
- The introduction lacks a well-structured research question or hypothesis. The reader is left uncertain about the central scientific aim.
- Study Area Definition
- The location of the investigated faults within the broader tectonic framework of the Alborz and Iranian Plateau is not sufficiently illustrated.
- A clear area of interest (AOI) map is needed, showing the relation of the investigated faults to the North Tehran Fault and other regional structures.
- Slip Rate Estimates and OSL Dating
- Slip rates are reported with vague qualifiers such as “ca. 0.5 mm/yr.” Uncertainty bounds and methodological details are missing.
- OSL dating results (Table 1) approach signal saturation and are explicitly acknowledged by the authors as possibly underestimated. Using these to derive slip rates is problematic without further validation.
- Error propagation in slip rate estimates is not properly addressed.
- Figures and Cartography
- Figures are of insufficient quality for publication. Some appear as low-resolution screenshots (e.g., Fig. 10b, 10c).
- Stratigraphic units labeled as “A Unit,” “C Unit,” etc., are confusing without a proper stratigraphic framework.
- Several figures (e.g., Fig. 5b) contain unnecessary labels that distract from the main observations.
- High-resolution, publication-quality figures prepared in GIS/Illustrator are required.
- Earthquake Catalog and Microseismicity
- The microseismicity analysis (Fig. 12) is problematic. It is not clear whether events were relocated. Without robust relocation, reported depths may have uncertainties >5–10 km, making interpretations of seismogenic layering unreliable.
- The polynomial fitting of hypocenters is not a robust method to infer fault geometry.
- Inconsistency with Regional Tectonics
- The model of a major south-dipping “Master Ray Fault” contradicts many previous studies that emphasize north-dipping thrusts as the dominant structures in the Alborz.
- The authors need to reconcile their interpretation with GPS shortening rates (4–5 mm/yr across Alborz) and with prior fault-kinematic models.
- Historical Earthquake Correlation
- The proposed association between the “Master Ray Fault” and destructive historical earthquakes (e.g., AD 855, AD 864) is speculative and lacks paleoseismological evidence.
- More rigorous evidence (archaeoseismology, trenching, historical intensity reanalysis) is required to substantiate this claim.
- Manuscript Structure and Writing
- The manuscript reads more like a technical report than a journal article. Organization is weak, logic is inconsistent, and redundant descriptions reduce clarity.
- A clearer, hypothesis-driven narrative is needed.
Minor Issues
- Terminology such as “north-vergent thrusting” and “south-dipping thrusts” is inconsistently applied and may confuse readers. A standardized terminology is recommended.
- Several grammatical and typographical errors remain (e.g., “ca.” used improperly, inconsistent tense usage). Careful proofreading is required.
- References to stratigraphic units (A, B, C) should be explained in more detail, preferably with a stratigraphic column figure or geological group name.
- Some statements in the introduction are overly general and not directly linked to the study (e.g., discussion of foreland basin systems). Streamlining would improve readability.
Recommendation
Given the current conceptual weaknesses, methodological issues, poor figure quality, and organizational problems, I recommend Rejection in its present form. A future submission could be reconsidered only if the authors:
- Provide robust evidence for the existence and geometry of the proposed “Master Ray Fault,”
- Improve chronological and slip rate analyses with better treatment of uncertainties,
- Relocate earthquake data and apply more rigorous seismological methods,
- Produce publication-quality figures,
- Rewrite the manuscript with a clear scientific hypothesis and logical structure.
Citation: https://doi.org/10.5194/egusphere-2025-500-RC3
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