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the Creative Commons Attribution 4.0 License.
| 11 Mar 2025
Building up a subsurface geological model in active offshore areas: constraints from legacy seismic reflection profiles and deep wells in the 2022 Fano-Pesaro Mw 5.5 earthquake sequence area (Adriatic Sea, Italy)
Abstract. Building a geological model in offshore areas is a complex task, due to the obvious absence of outcrops and thus the inaccessibility to the study site. The integration of key seismic reflection and borehole data is therefore fundamental, even if only available as legacy data on paper hard copy and/or characterized by an apparent low quality. However, such data are often the only ones available, and can still provide a high amount of detailed information for building a reliable geological model to compare and discuss with seismicity distribution in active areas. In this work, legacy seismic reflection profiles calibrated with boreholes are used to propose a new geological model of the frontal part of the Northern Apennines area struck by the 2022 Fano-Pesaro Mw 5.5 earthquake sequence (Adriatic Sea, Italy). The observed tectonic structures are originated by multiple décollements located at different depths and show a strong relationship between the faulting depth and the anticlines wavelength. Two structures, namely Pesaro and Cornelia anticlines, are interpreted as related to deep-seated thrusts, showing an en-echelon arrangement and thin-skinned deformation. A smaller wavelength structure, namely Tamara antiform, is interpreted to be related to shallow-seated imbricated fore-verging thrusts in the forelimb of the Pesaro anticline. We highlight the importance of constructing a well-constrained geological model by integrating legacy geological and geophysical data, aimed at offshore seismotectonic studies as well as at industrial applications, particularly in the context of energy transition.
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RC1: 'Review on egusphere-2025-941', Anonymous Referee #1, 11 Apr 2025
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The manuscript entitled “Building up a subsurface geological model in active offshore areas: constraints from legacy seismic reflection profiles and deep wells in the 2022 Fano-Pesaro Mw 5.5 earthquake sequence area (Adriatic Sea, Italy)” by Elham Safarzadeh and co-authors puts forward an interpretation of well-logs and seismic reflection profiles from the Adriatic offshore region in Northern Italy, an area affected by a seismic sequence linked to the activity of a compressional structure in 2022.
The authors draw attention to the presence of deeply rooted reverse faults, which they suggest control the wavelength of the major anticlines. They also note a shallow detachment within the Tertiary succession that appears to govern the development of shallower thrusts and smaller wavelength folds.
In their discussion, the authors compare their geological reconstruction with the distribution of seismicity during the 2022 seismic sequence, as presented in various catalogues.
The manuscript has a good structure, but the language should be improved, paying attention to numerous typos. I appreciated the authors' thorough well-log analysis and some interesting aspects of their seismic profile interpretation. However, several key areas within the current manuscript require careful attention.
As a first point, I suggest changing the title since no comprehensive geological model is presented. The authors propose the interpretation of a set of seismic profiles tied with well logs, but they do not propose a general interpretation of the tectonics of the region and/or provide subsurface maps as the reader would expect from the title's incipit.
Overall, the graphical quality of the seismic images is relatively low, both in the interpreted versions within the manuscript and in the uninterpreted versions in the supplementary materials. While I appreciate that the authors may have limited control over the quality of confidential profiles, I consulted the literature cited and found that the seismic profile crossing the Cornelia anticline (S3) is very similar, if not identical, to one of the profiles published in Maesano et al. (2023). Their supplementary materials provide an uninterpreted version of this profile with significantly better quality. Therefore, I suggest the authors base their interpretation on this higher-quality seismic profile and, more generally, utilise the best available version of seismic images whenever possible.
Regarding profile S1, there are a couple of points concerning the interpretation. At first glance, the chaotic seismic signal within the shallow splays in the TS may also involve some part of the Mesozoic succession to allow for section balancing. Also, concerning the S1 profile, I understand that the seismic image quality at the footwall of the thrust is poor. Still, the way the reflections of the succession are interpreted below the thrust appears inconsistent with the limited available data and could lead to the balancing problem that will be discussed subsequently.
Profiles S3 and S4 show a similar issue in interpreting the footwall reflections. Even though the authors map these reflectors as inferred (using dashed lines), they may not fully consider the sparse evidence that could help them produce a more accurate interpretation. Alternatively, they might omit the interpretation in these parts of the profiles if they deem them too speculative.
In any case, when examining the geological sections produced after the depth conversion of the seismic profiles, the effect of such approximations in the original interpretation is evident, as all three geological sections appear significantly unbalanced. I made some attempts to restore the sections to the first pre-tectonic layer, following the indications provided by the authors in Figure 2, and the results indicate a substantial imbalance across the entire pre-tectonic succession. This effect is unlikely to be related to out-of-section movement, as the authors state that the sections run orthogonal to the strike of the main structures (line 458). Therefore, it is most likely linked to some issues in the seismic interpretation, as suggested in the comments on the seismic profiles.
I fully appreciate the difficulties arising from the limited quality of the initial data. Still, a well-established principle in structural geology is that while a balanced section is not necessarily correct, an unbalanced one is certainly incorrect.
This point is particularly critical in a study such as this, given that the authors have attempted to compare their geological reconstruction with the distribution of seismicity in the area. The authors rightly point out the limitations associated with the localisation of seismicity in the Adriatic offshore area and the consequent uncertainties in both mainshock and aftershock locations. However, I believe it is paramount that this comparison is based on a validated geological reconstruction in the first instance, which is ultimately the declared scope of the work since the title.
Therefore, the manuscript and the geological model need a substantial major revision before publication. The revision should focus on producing geological sections where the imbalance is minimised and, where possible, quantified.
Below are some further specific comments:
Line 130 and Figure 2: "Calcare Di Asprigni" should read "Calcare Diasprigni".
Figure 6: On the S4 profile, the intersection with profile S5 is missing. On profile S5, the lower splays of PT between km 14 and km 28 do not have a perfect correspondence on profile S3 and could be improved. Specifically, the splays are traced below the Pesaro Mare 04 well on profile S5, but they appear to terminate before the well projection on profile S3.
Lines 481-482: “The fore-verging imbricated thrusts originated from the upper décollement of the PT within weak, marly rocks (ranging from the upper Miocene to Pleistocene), propagates both eastwards and upwards”. As discussed earlier, after attempting some restoration, I believe that this interpretation, as currently presented, could be one of the causes of the imbalance. I would not rule out the possibility that part of the Mesozoic-Tertiary succession is involved in these imbricate thrusts. While I am largely in agreement with the structural framework proposed by the authors, this is simply a suggestion for revising the interpretation in this particular area.
Lines 491-495: “These structural wavelength values, λₗ and λₛ, are larger than those obtained for corresponding structures in the Umbria-Marche area, where corresponding structures have wavelengths of 3.2 to 7.2 km for λₗ and 0.4 to 2.3 km for λₛ (Massoli et al., 2006), characterized by lower syn-tectonic sedimentation. Conversely, and they are smaller than those observed in the Po Plain, where higher syn-tectonic sedimentation contributes to even larger structural wavelengths, with λₗ ranging from 15.8 to 33 km and λₛ from 4.5 to 8.2 km (Massoli et al., 2006).”This sentence is somewhat unclear. Are the authors suggesting a correlation between the different wavelengths and the amount of syn-tectonic sedimentation?
Lines 508-511: “Our interpretation demonstrates that, unlike the PT, the CT lacks an upper shallower décollement. Instead, the ramp of the CT terminates blindly at a depth of 2 km within the base of the upper Pliocene turbiditic successions (Figs. 7b, 7c), and only one imbricated fore-verging thrust has been identified in S4.” This difference is quite evident, and I would expect further consideration regarding the reasons for the absence of the shallow décollement in the CT.
Line 535: “6.2 Seismotectonic implications”. In this section, the authors discuss their findings in relation to all previous work published on the 2022 seismic sequence and listed in the “State of the art” section (Line 175), except for the work by Maesano et al. (2023). Is there a specific reason for this omission? I believe a more comprehensive discussion could be beneficial for the readers.
Lines 582-583“This study highlights a possible minor role of the Cornelia thrust system during the 2022 earthquakes than previously thougth due to a more limited extent to the NW”. This sentence in the conclusion seems at odds with the Seismotectonic implications paragraph, where there is no explicit statement in this direction. Are the authors referring to a particular previous study? Some context might be missing, making this sentence not fully supported by the preceding discussion.
Citation: https://doi.org/10.5194/egusphere-2025-941-RC1
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