Persistent Deformation in a Post-Collisional Stable Continental Region: Insights from 20 Years of cGPS in Romania

Muntean, ﻿Alexandra; Petrescu, Laura; Ambrosius, Boudewijn; Borleanu, Felix; Nastase, Eduard Ilie; Munteanu, Ioan

doi:https://doi.org/10.5194/egusphere-2025-3103

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https://doi.org/10.5194/egusphere-2025-3103

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21 Jul 2025

| 21 Jul 2025

Persistent Deformation in a Post-Collisional Stable Continental Region: Insights from 20 Years of cGPS in Romania

Alexandra Muntean, Laura Petrescu, Boudewijn Ambrosius, Felix Borleanu, Eduard Ilie Nastase, and Ioan Munteanu

Abstract. The Carpathian Region, located at the edge of the East European Platform, presents a unique tectonic setting where major deformation associated with subduction and collision appears to have ceased around 8 million years ago. Yet vertical movements and present day seismicity continued afterwards, suggesting ongoing crustal deformation and challenging our understanding of intraplate earthquakes and the processes driving these phenomena in an area considered as a stable continental interior. In this study, we analyse over two decades of continuous GPS (cGPS) data from 143 permanent stations to estimate both horizontal and vertical crustal motions, constructing the most accurate model of crustal deformation in the region to date. The estimated velocity field indicates a southward drift of the South Carpathians and Moesia relative to Eurasia, with velocities ranging from 0.5 to 2 mm/yr. We detect a more complex pattern of vertical uplift and subsidence in the foredeep, challenging a previously held view that this region is solely subsiding. This pattern may reflect localized uplift in response to processes such as the Vrancea slab break-off beneath the South-East Carpathians. Crustal scale active faults accommodate the observed differential motion, fragmenting the foreland. Furthermore, using a regularized horizontal velocity vector field, we estimate strain rate variations, maximum shear strain, and dilatation patterns across Romania, which closely align with observed crustal earthquake mechanisms. This agreement validates our results and indicates a significant influence of surface plate kinematics on the observed seismicity, in addition to the deep Vrancea slab dynamics. Our findings provide fundamental insights into the causes of crustal deformation at the transition between active collision zones and stable continental platforms, enhancing our understanding of intraplate seismicity in regions traditionally considered tectonically stable.

Received: 03 Jul 2025 – Discussion started: 21 Jul 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Alexandra Muntean, Laura Petrescu, Boudewijn Ambrosius, Felix Borleanu, Eduard Ilie Nastase, and Ioan Munteanu

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RC1:
'Comment on egusphere-2025-3103', Stefan Leinen, 24 Jul 2025

General comments

The manuscript provides a substantial contribution to the research and understanding of the tectonic processes in the area of Romania and also in the context of the surrounding tectonic setting. This is based on a thorough analysis of GNSS data and respective time series analysis.
I assess the scientific significance with respect to data as excellent (1), as well as the presentation quality. Since in GNSS data processing, time series analysis, and velocity and strain field parameter estimation standard methods are used I rate the scientific quality as good (2).
I very much appreciate the high quality of the manuscript in terms of language and understandability which makes reading a pleasure and the review easy. I would like to mention that my expertise is in the field of GNSS data analysis and parameter estimation techniques, but less in geophysics.
In the following I will follow the review criteria as given by EGUsphere. At the end I address some points of discussion about details which might be addressed by the authors.

Specific comments (aspects 1. to 9. mentioned in the review criteria)
1. Does the paper address relevant scientific questions within the scope of SE?
Yes, since by analysis of long-term GNSS-derived position time series a much more detailed and sound picture of the defomations caused by tectonic motions is yielded. On this foundation the geophysical interpretation and understandings of involved processes can be very much enhanced.
2. Does the paper present novel concepts, ideas, tools, or data?
It provides novel data and information about the tectonics in the area.
3. Are substantial conclusions reached?
Yes, as given in sections 5 Results, 6 Discussion, 7 Conclusions.
4. Are the scientific methods and assumptions valid and clearly outlined?
Yes, the procedures are clearly stated and explained to the necessary detail.
5. Are the results sufficient to support the interpretations and conclusions?
Yes, as mentioned above.
6. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?
The data basis and the methods of calculation are explained to the necessary detail. Since the focus of the manuscript is to obtain insights in the tectonic ongoing processes, i.e. in the analysis and interpretation of the derived parameters (velocity field, strain field), this depth of explaining the calculations is sufficient. Data will be provided to interested scientists upon request as explained under Data availability, while details about used software is given under Code availability.
7. Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
Yes, especially with respect to previous work on the tectonics in the region of interest. However I can't claim to be familiar with all related literature. The authors clearly indicate their own findings and contrast them to the previous knowledge.
8. Does the title clearly reflect the contents of the paper?
Yes.
9. Does the abstract provide a concise and complete summary?
Yes.

Technical corrections (aspects 10. to 15. mentioned in the review criteria)
10. Is the overall presentation well structured and clear?

11. Is the language fluent and precise?
As already stated in the general comments the presentation quality is excellent. This includes the structure of the manuscipt, meaning the transition between sections in a logical way: from introduction, then data basis, data processing, estimation of parameters of interest, to analysis, interpretation and conclusion. The language is fluent, well understandable, and precise, and from my point of view there is no need for improvement.
12. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?

13. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
The focus of the manuscript is less on the mathematics involved. Therefore no formulas are given. Units in tables and figures are correct and clearly given. The quality of the figures is very good. This hold for the figures themselves, but also for the figure captions.
No parts of the manuscript should be reduced, combined, or eliminated, since the strucute of the manusript is very good (see point 10.).
14. Are the number and quality of references appropriate?
To my understanding yes. However I'm not familiar with all literature on geophysics in Romania and surrounding regions.
15. Is the amount and quality of supplementary material appropriate?
Yes. For the data base, i.e. all GPS stations, and also the derived grid, the information about network, velocity estimates and strain parameters are given. Also some more detail about the tectonic setting is given (figure S1).

Points of discussion, and on some details.
Figure 1: Abbreviation AM is not explained/introduced.
4. Analysis of the GNSS position (coordinates) time series: It seems that the estimation of velocities, jumps, annual and semi-annual parameters is based on a white-noise assumption, i.e. assuming no time correlation. However such time series usually contain significant stochastic time-correlation. Neglecting this might lead to over-optimistic confidence regions (as maybe the case in figure 3) and implausible statements about the significance of parameters. Have the authors considered this aspect?
4.1 For the modeled jumps in the time series: Have the authors tried to identify the reasons behind (antenna change or some local effect, ...)?
4.2: A reference to table S3 in the supplementary material is missing.
4.4: The reference mentioned for the STRAINTOOL is not given in the References of the manuscript.
Figure 3: Some large velocity vectors seem to deviate significantly from their neighbors (their motion pattern), e.g. ORAD, CLU2, TGMS. Shouldn't these deviations be addressed?

Citation: https://doi.org/10.5194/egusphere-2025-3103-RC1
- AC1: 'Reply on RC1', Alexandra Muntean, 03 Sep 2025
  
  We appreciate the reviewer's valuable comments, which have helped us improve the clarity and quality of the manuscript.
  
  Reviewer, Comment 1:
  Figure 1: Abbreviation AM is not explained/introduced.
  Response:
  Thank you for pointing this out. We have now introduced and explained the abbreviation “AM” (which stands for Apuseni Mountains) in the figure caption and in the main text we added the abreviation.
  
  Reviewer, Comment 2:
  4. Analysis of the GNSS position (coordinates) time series: It seems that the estimation of velocities, jumps, annual and semi-annual parameters is based on a white-noise assumption, i.e. assuming no time correlation. However such time series usually contain significant stochastic time-correlation. Neglecting this might lead to over-optimistic confidence regions (as maybe the case in figure 3) and implausible statements about the significance of parameters. Have the authors considered this aspect?
  Response:
  Thank you for raising this point. In the current analysis, the method is quite straightforward. Our algorithm estimates the velocity, seasonal signals and jumps in a single batch solution for each time series. The accuracies (sigmas) are derived from the WRMS of the residuals. They are computed as 2/3*WMRS divided by the length of the time series. This avoids overly optimistic estimates of the accuracy. It has proven to be a reliable and robust method, yielding realistic velocity and accuracy estimates (sigmas), comparable to literature solutions using different algorithms.
  
  Reviewer, Comment 3:
  4.1 For the modeled jumps in the time series: Have the authors tried to identify the reasons behind (antenna change or some local effect, ...)?
  Response:
  We thank the reviewer for this observation. The possible causes of the modeled jumps, including antenna changes and local site effects, are discussed in the manuscript (see Section 4.1, lines 168–173). We have now revised the text slightly to make this discussion more prominent and easier to locate.
  
  Reviewer, Comment 4:
  4.2: A reference to table S3 in the supplementary material is missing.
  Response:
  Thank you for your observation. We have revised the manuscript to include an appropriate in-text reference to Table S3 in the Supplementary Material. This should now clarify the connection between the main text and the supplementary content.
  
  Reviewer, Comment 5:
  4.4: The reference mentioned for the STRAINTOOL is not given in the References of the manuscript.
  Response:
  Thank you for pointing out the omission. We have now included the full references for STRAINTOOL in the References section as shown below.
  Anastasiou D.G., Papanikolaou X., Ganas A., and Paradissis D.: StrainTool: A software package to estimate strain tensor parameters. Zenodo; Version 1.1, https://doi.org/10.5281/zenodo.5501234, 2021
  Shen, Z. K., Wang, M., Zeng, Y., and Wang, F.: Strain determination using spatially discrete geodetic data, Bull. Seismol. Soc. Am., 105, 2117-2127, https://doi.org/10.1785/0120140247, 2015.
  
  Reviewer, Comment 6:
  Figure 3: Some large velocity vectors seem to deviate significantly from their neighbors (their motion pattern), e.g. ORAD, CLU2, TGMS. Shouldn't these deviations be addressed?
  Response:
  We appreciate the reviewer’s observation. As discussed in Section 4.2, we applied an outlier detection method to identify stations exhibiting significant deviations from the regional trend. This includes stations such as ORAD, CLU2, and TGMS. These stations are now represented as black vectors in an updated version of Figure 3.
  
  As mentioned, partially based on your comments, we made many changes in the text of the final manuscript to give better explanations. This version is not online (yet).
  
  Citation: https://doi.org/10.5194/egusphere-2025-3103-AC1
RC2: 'Comment on egusphere-2025-3103', Anonymous Referee #2, 10 Sep 2025

Persistent Deformation in a Post-Collisional Stable Continental Region: Insights from 20 Years of cGPS in Romania
Alexandra Muntean, Laura Petrescu, Boudewijn Ambrosius, Felix Borleanu, Eduard Ilie Nastase, Ioan Munteanu
Submitted to EGUsphere, 2025
This well-written paper describes results from 20 years of continuous GPS measurements in Romania. This is a useful addition to the existing literature. Some improvements can be made to the reference frame definition and the uncertainty estimation. The comparison with previous results is there, but could be expanded. I am not fully convinced by the vertical velocities and their interpolation. The interpretation is fine, with a range of hypothesis provided in terms of the underlying dynamics.
It may be useful to show seismicity on Figure 1, together with the location of GPS stations but without colors and site names. This would be particularly useful to accompany the introduction and tectonic setting sections of the paper. The details of the cGPS site names and network affiliations can be shown in the supplements, as they are not essential to the understanding of the paper.
My first significant comment concerns the definition of the Eurasia-fixed reference frame, briefly described in lines 148-150. First, the publication from which ``the most recently published rotation pole solution for that plate'' is used should be cited. Second, one needs to know how well the velocities presented here fit a Eurasia-fixed frame. In other words, do stations allegedly on stable Eurasia have velocities that do not statistically differ from zero? This is particularly important since the ``residual'' velocities discussed in the rest of the paper are very small, on the order of 1~mm/yr.
For instance, I am surprised that sites located on the East European Platform -- supposed to be stable Eurasia? or perhaps not? -- show a consistent NW-direct residual on the order of 1~mm/yr.
The authors may be using the Eurasia-ITRF14 angular velocities from Altamimi et al. 2017 (ITRF2014 plate motion model), but the consistency between Altamimi's frame definition and the authors is unlikely to be accurate at the 1~mm/yr level since they are (most probably) not using the same models.
The most rigorous way to determine such a frame would be for the authors to include in the analysis a number of sites located on stable Eurasia, for instance those used by Altamimi et al. 2017. Then the authors could compute their own Eurasia-ITRF14 angular velocity and use it to defined a Eurasia-fixed frame consistent with their analysis. But if the authors choose to use an independently-published angular velocity, then they must shown that residuals are close to zero at sites located on stable Eurasia.
My second significant comment concerns the estimation of velocity uncertainties, not explicitly described in the paper. One can assume from lines 156-158 that the uncertainty is that of a least-squares fit to the time series, assuming linear and seasonal terms plus offsets. If it is the case, then how do the authors deal with the well-known issue of time-correlated noise in cGPS time series and its impact on velocity uncertainty estimates? I suspect that the uncertainties estimated here actually underestimate more realistic ones that would account for time-correlated noise. That is an important issue in a region where residual velocities are very

small.
Several published options exist to fit GPS time series using a noise model that accounts for time-correlated noise, for instance with CATS (Williams GPS Solutions 2008) or Bos et al. J. of Geodesy 2008.
As a side note, the authors should quote the confidence interval they choose instead of using "sigma". Note that a 1-sigma uncertainty in 2 dimensions (east-north velocities for instance) correspond to a 39% confidence interval. A much safer 95% confidence interval would correspond approximately to 2.4-sigmasi in 2 dimensions.
Section "Estimating a gridded, smoother horizontal velocity field": how is the uncertainty on actual GPS velocities propagated to the gridded velocities? By the way, Figure 3 should show uncertainty ellipses for the gridded velocities.
I understand in section 4.4 that the strain rates are computed from the raw GPS velocities, not the gridded ones. Is that right? If so, what is the point of calculating gridded velocities? By the way, I assume that STRAINTOOL estimates velocities on a regular grid as well. Perhaps those should be shown on Figure 3? But I may be misunderstanding section 4.4 -- which perhaps calls for some clarification in the text.
Section 5.1 and throughout the paper: I suggest replacing "appear to show" by simply "show". The agreementi between solutions seems ok in the horizontal, but there seems to be some scatter in the vertical?
The comparison of the velocities in this work with those from Pina-Valdes et al. 2022 and Serpelloni et al. 2022 on figure 7 must show the 3 different solution in different colors.
I suggest the authors also review and cite D'Agostino et al. Earth and Planetary Science Letters 539 (2020), which intersects the study area presented here.
Line 285: I see no complexity in the strain rate field, which is rather smooth, with a consistent NS extension throughout and significant EW compression in the SW corner. I would actually like to read a complete interpretation of figure 5. Is there consistency between the earthquake focal mechanisms and the GPS-derived strain rate principal axes? What do we learn about the sens of slip on the main active faults? Are compressional events in Vrancea consistent with the principal axes of the geodetic strain rates?
Figure 5 could be improved by making the "beachballs" smaller, as they are quite cluttered. Also, is would be useful to show on the figure the depth of these events, as I assume that some are crustal but others much deeper. Perhaps color-coding the black quadrants of the focal mechanisms as a function of hypocenter depth would help?
I am a bit puzzled by the vertical velocities shown on Figure 4 and its interpolation. For instance, the uplift observed on the East Carpathians essentially depend on 2 sites with vertical velocity significantly different from the neighboring sites. Can we be certain that one is not looking at local anomalies due to monument stability or other local process? It is a bit troubling that this does not appear in Pina-Valdez' solution, for instance. The same holds for the SE Carpathians foredeep.
It seems to me that the authors are placing too much confidence in their interpretation on local vertical velocities, especially at local scale. This comment applies to the South Carpathians (lines 415-416), but also to the SE Carpathian foredeep.
Conclusion:
- The authors write "Our results mark a significant improvement in spatial coverage and resolution", "Our extended and more reliable data" -- I am wondering compared to which previous work?
- The authors write "The relative motions between these regions generate a complex strain rate pattern with zones of extension, compression, and shear, which closely align with observed regional seismic activity." What is meant by "align", especially since there is no seismicity map in the paper? For instance, do the authors mean "coincide" or also "share the same mechanisms"?
Throughout the paper: I suggest toning down (or removing) terms such as "fundamental", "vital", "crucial", "complex", "fresh", "significant". After all, this is only geodesy!

Citation: https://doi.org/10.5194/egusphere-2025-3103-RC2

Alexandra Muntean, Laura Petrescu, Boudewijn Ambrosius, Felix Borleanu, Eduard Ilie Nastase, and Ioan Munteanu

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https://doi.org/10.5194/egusphere-2025-3103-supplement

Alexandra Muntean, Laura Petrescu, Boudewijn Ambrosius, Felix Borleanu, Eduard Ilie Nastase, and Ioan Munteanu

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Short summary

We utilize two decades of continuous GPS data to investigate crustal deformation in Romania, a region that has long been regarded as tectonically stable. Despite the end of major collisions millions of years ago, deep earthquakes and active faults indicate ongoing strain. We estimate horizontal and vertical velocities, strain rates, and dominant deformation styles. Our results reveal localized uplift and extension in the foreland, linked to slab dynamics beneath the southeast Carpathians.


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