the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Oct 2025
Quantitative Geomorphic Analyses of Neotectonic Activity in the Central U.S. and Implications for Current Increased Seismicity
Abstract. This regional reconnaissance compares vector and scalar geomorphic indicators (transverse drainage basin asymmetry (TI), hypsometric integral (HI), and stream sinuosity index (SI)) in Kansas and Oklahoma to assess neotectonic geomorphology and explore the spatial association between geomorphic indices and basement structure. The study is motivated by both an observed increase in seismicity in the area between 2009 and 2019 and the gap in studies of crustal deformation in this region. In Kansas and Oklahoma, 1,697 and 950 4th-order basins, respectively, were analysed to assess TI to generate spatially averaged mean vectors. Geographic ‘domains’ with the preferred azimuthal direction of mean vectors were defined. In Kansas, six azimuthal domains were defined, whereas in Oklahoma, five azimuthal domains were defined. Most of the basin asymmetry can be attributed to regional climate conditions; however, an azimuthal domain containing increased seismicity in Oklahoma is consistent with neotectonic activity on faults of the Ozark Uplift. HI and SI pattern maps for both Kansas and Oklahoma were generated from hotspot analysis (LISA method) and the Total Sinuosity method combined with Empirical Bayesian Kriging (EBK). The distribution of HI hotspots and SI anomalies suggests ongoing tectonism. In eastern Kansas, active crustal deformation is indicated for an area containing increased seismicity within the Bourbon Arch and Nemaha Ridge fault zones. Another HI hotspot may relate to neotectonism of the Central Kansas Uplift fault zone, while a HI cold spot and low SI anomaly in Gray and Finney counties may indicate a subsiding neotectonic block. In Oklahoma, two possible neotectonic corridors coincide with high earthquake activity. A north-south corridor of high HI and SI values aligns with the Nemaha Ridge, extending into Kansas along the Humboldt Fault Zone. A high SI anomaly across the Cherokee Shelf and Arkoma Basin fault zones corresponds to an area of anomalous drainage basin asymmetry, consistent with neotectonism. These results are consistent with slow active tectonism in Kansas and Oklahoma, suggesting that the significant increase in earthquakes over the past two decades may have a natural source of seismicity.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-3205', Eric Gamble, 04 Dec 2025
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CC2: 'Sugestion on egusphere-2025-3205', Eduardo Salamuni, 21 Dec 2025
EGUSPHERE invited me to review the manuscript. My considerations follow, in addition to the PDF file with more detailed observations:
(1) The work is based on an interesting hypothesis that seeks the correlation between the asymmetry of hydrographic basins, the sinuosity index of rivers and the hypsometric integral of channels for the determination of local neotectonic movement processes.
(2) The work developed reveals a great observational and analytical effort on the part of the authors in an extensive area, involving two US states (Kansas and Oklahoma).
(3) There was a concern to show the reader the litho-structural characteristics of both states and in this particular, however, I suggest that there be changes in figure 8, as the geological maps of the two states are not compatible with each other, such as the displacement of the contact between the Permian and Pennsylvanian layers, in addition to other problems.
(4) The authors chose to place some graphs and maps as appendices. This caused the article to lose its fluidity, forcing the reader to constantly scroll between the main body and the appendix of the manuscript. I suggest that at least the figures in Appendices A, F, and H be moved into the text.
(5) Following the reinsertion of the figures mentioned above, I suggest a more in-depth analysis and discussion of what they represent as data, especially the figures in Appendix F.
(6) Regarding Appendix B, there was no appropriate description and analysis of the graphs. The reader needs to interpret the reason for their creation and their significance within the scope of the work. The authors need to be direct about their creation (method) and their direct result.
(7) Regarding earthquakes, it would also be desirable for the authors to discretize them by means of magnitude groups (e.g., 0-2 M; 2-4 M, 4-6 M) represented by different colors; this would assist the reader in analyzing the correlation.
(8) The Results and Discussion chapter should give rise to two separate chapters, one for Results and another for Discussion. This would allow, in a more objective way, to expand the analysis of the main hypothesis of the work in relation to the data. For example, the issue of basin asymmetry (via figures A1 and A2 moved to the results chapter), which could even receive background colors that would demarcate their greater or lesser asymmetry. On the other hand, in the Discussion chapter, a more detailed analysis could have been made regarding the fact that the enormous number of earthquakes in the two states would be natural or induced (apparently they are induced).
(9) The modifications suggested in the work are much more formal than substantive, as can be seen in the copy of the manuscript sent through this portal.-
EC1: 'Reply on CC2', Richard Gloaguen, 23 Dec 2025
This is a formal review. Authors, please address these comments. There was a misunderstanding and the reviewer posted his comments using this way.
Citation: https://doi.org/10.5194/egusphere-2025-3205-EC1
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EC1: 'Reply on CC2', Richard Gloaguen, 23 Dec 2025
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CC3: 'Comment on egusphere-2025-3205', Narayan Adhikari, 02 Jan 2026
This study uses a number of geomorphic indices to study neotectonic activity in Kansas and Oklahoma. To determine whether the fourth-order streams exhibit particular signs of neo-tectonic tilting, they examined their Transverse Topographic Symmetry, Hypsometric Integral, and Stream Sinuosity Index. They have examined the impact of additional variables that could result in a similar geomorphic signature, such as lithology and climate. To determine whether each subdomain has a particular connection to the neo-tectonic or other variables influencing it, they have divided their study field into numerous subdomains and analyzed whether each domain is influenced by a specific variable such as lithology, climate, or structure. Their methods are clear and replicable, and their conclusions are well-founded.
Citation: https://doi.org/10.5194/egusphere-2025-3205-CC3 -
RC1: 'Comment on egusphere-2025-3205', Anonymous Referee #1, 03 Jan 2026
This is a review of “Quantitative Geomorphic Analyses of Neotectonic Activity in the Central U.S. and Implications for Current Increased Seismicity” by Noor and coauthors. This study assesses neotectonic activity in Kansas and Oklahoma by integrating quantitative geomorphic analyses with lithology, climate, and seismicity. The study focuses on transverse drainage basin asymmetry, hypsometric integral, and the stream sinuosity index, derived from a 10-m DEM, and compares these landscape-scale datasets against previous geologic mapping, seismicity, and climate to interpret that there are a few regions in Kansas and Oklahoma with anomalous geomorphic indices that may be best explained by possible neotectonic activity. The interpretations and conclusions state that this is a reconnaissance-level regional study aimed at identifying possible regions for future study. The study has produced a large dataset with interesting observations and conclusions, and will be of interest to those studying landscape evolution and neotectonics. However, I think there are several major issues that should be resolved before the manuscript is suitable for publication. I outline these below under Major Comments, followed by Minor Comments by line number and on the figures and tables. I think this will eventually be a valuable contribution once the results and interpretations are presented more clearly.Major Comments:1. The background information is extensive, and much of the description of the geologic history could be trimmed to focus on what is clearly relevant for this study.2. The methods are very detailed and while this is great for readers to reproduce the analysis, much of this could go into Supporting Information to shorten the manuscript so readers can focus on the key observations and conclusions.3. The reasoning for defining the domains, both from the geomorphic indices and the neotectonic activity regions, should be explained more clearly up front. As written, it is unclear how or why domains are defined, and thus comparing between them and evaluating which may represent possible tectonic activity is challenging. In addition, the difference between the neotectonic domains in Table 3 and the reasoning behind each one should be more clearly explained in the text. For example, what defines a “sinking tectonic block” versus “fault movement”?4. Perhaps my largest issue with the manuscript in the current format is that the results and interpretations are disjointed and hard to follow. In addition, no hypsometric or sinuosity results are mentioned in detail or shown in the main text. The reader cannot evaluate these statements without data or figures to digest. I advise the authors to reformat the section on results and discussion. I suggest to begin with a clear description of the different domains and how they are defined, then discuss key differences between them from the different datasets and how or why certain ones may reflect more recent tectonic activity. Clear explanation of how the observations support interpretations that the geomorphic indices are caused by neotectonics or other factors would greatly benefit the reader and reinforce the main message of the manuscript.5. In general, the manuscript has many broad statements that need to be supported by citations or by showing specific data from this study. See Minor Comments below for some example statements that would benefit from citations.6. While the abstract states that the results may suggest that the increase in earthquakes over the past 2 decades may be natural, it is unclear how the results from this study lead to that conclusion. Consider adding more details to support this interpretation. From my reading of this manuscript, I agree that there are anomalies in the geomorphic indices that may be explained by possible neotectonics, but I cannot clearly see the link as to how that explains that the increased seismicity is linked to neotectonics.7. I encourage the authors to choose some specific examples from their text and create more detailed figures to support some of these statements. For example, some of the neotectonic regions mentioned in the discussion describe very specific landscape features, geology, etc., which are not visible at the scale of the current figures. The reader would greatly benefit from seeing the evidence for possible neotectonic activity in some of these domains more clearly illustrated.8. The broader implications and/or application of these tools or interpretations to other similar regions could be strengthened and expanded. What was learned here that may applied elsewhere? Do these results affect our understanding of seismic hazard in this region, and if so, how? How do the interpretations of these datasets compare with our understanding of the major structures or potentially active faults in the region (described in the background sections), or with InSAR or GNSS studies that may support on-going crustal deformation?Minor Comments:L35: Consider citing the updated 2023 US National Seismic Hazard Map (Petersen et al., 2023).L50-52: Consider citing a geologic map here, or some other reference, to support the statement that there are anomalous changes in Quaternary deposition in the region. What evidence is there from past work to support this statement?L53: Consider adding citations to this broad statement on the use of geomorphic indices to study neotectonics.L77-80: Add citations to support these statements on the age of the Central Plains Orogen.L77-89: In general, there are many statements in here that should have citations to support them, particularly regarding the timing of orogens or other deformational events in the region.L135: Please check for “Error! Reference source not found” and replace.L231-235: These statements require citations.L338-340: These statements seem out of place at the end of this section. Consider moving elsewhere for flow.L350: Please add a citation for this statement that south-facing slopes are subject to greater slope wash erosion.L384: Why is the transverse basin asymmetry index capitalized here and not in the above paragraph? Also, why is both TI and T-index used as abbreviations in this sentence? Are these the same?L444: Please check for “Error! Reference source not found” and replace.L560: Please check for “Error! Reference source not found” and replace.L564-570: More details may be needed here. Why is the z-test more appropriate given the sample size? What sample size is appropriate for a t-test? What do these tests show, in more detail? It is a little unclear what is being tested and what the evaluation criteria are.L576-577: More details are needed for the reader to understand this argument. How does this lack of correlation suggest that lithology alone is not a primary factor?L590: The reader would benefit from an overview of the domains and their main characteristics before details and comparisons between specific domains are discussed. For example, consider beginning with the locations of the domains in the study area, general stream network or lithologic characteristics, and what the geomorphic indices for each domain are. This would allow readers a solid background and understanding of the initial observations and analysis before comparisons between domains are discussed.L599: Consider changing the labels of the domains so that domain names are not repeated. It is confusing that there is a Domain A-E in each state. Perhaps number them or continue using letters, starting in the northwest and continuing towards the southeast?L603: This description of the stream pattern in Domain D would be more helpful if the reader knew how it differed from the stream patterns in other domains. Are they also trellised, or do they have a different pattern?L603-606: Perhaps I missed it, but why does a northwest azimuthal direction indicate neotectonic activity? Consider explaining more clearly for readers who may not easily follow this argument.L625-629: These observations should be annotated on a figure (the linear scarps). They cannot be seen at the scale of Figure 15.L625: These identified neotectonic regions would benefit from an overview and introduction, including how they were defined, before the detailed observations are presented.L654-656: What are the increased sinuosity values? It is hard for the reader to evaluate these statements without specific data to demonstrate the interpretation.L659: What are the lithologic patterns? Consider being more specific to help the reader understand this observation and interpretation.L683-703: This is a well-written and clear paragraph summarizing the key observations and interpretations!Figure 2: Add state labels and latitude/longitude to the figure to help readers better orient themselves.Figure 3: Add latitude/longitude to the figure. Add legend or describe linework in the caption (red lines, black lines with tick marks, black lines, etc.).Figure 6: Where is this cross-section from? Consider showing on a previous figure.Figure 11, Figure 12: Was the analysis performed on a lidar DEM? The text states it was performed on a 10-m DEM (I assume the same NED described in the text on L370), which is not typically a lidar-derived DEM and is instead derived from a variety of source datasets. Please clarify.Figure 14, Figure 15: It is unclear how these domains are defined?Figures A1 & A2: Consider just numbering the subbasins instead of having the label include “sub-basin”, it may make the figure less crowded and easier to read. Also consider increasing the font size or making the river network linework lighter.Figures B1-B13, C1-C9: Explain what the blue dots and red arrow represent in the figure caption.Table 1: Define the mapped fault zones in a footnote. How was the “present” or “none” defined for seismicity?Table 3: The evidence for “possible neotectonic element” is not clearly explained in the text. How do the authors distinguish between a “sinking block” and a “local uplift” and “fault movement”? The defining characteristics for these neotectonic elements are necessary for the reader to understand how the authors arrived at these interpretations.Citation: https://doi.org/
10.5194/egusphere-2025-3205-RC1
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I read the paper and I'm very impressed that these geomorphic indices are detecting landscape features that can be attributed to neotectonism.