the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Apr 2026
Crustal Structure of Java, Indonesia, from Ambient Noise Tomography: Implications for Regional Tectonics
Abstract. Although local scale investigation throughout parts of the java island has been conducted to understand about the subsurface condition, there are only a few tomography approaches to prove the previews geological model. In this study we perform ambient noise tomography (ANT) of entire Java Island to delineate subsurface and the relationship between shear wave velocity and basin structure in relation to tectonic influences. Our study utilize data from 114 permanent seismograph stations operated by The Indonesian Meteorological, Climatological, and Geophysical, and Geophysical Agency (BMKG)of continuous seismic record from January–June 2022 (6 month). We obtain more than 6000 potentials of Empirical Green's function and extract the Rayleigh Group Velocity for the period 3 s to 33 s. We perform manual pick for dispersion curves of 6328 of pair cross correlation to correct any artefact. The two step tomography process begins with group wave velocity maps, then we delineate the shear wave velocity structure beneath Java Island from 3 to 30 km from 217 sampling point 1D inversion. Profile Vs ranges from 1.3–4.1 ± 0.1 km/s with low Vs (1.3–2.2 ± 0.1 km/s) interpreted as basins including Bogor, Kendeng and Banyumas basins. On the other hand, the high Vs (2.2–4.1 ± 0.1 km/s) interpreted as crystalline basement or indicate stronger more rigid materials within the crust. The flexural pattern reveals a distinctive characteristic along Java Island, where the basin orientation shows regional variation. In western Java, the flexure trend indicates northward basin development, whereas in eastern Java the flexural axis shifts southward. In central Java, the transition between these two domains suggests a twisting or rotational deformation of the crust, reflecting a complex mechanical response of the Sunda Block to ongoing subduction processes beneath Java.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6291', Anonymous Referee #1, 02 Jun 2026
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RC2: 'Comment on egusphere-2025-6291', Anonymous Referee #2, 02 Jun 2026
The manuscript submitted focus on the imaging the Vs structure of the Java Island using a large seismometer array that covers the whole island. The authors used ambient noise tomography method to uncover several sedimentary basins and volcanic area, and also subduction geometry beneath the Java Island. While this is a very valuable information, with an already established method, the author fails to demonstrate the method’s application on their dataset. Additionally, there are many grammatical errors, incomplete paragraph and sentences, as well as redundancy in the text. My impression is that the author already have an idea/hypothesis that they want to put through, and tried to force it into the reader. This is fine of course, but repeating the same point over and over again across the manuscript is not as convincing. Some sentences are likely put in the paragraph just for the sake of it. However, I believe the dataset holds a very valuable insights to the subsurface structure of the Java Island. Therefore, I would suggest a major revision to this manuscript.
Below the authors can find my specific major comments, with the specific comments included in the annotated pdf file.
Major comments:
- Methodology
Reading the manuscript, I have a hard time following the methodology that the authors applied here. First of all, I think the author should detail their processing steps. Adding a flowchart of the workflow will go a long way to help the reader understand the authors processing steps. Additionally, many of the important processing parameters are not properly listed. For example, in section 3.1, the authors describe the cross-correlation method. However, things like the pre-processing steps (e.g., normalization, whitening, filtering) were not included in the text. These are important details that could affect the quality of the retrieved empirical Green’s function.
While they showed a picture of raypath density (Figure 3), the authors also did not specify the kind tomographic approach they used for the ray tracing. From the figure it looks like the author used a straight ray approach, but this should be specified in the text. Again, this is an important detail that could affect the robustness of the final Vs model presented.
- Structure of the manuscript
I have to reiterate again that I am having a hard time reading the manuscript due to the manuscript having redundancies , and unclear structure on how the authors are describing the methodology. The sequence of the processing steps are also looking out of place. For example, the authors between line 135 – 140 tried to describe the process of dispersion curve picking. However, it is not until line 145 – 152 the process to transform the EGF to frequency – group velocity are explained. This is only an example of how things are not well structured. Therefore, as I mentioned above, I would suggest the authors to include a flowchart, and restructure the manuscript, especially the methods section. - Inversion result
The authors describe a two-step inversion approach which I am unfamiliar with and no reference was given (Section 2.2). I believe the common approach to invert Group velocity dispersion curve is to deploy stochastic approach such as Neighbourhood Algorithm or Bayesian Inversion. These approaches are able to tackle the high dimensionality problem that the dispersion curve modelling came with. From the steps that the author described, it appears that the presented inversion is a linear inversion. How well does this approach work then in a highly non-unique and high dimensional inverse problem? The initial model used for the 2-steps inversion is also a very general AK135 model. The authors unfortunately gave no reference to this method so I cannot look into it in more detail. Many of the important inversion parameters are also missing. If the author persist on using a linear inversion approach, then an RMS evolution plot will be much appreciated. - Interpretation
This is where many of the redundancies can be found. The author mentioned many times that the observed model shows the Bogor, Banyumas, and Kendeng Basins. For instance, between line 205 and 235, this is already mentioned 3 times. While this is a good point to make from the observations, the authors mentioned this several times across the manuscript. I think it will be better for the manuscript if the authors can rearrange the manuscript so that this interpretation can be explained in length in one section rather than mentioning it in many paragraphs and make the sentences look out of place.
The authors also suggest several subduction model, and I have quite a hard time following the model the authors are trying to describe (in parts due to the incomplete sentences, and bad grammar). I would suggest the author to include a figure that shows the subduction geometry that they proposed. This will help the reader to understand the authors’ intention, and also helps the authors in describing their observations.
Furthermore, my impression is that the authors have a theory in mind of how the geometry look, and then focus on this theory to describe the model. For example, between line 246 and 248, the authors tried to summarize their findings. However, there are not many evidence presented in this manuscript that would support this exact sentence. Maybe I am missing many of the points the authors are trying to convey, but this is exactly why the manuscript needs to expand on this a bit more, first by fixing the structure, and also expanding on how they came to the conclusion at the end. I think the findings presented here carry many overinterpretation and bias to the authors initial idea.
- Methodology
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- 1
This study uses continuous seismic data recorded by 114 permanent BMKG stations across Java and its surrounding areas to image the crustal S-wave velocity structure (3–30 km depth) using ambient noise tomography. Based on the resulting velocity model, the authors primarily discuss the deep geometry of several sedimentary basins on Java and further relate these features to deep structural variations and along-strike heterogeneity in the subduction process. This is likely one of the most spatially comprehensive studies conducted in this region, and I recognize its potential publication value. However, the current manuscript has substantial shortcomings in language quality (missing predicates, inappropriate voice, and incomplete sentence structures occur throughout the paper), data processing and result presentation, as well as geologic interpretation. Therefore, I think the manuscript requires at least major revision before it can be reconsidered for publication. The following are my comments and suggestions.
Major Comments
(1) Language and Writing Quality
The authors should substantially improve the English throughout the manuscript. For example, the use of “present” in Line 66, “has beneficial revealed” in Line 73, and “sub surface profile” in Line 85 are inappropriate. In particular, Lines 100–125 contain numerous grammatical errors and awkward expressions. Although the scientific content is generally understandable, there are frequent problems including missing predicates, incorrect article usage, singular–plural inconsistencies, and incomplete sentence structures. A thorough language revision is necessary to improve the clarity and readability of the manuscript.
(2) Reliability of the Tomographic Results
Based on the currently presented data and figures, the reliability of the results is not sufficiently demonstrated.
Although Figure 4a shows the expected “V”-shaped cross-correlation functions, the lack of axis labels and scale information prevents readers from evaluating the initial group-velocity measurements. Furthermore, Figure 4d indicates that some outliers were manually removed, yet similar anomalous measurements still appear in the group-velocity maps (Figure 5). It is therefore unclear what dataset was ultimately used for inversion. This raises concerns regarding the reliability of the final results.
In addition, the authors employ a two-step inversion procedure, and both steps appear to be linear inversions. Important inversion parameters, such as damping and smoothing factors, are not described. How were these parameters selected? What is the distribution of misfits between the observed dispersion curves and those predicted by both the starting and final models? Is the velocity reduction near 39 km depth in Figure 7c a genuine feature? How sensitive is the dispersion fit to this anomaly?
These issues should be addressed through additional analyses and documentation.
Moreover, I strongly encourage the authors to make the cross-correlation functions publicly available rather than only releasing the final structural model. The former are essential for reproducibility and independent validation of the study.
(3) Geologic Interpretation
A) Lines 229–239: The authors argue that there is a significant difference in lithospheric flexural response between eastern and western Java. Is there any independent geologic or surface observational evidence supporting this interpretation?
B) Lines 234–236: The authors state: “This southward orientation suggests a distinct mechanical coupling and stress regime between the overriding Sunda Plate and the subducting slab, consistent with how the subduction system in this region has evolved over geological time.” How exactly has the subduction system evolved through geological time in this region? This statement is too general and requires supporting evidence and discussion.
C) Lines 241–248: Is the proposed segmentation of the deep structure supported by independent geophysical or geological observations, or is it inferred solely from the shallow Vs variations imaged in this study? If the former, appropriate references and observational evidence should be provided. If the latter, the interpretation appears speculative and may represent an overinterpretation of the results.
Minor Comments
(4) Please check the legend of the basemap in Figure 1. It does not appear to be written in proper English, and the logical relationships among the legend items are unclear.
(5) Lines 54–55: What exactly is meant by “seismic wave data”? Does this refer to travel times, dispersion measurements, or another type of seismic observation? Likewise, “at intermediate to greater depths” is vague and only provides a relative description. Please specify the depth range explicitly.
(6) Lines 60–68: I suggest marking the locations, and preferably the boundaries, of major basins such as the North Java Basin and Kendeng Basin in Figures 1 and 2.
(7) In the final sentence of the Introduction, the authors state that they are interested in geological structures such as volcanic arcs and major faults. However, both the Abstract and the Discussion focus almost exclusively on sedimentary basins, while the influence of volcanic and fault systems is largely ignored. This inconsistency is somewhat puzzling and should be clarified.
(8) Lines 90–91: Please label Sumatra, Java, Madura, and Bali Islands in Figures 1 and 2.
(9) Figure 2 caption: Where is the inset map referred to in the caption?
(10) Line 120: What do lambda, CC, and SNR represent? These abbreviations are not defined. Undefined terminology is not reader-friendly, particularly for non-specialists.
(11) Lines 154–156: How were group velocities converted to phase velocities? Does this involve an additional inversion procedure? More methodological details are needed.
(12) Figure 8 is difficult to interpret because the current axis scales do not allow readers to determine the grid dimensions. In addition, the inset appears identical to Figure 5. What additional information does it provide?
(13) Figure 11: The velocity cross-sections lack a color scale. A color bar should be added.
(14) On what basis do the authors define the 2.2 km/s contour as the base of the sedimentary layer? Please provide supporting references.