Dear Giovanni Martinelli

Thank you for your recognition of our work and valuable suggestions, which are very helpful for us to improve the quality of our manuscripts. Your two comments are exactly where we are lacking. At your suggestion, we plan to add a subsection to the discussion section for assessing the contribution of mantle degassing to EAFZ geothermal fluids. The supplementary content is as follows:

Contribution of Mantle Degassing to EAFZ Geothermal Fluids

Mantle degassing occurs extensively along fault zones, and the amount of volatile release can sometimes be comparable to the degassing associated with volcanic activity e.g. (Fischer and Aiuppa, 2020; Zhang et al., 2021). Sulfur-containing volatiles (such as SO₂ and H₂S) ascend along these fault zones and, upon reaching the shallow subsurface, mix with groundwater, where they are oxidized and migrate in the form of SO₄^2- in geothermal fluids. Therefore, the contribution of mantle degassing to the SO₄^2- content in geothermal fluids cannot be overlooked. To better assess the contribution of mantle degassing to SO₄ in EAFZ geothermal fluids, we need to consider the sources and modifications of geothermal fluids.

The deep-origin geothermal fluids in EAFZ are significantly diluted by shallow groundwater, masking the chemical signature of deeper fluid components. This dilution process introduces a large amount of dissolved oxygen, which facilitates the oxidation of H₂S to SO₄^2-. Lacking O₂ was detected in EAFZ geothermal gases suggested that the dissolved oxygen may have been consumed (Italiano et al., 2013; Yuce et al., 2014). However, it is important to note that H₂S, H₂, and CH₄ can all react with oxygen. Thermodynamic calculations indicate that CH₄ is more favorable than H₂S in oxidation reactions (ΔG° CH₄ = -818.1 kJ/mol, ΔG° H₂S = -494.2 kJ/mol, at 298 K and 1atm). In actual geothermal systems, however, the depletion of H₂S is more commonly observed than the depletion of CH₄, suggesting that H₂S may be oxidized before CH₄. To resolve this apparent contradiction, we propose the following possible explanations: 1) Oxidation of H₂S: While thermodynamic calculations predict CH₄ oxidation first, a small amount of H₂S might still be oxidized simultaneously with CH₄. Due to the much lower concentration of H₂S in geothermal systems compared to CH₄, H₂S is consumed more quickly, leaving CH₄ with a higher residual concentration. 2) Exogenous CH₄ Supply: In addition to mantle-derived CH₄, other sources of CH₄, such as biogenic CH₄ and thermogenic CH₄ (e.g., serpentinization), may contribute to the geothermal system. These external sources could increase the concentration of CH₄ in the geothermal fluids.

In the EAFZ, we observed significant contributions of biogenic and serpentinization-derived CH₄ but did not detect significant levels of H₂S (Italiano et al., 2013; Yuce et al., 2014). Therefore, we proposed that although H₂S may contribute to the geothermal system, its impact is likely limited due to its relatively low concentration. Inversely, the notable increase in SO₄^2- concentrations following seismic events is likely primarily controlled by the dissolution of shallow evaporitic layers (such as gypsum). All in all, while the oxidation of H₂S may contribute to SO₄^2- formation, distinguishing between H₂S oxidation and sulfate dissolution requires additional geochemical indicators, such as S isotopes and Ca isotopes, for more accurate assessments.

References

Fischer, T. P. and Aiuppa, A.: AGU Centennial Grand Challenge: Volcanoes and Deep Carbon Global CO₂ Emissions From Subaerial Volcanism-Recent Progress and Future Challenges, Geochemistry Geophysics Geosystems, 21, 2020.

Italiano, F., Sasmaz, A., Yuce, G., and Okan, O. O.: Thermal fluids along the East Anatolian Fault Zone (EAFZ): Geochemical features and relationships with the tectonic setting, Chemical Geology, 339, 103-114, 2013.

Yuce, G., Italiano, F., D'Alessandro, W., Yalcin, T. H., Yasin, D. U., Gulbay, A. H., Ozyurt, N. N., Rojay, B., Karabacak, V., Bellomo, S., Brusca, L., Yang, T., Fu, C. C., Lai, C. W., Ozacar, A., and Walia, V.: Origin and interactions of fluids circulating over the Amik Basin (Hatay, Turkey) and relationships with the hydrologic, geologic and tectonic settings, Chemical Geology, 388, 23-39, 2014.

Zhang, M., Xu, S., Zhou, X., Caracausi, A., Sano, Y., Guo, Z., Zheng, G., Lang, Y.-C., and Liu, C.-Q.: Deciphering a mantle degassing transect related with India-Asia continental convergence from the perspective of volatile origin and outgassing, Geochimica Et Cosmochimica Acta, 310, 61-78, 2021.

Dear Walter D'Alessandro

Thank you for your highly professional and constructive comments and suggestions, which are of great value to us in improving the quality of our manuscript. After carefully reading your comments, we have made a reply to your comments point-by-point under the discussion of all manuscript authors. The main replies are as follows:

Major revisions include:

1. Correction of sample collection time: We apologize for marking the wrong sampling time in Table 1 (marked time, March 2024, the actual sampling time, March 2023). The wrong timing brings huge ambiguity to the manuscript. After correcting the sampling time, the main line logic of the article is as follows: These evidences constitute a complete chain of causality from the source (evaporite) to the process (water-rock reaction balance disrupted by the earthquake) to the response (abnormal groundwater ion concentration).

1. Use "groundwater" instead of "geothermal water" to define the sample in this study. We collected 16 groundwater samples from SF and EAFZ within a month of the earthquake. The principle of sample collection is to collect if we can. Because the overall temperature is low, we think it is more reasonable to use "groundwater" instead of "geothermal water".
2. We have given a complete explanation of the pre-earthquake hydrochemical data in the manuscript.
3. We supplement the analysis method and data quality control description
4. We rearranged the logic of the article to make the expression clearer
5. We have made a full explanation of some misunderstandings
6. We explain the possible “overestimation of the heat storage temperature” and analyze that the heat storage temperature estimate has little effect on the conclusion of our core conclusions.
7. We plan to conduct additional experiments on the samples, including radioactive Sr isotopes and S isotopes, to support our argument with more evidence.

Since there are diagrams in the complete reply draft, we put the complete reply draft in the form of an attachment on the website system. If you have any questions or suggestions about the manuscript, we sincerely invite you to keep discussing with us. Thank you for constructive review comments.

Thank you and best regards.

Dear Hafidha Khebizi

Thank you for your recognition of our work and constructive suggestions. This is very helpful for us to improve the quality of the manuscript, and also brings confidence for us to continue to explore. Thank you for sharing the very rewarding work you do. We get a lot of inspiration from your work. We would like to express my heartfelt thanks.

We've responded to each of your comments, as detailed below:

Note: Italic blue is the comment. Black is the reply.

Dear authors and colleagues of the scientific community,

I congratulate the authors for their interesting work entitled Gypsum as a potential tracer of Earthquakes: a case study of the Mw7.8 2 earthquake in the East Anatolian Fault Zone, southeastern Turkey, and I hope it will be published soon. To find out the relationship between geothermal fluid anomalies and earthquakes, the authors performed a systematic hydrogeochemistry and isotopic analysis of the geothermal fluids in the East Anatolian Fault Zone (EAFZ). The results show that earthquakes reconstructed these geothermal fluids.

Reply: Thank you for your recognition of our work. Thank you.

Considering gypsum as an earthquake tracer is excellent reasoning for analysing the impact of anomalies after the earthquake, and the work could be a great reference for future studies related to the earthquake.

Reply: Yes, through the analysis of groundwater after the earthquake, we discovered the potential value of gypsum as an earthquake warning. It is hoped that this work will attract the attention of more researchers and colleagues, and incubate more meaningful achievement.

To enrich this excellent analysis, I have some remarks concerning the implication of macroscopic and microscopic aspects of geothermal fluids before and after the earthquake, notably the relation with the structural geology of the region. For this, some questions seem important to be asked.

First, from a macroscopic point of view, it is necessary to understand, in the normal case (before the earthquake), from a geological point of view, if the existing deformations (faults) already have effective structures for the infiltration of meteorological waters and the implication of the disposition of the thermal springers according to the faults. After the earthquake, is there any sampling from Miocene groundwater and soil? Is there recent salt precipitation in the Miocene and upper Eocene-Oligocene soil and/or in the soil of the surrounding springer sources? Is there a rise in the ground level due to fault action, and are there marine intrusions that occurred after the strike-slip? Is there significant contamination of the water table (increased electrical conductivity)?

Reply: Hot springs and fault zones are often associated. Hot springs are considered as one of the potential means of earthquake warning. A large number of research results have been published in Japan, the United States, Iceland, Spain, China, Turkey... ... In EAFZ, many hot springs have been systematically studied, and the results show that these hot springs contain material supply from deep crust and even mantle. Therefore, it is highly possible to obtain valuable information by conducting post-earthquake hydrochemical and isotopic analyses of these hot springs.

Unfortunately, we only collected water samples after the earthquake and did not analyze soil samples. Your comment is a very good suggestion, reminding us that detailed analysis of surrounding rock may be needed in future work. Thank you.

Salt precipitation and electrical conductivity (EC). Before we can answer your question, we need to explain an error in the manuscript. Our sample was taken in March 2023 (within one month after the earthquake). In the video 1 we provided, the macro abnormal changes of HS14 were diluted by the adjacent stream, coupled with the fact that the samples were taken within one month after the earthquake and no soil samples were collected, we could not accurately determine whether salt precipitation existed. By comparing the EC of the same hot spring during the seismically quiet period and the seismically active period, we found that the EC of HS14 increased slightly (varying from 990 to 1305). Data of EC pre-earthquake from Yuce, G., Italiano, F., D'Alessandro, W., Yalcin, T. H., Yasin, D. U., Gulbay, A. H., Ozyurt, N. N., Rojay, B., Karabacak, V., Bellomo, S., Brusca, L., Yang, T., Fu, C. C., Lai, C. W., Ozacar, A., and Walia, V.: Origin and interactions of fluids circulating over the Amik Basin (Hatay, Turkey) and relationships with the hydrologic, geologic and tectonic settings, Chemical Geology, 388, 23-39, 2014.

Seawater intrusion was evident after the earthquake. Na⁺ and Cl⁻ of HS14, HS15 and HS16 increased significantly, indicating the possible existence of seawater intrusion (Fig. 6 manuscript).

Rise in the ground level due to fault action is common. We have made a detailed study on the post-earthquake surface rupture and post-earthquake risk analysis. Article link：Liang, P., Xu, Y., Zhou, X., Li, Y., Tian, Q., Zhang, H., Ren, Z., Yu, J., Li, C., Gong, Z., Wang, S., Dou, A., Ma, Z., and Li, J.:  Coseismic surface ruptures of MW7.8 and MW7.5 earthquakes occurred on February 6, 2023, and seismic hazard assessment of the East Anatolian Fault Zone, Southeastern Turkiye, Science China Earth Sciences, doi: 10.1007/s11430-024-1457-7, 2024.

Screenshot from Liang et al., 2024 doi: 10.1007/s11430-024-1457-7 (If the picture cannot be displayed, please check it in the attachment, thank you).

From a microscopic point of view, gypsum is easily and quickly influenced by contact with water, thanks to its physicochemical characteristics, in particular its very high dissolution rate and its solubility in water that make it an excellent tracer of hydrochemical anomaly but also a tracer of lithological instability (Khebizi et al., 2022; Khebizi et al., 2023). For this, I am pleased to invite you to read the part concerning the gypsum implication on the lithological instability in my article published in Larhyss Journal and my oral communications, which expose, for the first time in Algeria, a new concept of the lithological vulnerability of the subsurface. Although the study areas differ, the analysis presented in my work shows the indication of gypsum dissolution at the regional scale as an excellent major risk indicator. The lithological vulnerability of the subsurface concept can be applied to different situations around the world, notably the case of earthquakes. It highlights the hydrodynamic anomalies' relation with the structural and geological context of the area to be studied. 

Thank you very much for your sharing. It's a fantastic set of work. From my personal point of view, I can't agree with you more. Gypsum's very high dissolution rate and solubility in water can be used for risk warning of earthquakes and geological disasters. Thank you again for your information. Your work gives us great encouragement and confidence.

Second, if there is a remarkable increase in calcium concentration in water after the earthquake, how do you explain the reaction of carbonate dissolution and the origin of CO2? Is it linked to magmatic activity? In this case, is there a signature of other gases on other cations? Or is it only related to carbonate since the calcite dissolution is linked to the mineral’s surface to be in direct contact with water?

In my opinion, Ca may come from carbonate or igneous rocks. In order to accurately restrict the source area of Ca, we are also considering introducing Ca isotopes to distinguish its sources. Ca isotopes in carbonate rocks are lighter than those in igneous rocks and mantle. Ca isotope has a good potential in the source region that restricts Ca.

The index of CO₂ source region is very mature. Geothermal gases are well studied at EAFZ. The C isotope study of CO₂ shows that CO₂ is controlled by deep carbon and inorganic carbonate (−5.6 to −0.2‰) (Italiano, F., Sasmaz, A., Yuce, G., and Okan, O. O.: Thermal fluids along the East Anatolian Fault Zone (EAFZ): Geochemical features and relationships with the tectonic setting, Chemical Geology, 339, 103-114, 2013.). He isotope analysis also shows a large proportion of the mantle.

Explanation of the specific process: gypsum dissolution and carbonate dissolution are together. In the manuscript, PHREEQC was used to simulate the water-rock reaction process (Fig. 7). The results show that gypsum dissolution alone is not enough to explain the Ca content in the samples, indicating that calcite and other minerals are involved in the water-rock reaction. Combined with previous studies, we believe that CO₂ from deep water is first dissolved in water, and then reacts with gypsum or calcite. CO₂ is associated with magma, but does not form volcanic eruptions and may only exist in deep areas of partial melting.

Allow me to add that the underground water circulation, which is controlled by faults and hydraulic parameters (permeability), determines water-rock equilibrium. In this case, water-rock equilibrium depends on the host rock spatial disposition of rock that guides water mineralization and the different processes. Consequentially, the water-rock equilibrium changes from one area to another due to changes in water mineralization according to the host rock lithology. For this, the information that can be taken from the geological map is that springer’s water is related to ophiolite rocks. So, I think water geochemistry indicates similar water-rock interactions for all sources. However, a mineral’s enrichment zoning can occur due to (i) the meteorological conditions, (ii) the proximity of the springer water from seawater, and/or (iii) the distance from the upstream. The earthquake reconstructed these geothermal fluids depending on the energy released which controls hydrothermal circulation and amplifies interactions with the surrounding environment whether at depth or on the surface.  For this, vulnerability zoning in a horizontal and vertical direction can be done according to chemical variation, notably gypsum and probably halite enrichment. It can be indicated as shown in Fig. 8.

I can't agree with you more. Water-rock reaction is affected by meteorology, rock properties, permeability, porosity, temperature, pressure... Multiple factors control. At present, our work is limited to the analysis of water chemistry and isotopes, and there is a lot of work to be done in the future. These works involve not only geochemistry, but also rock mechanics, numerical simulation and other interdisciplinary fields, and we hope to have more like-minded colleagues to explore together.

Earthquake warning is the most difficult problem faced by mankind. Groundwater is considered as one of the means to explore earthquake early warning. However, groundwater in its natural environment is very complex. There is still a long way to go to explore the relationship between groundwater and earthquakes.

Finally, the discussion on this topic is very significant, and the structural and lithological vulnerability and their tracers after the earthquake using vulnerability mapping of the Turkey earthquake seems very interesting for future work.

Thank you for your recognition of our work, your recognition is our driving force forward. Sincere thanks and best wishes.