Co- and post-seismic Deformation Mechanisms of the M_W 7.3 Iran Earthquake (2017) Revealed by Sentinel-1 InSAR Observations

Round 1

Reviewer 1 Report

Dear authors,

Here to you my suggestions:

 - lines 173-175, I think you should specify here what do you mean for direct solution.

- Figure 4, please provide in the caption the meaning of the positive and negative values for each component of the displacement. E.g., if positive in the EW component means movement toward east. I would also suggest to add the a,b,c letters for each sub-figure.

 - line 213, I suggest to write 'We selected eight dates'

 - Section 5.1 and 5.2 must go before section 4.3 as they refer to the co-seismic deformation associated to the earthquake so to be coherent with the discussions. The same applies to the Conclusions.

Author Response

Response to Reviewer 1 Comments

Dear Reviewer：

We thank so much for the constructive and helpful comments. We have made corresponding changes based on your suggestions. The followings are the detailed responses to the comments and suggestions.

Best regards

Authors

Point 1: lines 173-175, I think you should specify here what do you mean for direct solution.

Response 1: Thank you for your constructive suggestion. We have rewritten the section 4.1.2 (lines 169-201) in the revised manuscript according to your suggestion. In order to specify the mean for direct solution, the following sentence was inserted.:

“The direct solution only requires the inverse of Equation (4). Owing to the side-looking geometry of the SAR sensor and its near-polar orbiting satellite platform, InSAR measurements are extremely insensitive to the north-south component; error amplification can be found in the combination of multiple InSAR measurements from different viewing geometries, and the most exaggerated errors are in the north-south component [27].” (lines 184-189 in the revised manuscript)

Point 2: Figure 4, please provide in the caption the meaning of the positive and negative values for each component of the displacement. E.g., if positive in the EW component means movement toward east. I would also suggest to add the a,b,c letters for each sub-figure.

Response 2: Thank you for your valuable advice. We have changed the caption of Figure 4 as following, and the ‘a, b, c’ letters were added for the sub-figure.

“Figure 4. Interferograms of 3D co-seismic deformation. (a) EW direction, positive and negative values represent eastward and westward movements, respectively. (b) Up-Down (UD) direction, positive and negative values represent uplift and subsidence movements, respectively. (c) NS direction, positive and negative values represent northward and southward movements, respectively. Black lines are faults; red stars represent the epicenter.”

Point 3: line 213, I suggest to write 'We selected eight dates'

Response 3: Thank you for your careful read. We have replaced “We selected eight epochs” with “We selected eight dates” in the text.

Point 4: Section 5.1 and 5.2 must go before section 4.3 as they refer to the co-seismic deformation associated to the earthquake so to be coherent with the discussions. The same applies to the Conclusions.

Response 4: Thank you for your constructive suggestion. We have adjusted the structure in the revised manuscript according to your suggestions. Headings were listed in the following:

4.1 Co-seismic Deformation Field

4.1.1 LOS Co-seismic Deformation Field

4.1.2. Three-dimensional Co-seismic Deformation Field

4.2 Fault Geometry and Slip Distribution

4.2.1 Uniform Slip Model

4.2.2 Distributed Slip Model

4.3 Two-dimensional Post-seismic Deformation Time Series

As well, we have adjusted the conclusions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Although this article describes a standard approach to measure and model the co-seismic deformation pattern induced by a big earthquake, which it is already seen in many other similar studies, it shows some interesting results on the MW 7.3 Iran earthquake that deserve publication on RS after a minor revision.

My main comments and suggestions are:

·       In the introduction the authors must declare and describe what has already been done about this earthquake, what has not yet been understood and what will concern the topic of their article. This will permit readers to immediately understand the features of novelty of this article;

·       Line 38: Add the citation of the fig.1, to show the location on the map of the earthquake;

·       Line 41: Delete ‘focal mechanism’. The surface deformation is not directly, related to the focal mechanism;

·       Lines 46, 51, 57: Replace ‘scholars’ with other terms (eg researchers or similar);

·       Line 47: Replace ‘deformation fields’ with ’surface displacement’;

·       Lines 52-54: ‘indicates that it is an oblique thrust slip’: this sentence is not correct, because it is not possible to discriminate the type of faulting from dip and strike, you need the rake to determine it.

·       Line 79: the aftershocks do not seem so distributed along the MFF;

·       Line 148: Replace ‘interference pairs’ with interferograms’;

·       Figure 8: the panel ‘c’ must report a different color bar from the data and model panels, otherwise its values are to be too much oversaturated and the resulting values are not perceptible.

Author Response

Dear Reviewer：

We thank so much for the constructive and helpful comments. We have made corresponding changes based on your suggestions. The followings are the detailed responses to the comments and suggestions.

Best regards

Authors

Point 1: In the introduction the authors must declare and describe what has already been done about this earthquake, what has not yet been understood and what will concern the topic of their article. This will permit readers to immediately understand the features of novelty of this article;

Response 1: Thank you for your valuable suggestion.

To help readers better understand our work, the following sentences were inserted into the revised manuscript:

“Although previous studies have monitored co-seismic deformation and inverted the co-seismic source parameters, they have significant differences in whether there are one or two slip centers in the earthquake.” (lines 50-52 in the revised manuscript)

“Post-seismic deformation carries key information for understanding stress increases induced by the mainshock and the mechanism of aftershock triggering. However, few studies have focused on the post-seismic deformation and relationship between the mainshock and aftershocks.” (lines 55-57 in the revised manuscript)

Point 2: Line 38: Add the citation of the fig.1, to show the location on the map of the earthquake;

Response 2: Thank you for your suggestion. I have made corresponding correction and redrawn the Figure.1.

Point 3: Line 41: Delete ‘focal mechanism’. The surface deformation is not directly, related to the focal mechanism;

Response 3: Thank you for your careful read. “focal mechanism” has been removed from that sentence.

Point 4: Lines 46, 51, 57: Replace ‘scholars’ with other terms (eg researchers or similar);

Response 4: Thank you for your advice. We have replaced “scholars” with “researchers” in revised manuscript.

Point 5: Line 47: Replace ‘deformation fields’ with ’surface displacement’;

Response 5: Thank you for your suggestion. We have replaced “deformation fields” with “surface displacement” in the corresponding position.

Point 6: Lines 52-54: ‘indicates that it is an oblique thrust slip’: this sentence is not correct, because it is not possible to discriminate the type of faulting from dip and strike, you need the rake to determine it.

Response 6: Thank you for your constructive suggestion. We have rewritten the sentence at lines 50-52, and removed “indicates that it is an oblique thrust slip” from the text:

“Different researchers have presented solutions for the focal mechanism, as listed in Table 1 [9–11]. Although previous studies have monitored co-seismic deformation and inverted the co-seismic source parameters, they have significant differences in whether there are one or two slip centers in the earthquake.”

Point 7: Line 79: the aftershocks do not seem so distributed along the MFF;

Response 7: Thank you for your advice. Considering that the existence of this sentence is not necessary, we have removed the following sentence from the text.

“As shown in Figure 1, most of the aftershocks were distributed along the MFF. We assumed that the Iran earthquake may have been triggered by the movement of the MFF based on the high-precision relocation of the aftershocks.”

Point 8: Line 148: Replace ‘interference pairs’ with interferograms’;

Response 8: Thank you for your suggestion. We have replaced “interference pairs” with “interferograms” in the corresponding position.

Point 9: Figure 8: the panel ‘c’ must report a different color bar from the data and model panels, otherwise its values are to be too much oversaturated and the resulting values are not perceptible.

Response 9: Thank you for your valuable suggestion. I have redrawn the Figures.8 and 9 with different color bar. Due to the structure in the revised manuscript was adjusted, Figure 8 and Figure 9 was renamed Figure 5 and Figure 6, respectively. See Figures.5 and 6 in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript investigates coseismic and postseismic deformation of the 2017 Iran-Iraq earthquake by Synthetic Aperture Radar Interferometry. Given that there are already a few previous studies on the coseismic deformation of this earthquake (Feng et al., 2018; Vajedian et al., 2018; Wang et al., 2018; Ding et al., GJI, 2018, of which Ding et al. is not cited in the manuscript), the main originality of this work should stem from the investigation of postseismic deformation. However, the authors concentrate on coseismic deformation, making it difficult to find originality of this work. Here is some suggestion that may make the work stronger. Addressing these comments will require a substantial reorganization of the manuscript. 

1. As mentioned above, given that a few previous studies on the coseismic deformation already exist, this study should focus more on postseismic deformation. The authors should clearly demonstrate what insights can be added in this study on the coseismic deformation. Otherwise, the coseismic deformation part must be omitted. 

2. Related to the comment above, the argument on postseismic deformation should be expanded. For example, Section 4.3 should be expanded to explain why poroelastic rebound and viscoelastic relaxation are excluded for the primary driver of the postseismic deformation. If the authors consider that the afterslip is the primary driver for the postseismic deformation, the distribution of postseismic slip and possibly its temporal evolution should be presented to compare with the coseismic slip distribution and the evolution of aftershocks. 

3. Figure 1: Aftershock distribution should be shown by a more blown-up view. 

4. An extensive language editing seems to be required. I did not attempt to edit the wording because I suppose it will be done by the copy editing office of the journal. 

Author Response

Dear Reviewer：

We thank so much for the constructive and helpful comments, which will greatly improve my work. We have made corresponding changes based on your suggestions. The followings are the detailed responses to the comments and suggestions.

Best regards

Authors

Point 1: As mentioned above, given that a few previous studies on the coseismic deformation already exist, this study should focus more on postseismic deformation. The authors should clearly demonstrate what insights can be added in this study on the coseismic deformation. Otherwise, the coseismic deformation part must be omitted.

Response 1: Thank you for your constructive suggestion. There are two reasons why we study on the co-seismic deformation. First, we required co-seismic deformation to study the relationship between co- and post-seismic displacements. Second, previous studies still have significant differences in whether there are one or two slip centers for the earthquake.

The following sentences were inserted in the revised manuscript:

“Different researchers have presented solutions for the focal mechanism, as listed in Table 1 [9–11]. Although previous studies have monitored co-seismic deformation and inverted the co-seismic source parameters, they have significant differences in whether there are one or two slip centers in the earthquake.” (lines 49-52 in the revised manuscript)

“Our slip model implies the presence of two asperities in the shallow and deep subsurface; this is compatible with the result of Feng et al. and Ding et al. [9,11]. In contrast, the model proposed by Vajedian et al. [10] implies a single centralized slipping. It was worth noting that the slip model of the USGS was altered from single centralized slipping to two asperities (last updated 2018-10-17).” (lines 287-291 in the revised manuscript)

Point 2: Related to the comment above, the argument on postseismic deformation should be expanded. For example, Section 4.3 should be expanded to explain why poroelastic rebound and viscoelastic relaxation are excluded for the primary driver of the postseismic deformation. If the authors consider that the afterslip is the primary driver for the postseismic deformation, the distribution of postseismic slip and possibly its temporal evolution should be presented to compare with the coseismic slip distribution and the evolution of aftershocks.

Response 2: Thank you for your valuable suggestion.

Figure 11. shows the relationship between the co- and post-seismic displacement and the temporal evolution of afterslip and aftershocks of the Iran earthquake are consistent.

As for the distribution of post-seismic slip and possibly its temporal evolution, we have done some described in the manuscript. For example:

“The time series of the 2D post-seismic deformation showed the same trend as the 3D co-seismic deformation in the UD and EW directions (Figures 4(a), 4(b), 9, and 10).”, see lines 315-316 in the revised manuscript.

“Our results show that 3D co-seismic deformation was distributed on two sides of the epicenter in the UD and EW directions (Figure 4), while post-seismic deformation was southwest of the epicenter (Figures 9 and 10).”, see lines 405-408 in the revised manuscript.

Besides, we have adjusted the structure in the revised manuscript. Section 5.1 and 5.2 have gone before section 4.3. And the following sentences were inserted in the revised manuscript.

“Post-seismic deformation following large earthquakes represents the redistribution of co-seismic stress changes in host rocks. Thus, continued study and geodetic observations are necessary to understand the gestation of earthquakes and the detailed post-seismic deformation behaviors. In this study, we obtained the surface deformation within 220 days of the mainshock and investigated the spatial pattern of aftershocks, which is an important prerequisite for further understanding the nature of the causative fault.”, see lines 301-306 in the revised manuscript.

“Viscoelastic relaxation has a minor effect on near-field post-seismic displacement, especially in the short term [37]. Therefore, the effect of viscoelastic relaxation was not considered in this study.”, see lines 309-311 in the revised manuscript.

“This similarity of co- and post-seismic interferograms suggests similar subsurface processes during both periods; specifically, it suggested that the dominant post-seismic process over the observation period was afterslip. As the same time, the references of [9] show that the poroelastic rebound had a minor effect for the Iran earthquake. Both co- and post-seismic deformation show two predominant patches. The post-seismic deformation steadily migrated to the southwest.”, see lines 316-321 in the revised manuscript.

“In order to reveal the relationship between afterslip and aftershocks, we collected the aftershock dataset (MW > 3.0) up to 220 days after the mainshock (from http://irsc.ut.ac.ir/bulletin.php).”, see lines 328-330 in the revised manuscript.

“Conjoint analysis of co- and post-seismic displacement (Figure 11; divided by the 7 days after the mainshock) showed that post-seismic displacement had the same direction of motion as the co-seismic displacement. The rate of afterslip was highest immediately after the mainshock, and then decreased with time. The cumulative deformation increased gradually. Together, these observations supported the scenario that afterslip on the fault plane was the most likely mechanism responsible for the near-field post-seismic deformation, rather than poroelastic rebound.”, see lines 363-368 in the revised manuscript.

 “The temporal evolution of afterslip and aftershocks of the Iran earthquake are consistent. With the deceleration of afterslip, the number of aftershocks fell, indicating a correlation between afterslip and the number of aftershocks.”, see lines 372-375 in the revised manuscript.

Point 3: Figure 1: Aftershock distribution should be shown by a more blown-up view.

Response 3: Thank you for your valuable suggestion. I have redrawn the Figure 1., showing in accessory.

Point 4: An extensive language editing seems to be required. I did not attempt to edit the wording because I suppose it will be done by the copy editing office of the journal.

Response 4: Thank you for your advice. The revised manuscript has undergone English language editing by Editage. The text has been checked for correct use of grammar and common technical terms, and edited to a level suitable for reporting research in a scholarly journal. A certificate of English language editing is shown in accessory.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

I saw a great improvement of this manuscript in response to the comments from the previous version. I am satisfied with most of the part, but I still have one concern. The authors seem to claim that this study offers a state-of-the-art coseismic slip distribution of the 2017 Iran-Iraq earthquake. To justify this claim, the authors need to explicitly describe the difference between the present study from previous ones. Is the dataset novel or is the methodology novel?

Author Response

Thank you for your constructive suggestion. There is not novel in the dataset, we just considered some details in data processing and the linear inversion.

First, in data processing, a nonlinear least-squares adjustment of the observed phase over presumably stable areas was used to refine the nonlinear residual orbit error. As the same time, we used a regression analysis between the atmospheric delay phase and the topography to correct the atmospheric delay phase. see lines 117-124 in the revised manuscript.

Second, in order to perform an exhaustive search in the potential space, we set a larger range and expanded the fault along the strike direction and down the dip direction to 90 and 60 km, respectively.

We have rewritten the sentence at lines 273-275 and lines 288-293.

“In order to perform an exhaustive search in the potential space, we set a larger range and expanded the fault along the strike direction and down the dip direction to 90 and 60 km, respectively.”

“In this study, the results demonstrate that the presence of two asperities in the shallow and deep subsurface; these are compatible with the results of Feng et al. and Ding et al. [9,11]. In addition, some results from the previous publications indicate there is one slip center in Iran earthquake [10,35]. However, it was worth noting that the slip model of the USGS was altered from single centralized slipping to two asperities (last updated 2018-10-17).”