Instantaneous LEM back-analyses of major rockslides triggered during the 2016&ndash;2017 Central Italy seismic sequence

Verrucci, Luca; Forte, Giovanni; De Falco, Melania; Tommasi, Paolo; Lanzo, Giuseppe; Franke, Kevin W.; Santo, Antonio

doi:https://doi.org/10.5194/nhess-2022-256

Preprints

https://doi.org/10.5194/nhess-2022-256

Preprints

24 Oct 2022

| 24 Oct 2022

Status: a revised version of this preprint was accepted for the journal NHESS and is expected to appear here in due course.

Instantaneous LEM back-analyses of major rockslides triggered during the 2016–2017 Central Italy seismic sequence

Luca Verrucci, Giovanni Forte, Melania De Falco, Paolo Tommasi, Giuseppe Lanzo, Kevin W. Franke, and Antonio Santo

Abstract. Among the almost 1400 landslides triggered by the shocks of the 2016–2017 Central Italy seismic sequence, only a limited number, all classifiable as rockslides, involved volumes larger than 100 m³. Four of these failures, including the three largest, were described in terms of structural and geomechanical investigations in a previous study. In this paper, the mechanics of these failures under seismic actions are investigated. The estimated acceleration time histories at the rockslide sites were evaluated through a 2D simplified numerical model accounting for the attenuation phenomena and for the topographic effect of the rock cliffs from which the slide detached. Instantaneous stability analyses were carried out to obtain insights into the variability of the instantaneous margin of safety along the development of the shocks over the entire spectrum of mechanisms that could be activated. Finally, some general suggestions on the pseudo-static verification method for three-dimensional cases are proposed, which represent useful indications to hazard evaluation at local and regional scale.

Received: 12 Oct 2022 – Discussion started: 24 Oct 2022

Luca Verrucci et al.

Status: closed

RC1:
'Comment on nhess-2022-256', Anonymous Referee #1, 17 Dec 2022

General comments:

The manuscript presents an interesting approach for analyzing earthquake-induced wedge failures, modeling the evolution of the factor of safety during earthquakes using instantaneous pseudo-static analyses, while taking into account real seismic records and topographic amplification for the ground motion inputs. The methodology and results are a contribution to the knowledge of coseismic landslides. The paper relies too much on previous publications from some of the authors, such as it lacks the necessary context for the reader. Some of the assumptions made for the analyses and some figures need some further justification or explanation (see below). These changes can be achieved with a moderate revision.

Specific comments:

Lines 59 to 62: This paragraph is insufficient. A summary of the geology, seismotectonic setting including the faults where the earthquakes originated, and a description of the 2016 earthquake sequence are required. The reader needs some context to understand the analyses without referring to other publications.

Line 74: Check redaction. "Ground modification induced by stratigraphic conditions were not considered because all slides are in bedrock...."?

Line 90 and Table 3: explain the scaling procedure to obtain S

Lines 98 to 105: How steep are the slopes? You may add that slope gradient data in Table 3, or some cross sections of the landslides, to justify that the vertical cliff model is a reasonable approximation for all the analyzed landslides.

Line 108, Figure 5 caption and elsewhere: you use the term "horizontal rigid outcrop" to refer to site of the reference ground motion for topographic amplification calculation. I presume this is at the base (the top is also horizontal), and at some distance from the cliff (it can be attenuation at the slope toe). Please clarify this location and maybe use a different name for it.

Line 139: check the phrase "the inverse Fourier transform to the (1) and....", should be "to equation (1)"?

Lines 144 and 145, Figure 5. You say that "The alteration of the motion is usually significant for periods lower than 1-2 s, while it is negligible for periods higher than the fundamental period T0...". However, in Fig. 5 it does not look negligible between To and 2 seconds in some of the charts, which agrees your first phrase. Check the description and correct the second part if needed.

Lines 174 to 182. Please comment on the validity of factors of safety below 1.0 after the first time this value is reached and some sliding occur.

Lines 196 to 201: Please add which software or code did you use to make the factor of safety calculations.

Lines 201 to 203: The last sentence of this paragraph could go in the Discussion section

Lines 215 and 216: "the geometric layout of the rockslide scar suggests that the wedge should have experienced displacements as large as to

break a constraining rock spur at its highest part" That´s vague, how large is that? an estimate at least? What size is the "spur"?

Lines 237 to 246: The whole paragraph should better go in the Discussion section.

Figure 3: Please enlarge the size of blue dots, they are hardly distinguished.

Figure 7 caption: Indicate what is the meaning of the black lines at the top of each chart (mechanism).

Table 2: I suggest adding the distance between the seismic station and the landslide of interest for which you use the ground motion records.

Table 2: Better use units of "g" for accelerations, to be consistent with Table 3.

Table 3: add average slope gradient at each rockslide site

Table 5: Add the cohesion values, even if it is the same for all.

Citation: https://doi.org/10.5194/nhess-2022-256-RC1
- AC1: 'Reply on RC1', Giovanni Forte, 18 Jan 2023
  
  Dear Reviewer,
  on behalf of all the Authors, I sincerely thank you for your time and the suggestions provided, which will permit us to improve the quality of the manuscript.
  You can find in the attachment the responses to your comments.
  Best Regards
  Giovanni Forte
  
  Citation: https://doi.org/10.5194/nhess-2022-256-AC1
RC2:
'Comment on nhess-2022-256', Anonymous Referee #2, 10 Jan 2023

In this paper, the authors examine four wedge failures triggered by earthquakes that occurred in Central Italy. The authors use 2D mechanical models to better understand the stability of the examined hillslopes under the influence of seismic forces. I believe the manuscript could be significantly improved by reviving the general structure of the manuscript. In the current version of the manuscript, data, method and results are presented together and that makes it a bit difficult to follow sometime. The research gap(s) and research question could be better emphasized. Also, the relevant literature could be enriched.

Below I've included line by line suggestions and highlighted all these points.

Line 13. “occurred” This is a statement based on reported events so, it is better saying “Most of the reported/documented landslides”

Line 23. “Lombardo et al. 2021, Quinton Aguilera et al. 2022” I do not think these are papers reporting rockslide volumes. Could you please replace them with relevant articles?

Lines 23-24. “for the energy released by these seismic events (moment magnitude, < 6.5)” Could you please revise this line, I could not get what you mean here.

Lines 25-27. You are referring to co-seismic rockslides reported in Central Italy, right? Please indicate that here again for clarification. Also, it would be better if you give citations to the literature. What is the source of these descriptions?

Line 31. “local” For strong earthquakes, it could be also important for regional scale assessments, no?

Lines 30-35. Yes, you can say that this is an important concept for co-seismic landslide hazard assessment. I believe this is the point where you should emphasize the gap in the literature that you are aiming to address with this research. After emphasizing the research gap, instead of saying “These considerations sparkled investigation of the failure stages of the largest rockslides …”, please be more specific and indicate the specific research question that you are targeting in this paper. And then, you can mention what you did and how you did that goal you described. Just a small suggestion to improve the flow of the manuscript and to clarify your point.

Lines 41-57. Please trim this part because part of the content presented here would be better if you present it in the method section.

Line 58. I recommend using the traditional structure as Intro, study area/materials/data, method, discussion and conclusions.

Lines 59-60. I agree with the authors that detail information regarding geology, rock mass structure and so on may not need to be presented here if there is already a paper describing the very same. However, you can still briefly introduce your study area briefly. Please start by introducing your study area then after your brief summary you can mention Forte and others for further details. And I believe you should tell us why you chose those four landslides in particular. Do you have detailed geotechnical information about them? Please elaborate.

Line 66. Figure 2 is not something you adapted from the literature, right? If this is something you generated as part of this research, please keep it for your result section because this is also part of your results; you carried out some kinematic analyses and generated those plots. First, mention in the method section and then present in your results section.

Line 70. This should be your method section now. Of course, you can add sub-headings.

Lines 76-82. This paragraph could be moved to the previous section (i.e., study area/materials/data) as you are presenting your landslides here.

Line 87. Remove “.”

Lines 104-105. Is this an important literature gap that you aim to address? If this is the case, please mention it in your introduction.

Lines 110-112. Could you please cite the relevant literature? You can provide more details through Supplementary Materials maybe. Specifically, topographic amplification deserves a better summary. It should be also better elaborate in the introduction. The given literature on this topic should be improved.

Line 122. Assimaki et al. (2005) are focusing on soil, right? In this regard, can we assume that the amplification mechanism will be the same or similar?

Lines 123-130. These are part of your results, aren’t they?

Line 139. Could you please elaborate on these assumptions? Based on what? The same comment is valid for the whole section.

Section 4. Please consider the very same comment I mentioned above. The parts that you present your method or data could be differentiated from your results and also assumptions and knowledge you obtained from the literature could be better explained.

Citation: https://doi.org/10.5194/nhess-2022-256-RC2
- AC2: 'Reply on RC2', Giovanni Forte, 24 Jan 2023
  
  On behalf of all the Authors, I thank the reviewer for her/his time in reviewing our manuscript.
  In the supplement, you can find the responses to the raised observations.
  Cordially
  Giovanni Forte
  
  Citation: https://doi.org/10.5194/nhess-2022-256-AC2

Status: closed

RC1:
'Comment on nhess-2022-256', Anonymous Referee #1, 17 Dec 2022

General comments:

The manuscript presents an interesting approach for analyzing earthquake-induced wedge failures, modeling the evolution of the factor of safety during earthquakes using instantaneous pseudo-static analyses, while taking into account real seismic records and topographic amplification for the ground motion inputs. The methodology and results are a contribution to the knowledge of coseismic landslides. The paper relies too much on previous publications from some of the authors, such as it lacks the necessary context for the reader. Some of the assumptions made for the analyses and some figures need some further justification or explanation (see below). These changes can be achieved with a moderate revision.

Specific comments:

Lines 59 to 62: This paragraph is insufficient. A summary of the geology, seismotectonic setting including the faults where the earthquakes originated, and a description of the 2016 earthquake sequence are required. The reader needs some context to understand the analyses without referring to other publications.

Line 74: Check redaction. "Ground modification induced by stratigraphic conditions were not considered because all slides are in bedrock...."?

Line 90 and Table 3: explain the scaling procedure to obtain S

Lines 98 to 105: How steep are the slopes? You may add that slope gradient data in Table 3, or some cross sections of the landslides, to justify that the vertical cliff model is a reasonable approximation for all the analyzed landslides.

Line 108, Figure 5 caption and elsewhere: you use the term "horizontal rigid outcrop" to refer to site of the reference ground motion for topographic amplification calculation. I presume this is at the base (the top is also horizontal), and at some distance from the cliff (it can be attenuation at the slope toe). Please clarify this location and maybe use a different name for it.

Line 139: check the phrase "the inverse Fourier transform to the (1) and....", should be "to equation (1)"?

Lines 144 and 145, Figure 5. You say that "The alteration of the motion is usually significant for periods lower than 1-2 s, while it is negligible for periods higher than the fundamental period T0...". However, in Fig. 5 it does not look negligible between To and 2 seconds in some of the charts, which agrees your first phrase. Check the description and correct the second part if needed.

Lines 174 to 182. Please comment on the validity of factors of safety below 1.0 after the first time this value is reached and some sliding occur.

Lines 196 to 201: Please add which software or code did you use to make the factor of safety calculations.

Lines 201 to 203: The last sentence of this paragraph could go in the Discussion section

Lines 215 and 216: "the geometric layout of the rockslide scar suggests that the wedge should have experienced displacements as large as to

break a constraining rock spur at its highest part" That´s vague, how large is that? an estimate at least? What size is the "spur"?

Lines 237 to 246: The whole paragraph should better go in the Discussion section.

Figure 3: Please enlarge the size of blue dots, they are hardly distinguished.

Figure 7 caption: Indicate what is the meaning of the black lines at the top of each chart (mechanism).

Table 2: I suggest adding the distance between the seismic station and the landslide of interest for which you use the ground motion records.

Table 2: Better use units of "g" for accelerations, to be consistent with Table 3.

Table 3: add average slope gradient at each rockslide site

Table 5: Add the cohesion values, even if it is the same for all.

Citation: https://doi.org/10.5194/nhess-2022-256-RC1
- AC1: 'Reply on RC1', Giovanni Forte, 18 Jan 2023
  
  Dear Reviewer,
  on behalf of all the Authors, I sincerely thank you for your time and the suggestions provided, which will permit us to improve the quality of the manuscript.
  You can find in the attachment the responses to your comments.
  Best Regards
  Giovanni Forte
  
  Citation: https://doi.org/10.5194/nhess-2022-256-AC1
RC2:
'Comment on nhess-2022-256', Anonymous Referee #2, 10 Jan 2023

In this paper, the authors examine four wedge failures triggered by earthquakes that occurred in Central Italy. The authors use 2D mechanical models to better understand the stability of the examined hillslopes under the influence of seismic forces. I believe the manuscript could be significantly improved by reviving the general structure of the manuscript. In the current version of the manuscript, data, method and results are presented together and that makes it a bit difficult to follow sometime. The research gap(s) and research question could be better emphasized. Also, the relevant literature could be enriched.

Below I've included line by line suggestions and highlighted all these points.

Line 13. “occurred” This is a statement based on reported events so, it is better saying “Most of the reported/documented landslides”

Line 23. “Lombardo et al. 2021, Quinton Aguilera et al. 2022” I do not think these are papers reporting rockslide volumes. Could you please replace them with relevant articles?

Lines 23-24. “for the energy released by these seismic events (moment magnitude, < 6.5)” Could you please revise this line, I could not get what you mean here.

Lines 25-27. You are referring to co-seismic rockslides reported in Central Italy, right? Please indicate that here again for clarification. Also, it would be better if you give citations to the literature. What is the source of these descriptions?

Line 31. “local” For strong earthquakes, it could be also important for regional scale assessments, no?

Lines 30-35. Yes, you can say that this is an important concept for co-seismic landslide hazard assessment. I believe this is the point where you should emphasize the gap in the literature that you are aiming to address with this research. After emphasizing the research gap, instead of saying “These considerations sparkled investigation of the failure stages of the largest rockslides …”, please be more specific and indicate the specific research question that you are targeting in this paper. And then, you can mention what you did and how you did that goal you described. Just a small suggestion to improve the flow of the manuscript and to clarify your point.

Lines 41-57. Please trim this part because part of the content presented here would be better if you present it in the method section.

Line 58. I recommend using the traditional structure as Intro, study area/materials/data, method, discussion and conclusions.

Lines 59-60. I agree with the authors that detail information regarding geology, rock mass structure and so on may not need to be presented here if there is already a paper describing the very same. However, you can still briefly introduce your study area briefly. Please start by introducing your study area then after your brief summary you can mention Forte and others for further details. And I believe you should tell us why you chose those four landslides in particular. Do you have detailed geotechnical information about them? Please elaborate.

Line 66. Figure 2 is not something you adapted from the literature, right? If this is something you generated as part of this research, please keep it for your result section because this is also part of your results; you carried out some kinematic analyses and generated those plots. First, mention in the method section and then present in your results section.

Line 70. This should be your method section now. Of course, you can add sub-headings.

Lines 76-82. This paragraph could be moved to the previous section (i.e., study area/materials/data) as you are presenting your landslides here.

Line 87. Remove “.”

Lines 104-105. Is this an important literature gap that you aim to address? If this is the case, please mention it in your introduction.

Lines 110-112. Could you please cite the relevant literature? You can provide more details through Supplementary Materials maybe. Specifically, topographic amplification deserves a better summary. It should be also better elaborate in the introduction. The given literature on this topic should be improved.

Line 122. Assimaki et al. (2005) are focusing on soil, right? In this regard, can we assume that the amplification mechanism will be the same or similar?

Lines 123-130. These are part of your results, aren’t they?

Line 139. Could you please elaborate on these assumptions? Based on what? The same comment is valid for the whole section.

Section 4. Please consider the very same comment I mentioned above. The parts that you present your method or data could be differentiated from your results and also assumptions and knowledge you obtained from the literature could be better explained.

Citation: https://doi.org/10.5194/nhess-2022-256-RC2
- AC2: 'Reply on RC2', Giovanni Forte, 24 Jan 2023
  
  On behalf of all the Authors, I thank the reviewer for her/his time in reviewing our manuscript.
  In the supplement, you can find the responses to the raised observations.
  Cordially
  Giovanni Forte
  
  Citation: https://doi.org/10.5194/nhess-2022-256-AC2

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Short summary

Stability analyses in static and seismic conditions were performed on four rockslides occurred during the main shocks of the 2016–2017 Central Italy seismic sequence. These results also indicate that specific structural features of the slope must carefully be accounted for in evaluating potential hazard on r transportation infrastructures in mountainous regions.


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