Analysis of the Influence of Structural Geology on the Massive Seismic Slope Failure Potential Supported by Numerical Modelling

Round 1

Reviewer 1 Report

Dear authors, 

I will start by saying that every time I receive a manuscript to be reviewed from you or part of you, it is always a pleasure to see how much field activities are taken in high regard and used in the context of modern geosciences.

I have seen way too many research articles where fieldwork is not even considered among the requirements.

So, I am very pleased with your work and with the way you framed it.

Below I will provide a brief summary of your research and add my comments / suggestions for improvement, which are minor.

You have conducted a series of campaigns in four sites scattered across central and eastern Europe, where large rockslides occurred in the past. The information, mainly of structural nature, is then used to compare the settings and the potential reasons that have lead to those failures, also in comparison with other rockslides in Tien Shan, where you have a similar if not a better knowledge overall, especially in term of the trigger. This framework is well described with figures showing the work you have done and the interpretations that arose from it. And, it is further complemented by numerical simulations. This is all voted to test whether earthquakes can be the causes for such large rockslides and what can be the contribution of structural features.

So, the research question is solid and the tools you used are both basic but fundamental for any geoscientist and advanced when it comes to visualization tools and physically-based models. The text reads nicely, even with a bit of storytelling flavor for the reader, which is never a bad thing. The quality of the figures is excellent and overall, this work is a rare case where I would even suggest an acceptance to begin with. 

However, there is always something that can be improved. Figure 1 and 3 have the units wrong. In Figure 1 the reader can see "Kilometers", which should be either "kilometers" or km. And, in Figure 3 the same issue exists for "Meters". 

My other suggestion relates to the way you conclude the manuscript. I have really enjoyed the reading as if it was a sort of geo-story book, lloking for clues on the responsible for such rockslides. But, I find the conclusion to be a bit disappointing. In the end, and it is normal do not get me wrong, you could not strictly prove the type of trigger, assumed to be an earthquake, nor its contribution to each of the failures you examine in Switzerland & Romania. So, my suggestion is, what could you do more than this to prove your assumption? Why don't you add a final sentence where you provide your expert opinion if this aim could even be achieved and if yes, what more can be done (maybe landscape reconstruction methods, or detailed geophysical 3D surveys for a better characterization of the medium and advanced earthquake simulations.)

Other than this, I just want to thank again the authors for the interesting reading. 

Good luck with the next phase and kind regards, 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Review of “Analysis of the Influence of Structural Geology on the Massive Seismic Slope Failure Potential supported by Numerical Modelling” by Emilie Lemaire and co-authors

Lemaire and coauthors present 3d geological models of rockslides including structural data (bedding, faulting fractures and joints orientation) accompanied by a series of numerical models with the aim to understand if the landslides were potentially triggered by earthquake and to identify the set of diagnostic criteria to ascertain that they were seismically triggered. The results are relevant for paleoseismological studies and geohazard assessment in mountainous terrain and thus, the paper is worth publishing. I find that the paper is relatively well structured (despite I miss a proper methods section) and, although English is not my native language, I feel it is relatively well written (though I took the liberty to make a few corrections).

There are however several issues that can be improved:

First, it is not clear which diagnostic criteria indicating that the landslides were triggered by earthquakes result from this work or were defined in previous work. According to section 2.1 (L85-86), it seems a series of criteria were already defined, at least for the Tamins rockslide. The new criteria and those already defined in previous studies needs to be clarified.

Secondly, the authors attempt to compare landslides in the Alps and Carpathians, for which the trigger mechanism is not well known, with catastrophic slope failures in the Tien Shan Mountains that the authors state have “ascertained or very likely seismic origin”. The problem is that the trigger mechanism for some of the landslides in the Tien Shan Mountains is as known as that for the Alps and Carpathians. For sound comparison, I suggest the authors limit the number of cases in the Tien Shan Mountains to those for which the trigger mechanism is well constrained or they change the cases for which there is no full confidence in trigger mechanism by other examples in which such trigger mechanism is well known. The issue here is also that the authors make little to almost no use of these examples to discuss the deformation patterns of seismically induced rock slope failure or to provide insights on the trigger mechanism of landslides in the Alps and Carpathians, so the authors should strengthen such comparison.

Third, the authors make inferences on the stability of the slope based on the displacement fields of Figs. 14-17 (e.g., in L413-415). The authors need to express how instability is defined. Is there a threshold displacement that marks the onset of landsliding?

Fourth, the conclusions are rather long, difficult to understand and guided by local examples. I recommend the authors to make clear statements on the characteristics that identify a landslide as triggered by earthquakes and how preexisting rock structures influence the stability of a slope.

Other minor comments:

L22-23. The exact cause of slope failure is often unclear. à That must obviously predominantly refer to prehistorical slope failures.

L74.  “.. number of 500 to 800 earthquakes per year with magnitudes of 2.5 or more” --> Explain how relevant those earthquakes can be for slope failure in rocky slopes. Which intensity or ground shaking is needed for such slope failures to occur?

L94. Use of scarp vs scar. I would favor the latter here.

L95-97 this slope failure, after collision with the opposite valley flank, … --> Please specify if and how much the rockslide climbed on this flank as this may provide hints on its velocity.

L102-103. field investigations does not suggest --> field investigations do not suggest

Fig. 3. Indicate stratal dip, add fault marks on hanging-wall and sense of landslide motions or landslide headscarp. Low --> Lower (or add dot); Mid --> Middle (or add dot)

L143. 1 Mio. m3 --> 1 Mm^3 or 10^6 m^3

L178. There is no previous reference to the Bielogorka rock avalanches in the MS.

L191-192. a series of paleoseismic events are outlined by Korzhenkov et al. --> on the basis of what?

L208. The Karasuu rock avalanche is Kara-Kul in Fig. 4?

Fig. 7 (caption). … the lower left panel if a zoom on the three stereonet … -->… the lower left panel OF a zoom on the three stereonet …

L286. The Balta rockslide is a typical anti-dip slope case--> anti-dip with respect to what? Bedding? Joints?

L306-307. … combining two numeric approaches: the distinct element method (DEM) and the finite-difference method (FDM). --> It is unclear which method was actually used in each part of the analysis

L314. Which criteria was used to create the potential surface before failure of the Balta rockslide? Is there cross-sectional volume balance in the pre- and post-failure sections in Fig. 12?

L319. …, the slope angle was decreased or increased, respectively --> by which factors

L355-357. The dynamic models are shown in Figure 15 and are presented in tabular form separated in columns according to the slope morphology and by rows according to the structure. --> delete and cite the figure in the following sentence.

Fig. 15. It is curious that the dynamic case in Structure 7 seems to have lower displacement than the static case?

Author Response

Please see the attachment.

Author Response File: Author Response.docx