Thanks for the revision of the manuscript. We have considered your comments and suggestions. We think that it will help in improving the paper. A point-by-point answer to the comments is following.

“(…) I think that a significant part of reported findings could be better explained if further scientific literature is considered like (for example) Cisternas, A., P.Godefroy, A.Gvishiani, A.I.Gorshkov, V.Kossobokov, M.Lambert, E.Ranzman, J.Sallantin, H.Soldano, A.Soloviev, and C.Weber (1985). A dual approach to recognition of earthquake-prone areas in the western Alps. Annales Geophysicae, 3, 2: 249-270, etc. The present-day seismological debate should be hinted, since the submitted manuscript may strongly contribute to it. I found Authors like Nappo unreported in References. Please check all References. It’s also suggested a review by a native mother-language translator.”

Thanks for the suggestions. The work after Cisternas et al. 1985 was not upon our knowledge. We have revised the main findings of this work and found some convergence between the possible source location and the location of some object detected by means of the pattern recognitions approach. We will cite this work as a further support to our findings.

Additionally, some literature, supporting our findings, have been unintentionally overlooked. We are referring to some recent works or already published papers describing in detail the evidence from other Alpine lakes for a synchronous earthquake. We are providing below a list of the added papers and some lines summarizing their interesting findings. The same lines of evidence will be added in the revised version of the manuscript.

Nigg et al. (2021)

The detailed description of a large delta collapse in Lake Sils (Switzerland) dated back at ca. 700 CE.

Bellwald et al – in press

A really “hot off the press” manuscript that is describing in detail an event at Lake Silvaplana (Switzerland) which is synchronous with our evidence. This manuscript is also citing our pre-print as a further support to their interpretation. The Authors claim as a possible seismogenic source the Engadine Line, north of the Insubric Line.

Tibaldi and Pasquarè (2008)

This work describes the geomorphological and geological evidence for a Holocene and historical re-activation of the Engadine Line, as a possible seismogenic source.

The papers described above prefer a location for the possible seismogenic source located in the Central Alps or close to the Insubric Line. The alternative hypothesis it that the source is located, with a slightly higher magnitude, at the foothills of the Southern Alps, close to Lake Como and city.

Possible evidence, in this line, comes from the work after Fanetti et al. 2007.  In this work a major fluvial diversion is dated back at the VI Cent. CE with the location of the outlet of major stream (Breggia river), shifted to the south for the first and only time since Lateglacial. Notwithstanding the poor chronological constraints to such an event, it is noteworthy the consistency of the timing. Historical sources report major floodings in the whole northern Italy at that time, a period known in scientific literature as the Late Antique Little Ice Age (Büntgen et al. 2016). Nonetheless, we cannot exclude that cascade like events involving the occurrence of a strong earthquake with the consequent occurrence of numerous landslides, remobilization of large volumes of debris, available for water transportation and intense and prolonged periods of rainfalls could have triggered a chain of events inducing this major shift in the Breggia riverbed (e.g., Tang et al. 2012; Dellow et al. 2017; Fan et al. 2018).

CITED REFERENCES

Bellwald, B., Nigg, V., Fabbri, S. C., Becker, L. W. M., Gilli, A., & Anselmetti, F. S. Holocene seismic activity in south‐eastern Switzerland: Evidence from the sedimentary record of Lake Silvaplana. Sedimentology.  in press

Büntgen, U., Myglan, V. S., Ljungqvist, F. C., McCormick, M., Di Cosmo, N., Sigl, M., ... & Kirdyanov, A. V. (2016). Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD. Nature geoscience, 9(3), 231-236.

Dellow, Sally, Chris Massey, Simon Cox, Garth Archibald, John Begg, Zane Bruce, Jon Carey et al. "Landslides caused by the Mw7. 8 Kaikōura earthquake and the immediate response." (2017).

Fan, Xuanmei, C. Hsein Juang, Janusz Wasowski, Runqiu Huang, Qiang Xu, Gianvito Scaringi, Cees J. van Westen, and Hans-Balder Havenith. "What we have learned from the 2008 Wenchuan Earthquake and its aftermath: A decade of research and challenges." Engineering Geology 241 (2018): 25-32.

Fanetti, D., & Vezzoli, L. (2007). Sediment input and evolution of lacustrine deltas: The Breggia and Greggio rivers case study (Lake Como, Italy). Quaternary international, 173, 113-124.

Nigg, Valentin, Stephan Wohlwend, Michael Hilbe, Benjamin Bellwald, Stefano C. Fabbri, Gregory F. de Souza, Florian Donau, Reto Grischott, Michael Strasser, and Flavio S. Anselmetti. "A tsunamigenic delta collapse and its associated tsunami deposits in and around Lake Sils, Switzerland." Natural Hazards 107, no. 2 (2021): 1069-1103.

Tang, C., van Asch, T. W., Chang, M., Chen, G. Q., Zhao, X. H., & Huang, X. C. (2012). Catastrophic debris flows on 13 August 2010 in the Qingping area, southwestern China: the combined effects of a strong earthquake and subsequent rainstorms. Geomorphology, 139, 559-576.

Tibaldi, A., & Pasquarè, F. A. (2008). Quaternary deformations along the ‘Engadine–Gruf tectonic system’, Swiss–Italian Alps. Journal of Quaternary Science: Published for the Quaternary Research Association, 23(5), 475-487.

Thanks for your comments to the paper. We have considered all the comments and a point-by-point answer is given below. In the revised version of the manuscript all the points will be addressed, as described below.

Section 3.3 GIS buffer analysis for source location. It is puzzling enough that in this sub-section the authors explain several issues for the assignment of seismic intensities and magnitude. Therefore, the title of the sub-section does not adequate summarize the content of the sub-section.

We are not sure to have fully understood this point. The assignment of threshold intensities for positive and negative evidence is a crucial point to run the inversion procedure. For a full description of this point, you could check Kremer et al. 2017. We will consider to modify the title of the sub-section like this: “Section 3.3 GIS buffer analysis for source location: issues for the assignment of macroseismic intensity and magnitude”

In addition, the description of the procedure they followed is not clear enough. The authors claim (l. 205): “As a preliminary step all the positive and negative evidence for a synchronous effect, triggered by the same earthquake, are collected”. However, they do not explain what they mean with the terms “positive and negative evidence”. As a consequence, the next statement is not well understood: “Then, lower and upper threshold Intensity values for triggering the considered effect are postulated: lower threshold values need to be exceeded at positive evidence locations; on the contrary, upper threshold values has to be not exceeded at negative sites”. In addition, the authors should explain the meaning of  “Intensity VI2⁄10” and “Intensity VI5/10”. I am afraid that the reader would not follow the procedure unless all the above points are clarified.

We recognize that not all the readers can be fully aware of the details of inversion procedure here adopted, and in the other quoted works as well. For sake of clarity we’ll update the text with a more detailed description of the adopted methodology and we’ll give a set of references with the full description of the sensitivity analyses performed so far on inversion procedures.

We here underline that changes in the threshold intensities for the positive evidence will result in different estimates of the inverted Magnitude, but with similar spatial distributions. Conversely, increasing or reducing the threshold value for the negative evidence, will result in a reduction or increase, respectively, of the area potentially hosting the seismogenic source.

272 “From the evidence above, we can finally assess that i) the fluidization observed in Section 02FL is triggered by ground shaking”. However, this statement is not supported by any relevant explanation. The authors should provide such explanations.

Thanks for pointing out this issue. Indeed the interpretation arising from the data should be better described in the text. In particular, the interpretation is derived from the absence of other possible triggering processes for the fluidization at the site. Specifically, following for instance Owen et al 2011, a context-based approach has allowed to exclude other triggering factors (e.g., storm waves, tsunamis, tidal shear, periglacial processes and groundwater seepage) and to exclude as well possible deformations due to a sudden vertical load due to the deposition of a coarse bed onto saturated sands. The only load deformation structures, clearly visible in the sequence at the top of the fluidized bed, are in fact deforming the convoluted bed, thus postdating the fluidization.

In the revised manuscript, we will give an explicit and more detailed description of the statements above.

277 “This age constrain comes from the assumption that the whole fluidized interval has been deformed by a single event”. Again explanations are needed as regards the assumption for a single event.

Strictly reasoning, assumptions should not be demonstrated but should be reasonable and useful for the adopted investigation rationale, following the Occam’s razor postulate. In this case, the adopted assumption is both. It is reasonable considering that all the bed in the fluidized sequence are deformed (i.e., there is no evidence for a pause between two shaking events). There is no need for the introduction of more than one fluidification event. A couple of statements explaining this reasoning will be added in the text.

281 “The Roman Baths preserve some peculiar damages on the walls and structures that can be interpreted as Earthquake Archeological Effects”. However, other causes of the observed damages should be discussed and ruled out before the earthquake is accepted as the only causative event.

Yes. We agree on this point. Indeed, the only evidence from the Roman Baths, already observed and discussed in previous archeological reports, is not enough to infer the occurrence of an historical earthquake shaking at the site. These observations, conversely, assume a more compelling meaning when compared with the evidence at the Via Manzoni excavations.

Specifically, some of the observed damages at the Roman baths could be also due to the effects of i) a differential compaction caused by subsidence (i.e., fractures in the walls and gentle folding); ii) damages due to the shallow depth of the ruins, below a previous gas station (partially collapsed arch). Chipped stone corners are more probably linked to ground shaking and, most importantly, the collapsed wall, stick in a vertical position right next to its original collocation, is not consistent to any toppling-like kinematics due to typical wall collapse.

We’ll add to the text some statements summarizing the lines of evidence above.

I’m not sure that the sub-section “5.2 Potential source location” is appropriate to be included in the Discussion section. This rather fits to the Results section since it is extensively devoted to estimated magnitude and location of the hypothetical earthquake.

Thanks for the suggestion, yes, we will move the section under the RESULTS Chapter.

Other comments may include the need for better editing and linguistic improvement of the manuscript, e.g., replace “ancient earthquake” by “historical earthquake” in several places, “historical events clusters” by “historical seismic clusters” (l. 82). In addition, in l. 174-175 there is an orphan short statement “Analysis of the archeoseismological evidence…”, which should be corrected. Also, in Abstracts and Conclusions the main result about the investigated palaeoearthquake the authors should be referred to by more conservative wording, e.g. hypothetical or possible earthquake. The title of the article does not reflect adequately the content of the article. Namely, the wording “ancient forgotten earthquake” is inappropriate. The term “forgotten” does not apply since the supposed earthquake is revealed for the first time. The term “ancient earthquake” is also proposed to be replaced by “palaeoearthquake”. Then, an improved title could be “The footprint of a VI Cent. A.D. palaeoearthquake in the European Western Southern Alps.

Thanks also for these suggestions. Yes, we will follow the comments and make the requested changes to the title; the entire manuscript will undergo a careful reading for improving clarity and readability.

Cited works

Kremer, K., Wirth, S. B., Reusch, A., Fäh, D., Bellwald, B., Anselmetti, F. S., ... & Strasser, M. (2017). Lake-sediment based paleoseismology: Limitations and perspectives from the Swiss Alps. Quaternary Science Reviews, 168, 1-18.

Owen, G., & Moretti, M. (2011). Identifying triggers for liquefaction-induced soft-sediment deformation in sands. Sedimentary Geology, 235(3-4), 141-147.