Structural Complexity and Seismogenesis: The Role of the Transpressive Structures in the 1976 Friuli Earthquakes (Eastern Southern Alps, NE Italy)

Round 1

Reviewer 1 Report

GENERAL COMMENTS

The Authors reconstructed the seismotectonic setting of the area comprising the northeastern Friuli

Plain and the Julian Prealpine border (NE Italy) by integrating geological and seismological data. They apply a multidisciplinary approach for reconstruction the 3D –geometry of the Susans-Tricesimo Thrust System elaborating four geological cross sections from the interpretation of ENI industrial seismic lines. Moreover, they investigated the seismogenic volume of central-eastern Friuli area through the analysis of the hypocentral distribution of the earthquakes of the latest 50 years (1976–1977- and 1978–2019-time intervals). The seismotectonic model resulting from their study highlight strong consequences that should not be underestimated for the evaluation of the seismic hazard in Friuli.

 

The paper is scientifically very interesting, mainly for future research on the topic of the seismic hazard assessment of the Friuli.

Anyway, I would suggest some improvement the text for which I recommend minor revision.

The main points to be considered are the following ones:

1. Introduction

Line 32: Northeastern  and not NE

I suggest to Authors to better finalize their research. They should highlight that their seismotectonic model should provide a contribution to the seismic hazard assessment of the Friuli.

Figure 1:

• the Authors should modify the caption: Seismotectonic map of Friuli region and western Slovenia;
• the Authors should remove the references of instrumental epicentres, historical seismicity and instrumental seismicity from the figure and report them in the caption;
• the Authors should replace the Italy sketch reported in figure with the Italy tectonic sketch and increase its size.
• In addition, the resolution of the figure should be improved, the size of the toponyms and fault names should be increased and the municipal boundaries should be inserted. Even the tectonic structures are badly seen.

2. Geological and seismotectonic setting

Lines 109-120: The Authors should add a figure showing the information describe in the text (e.g. the epicentres of different authors cited, the hypothesis of the activated faults). I think that it is not enough to mention the papers.

3. Methodology

3.1 Structural model reconstruction

Line 224: The Authors should add the reference of the 3D Move Software

3.2 Seismicity distribution analysis

The Authors should replace the title of the sub-chapter with “Instrumental seismicity analysis”

Line 255: The Authors should replace the sentence “The earthquakes distribution analysis” with “the spatial and temporal earthquake distribution analysis”

4. Results

4.1.1 chapter:

I suggest to schift figure 2 in the capter 3.1. In addition, the resolution of the figure should be improved, the size of the toponyms and fault names should be increased.

In the figure 3, for the BB’ section is missing the seismic profile why?

4.2.2:

Figure 11: The authors should modify the caption of figure 11: A) depth 0-7 km; B) depth 7-13 km; C) depth 0-7.

Lines 642: Please insert Figure 11

5. Discussion: The Authors should replace 4 with 5
6. Conclusions: The Authors should replace 5 with 6

The Authors should also stress the importance of their paper for the seismic hazard of the Friuli region.

Lines 719-720: The Authors should remove the two last lines

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The paper brings a nice contribution for understanding the active tectonics of the NE Italian and NW Slovenian Alps, and a new scheme of the seismotectonics setting of the 1976-77 Friuli earthquake sequence. The work blends seismic profile interpretation, appraisal of local structural geology and distribution of relocated seismicity to detect the sources of recent and current seismicity. As such, it merits publication in Geosciences.

I reported minor comments and suggestions in the annotated manuscript.

My only concern regards the deep interpretation of active structures based on the earthquake hypocentral distribution. Although most seismic events (both for the 1976-77 sequence and the 1978-2019 seismicity) are confined between the ST thrust and the high-angle Slovenian strike-slip faults, the identification of activity on single structures is not straightforward and different interpretations can be put forth. Maybe the authors should be less definitive in their interpretation and state their degree of uncertainty. I agree that, based on results presented, the activation of the Buia thrust as proposed by Cheloni et al. 2012 is not required; however, I would like to see a more detailed description of the reasons why this thrust was previously considered the seismogenic source.uired

I also find some details of the local geology, particularly concerning the Plio-Quaternary units, appreciable only for those accustomed to the issue. Some quoted papers on local geology are dispersed into gray literature. Moreover, many surface geological aspects are not exploited further in the paper.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors,

the manuscript “Structural complexity and seismogenesis: the role of the transpressive structures in the 1976 Friuli earthquakes (Eastern Southern Alps, NE Italy)” proposes an interpretation to the development and evolution of the 1976 seismic sequence occurred in Northeastern Italy.

Although the manuscript is correctly written and figures are clear, the data on which the Authors' arguments are based are questionable: the seismic lines can be interpreted up to a depth of 5 km, as admitted by the Authors; the traces of deep structures are speculations of the Authors (in fact, they do not specify in the text how they were traced); the trend of the seismicity does not follow the trend of the interpreted structures; the locations and epicentral depths of the events of the seismic sequence (1976-77) were determined with an inadequate number and arrangement of stations and in any case different from those of the post-1977 seismicity. In addition, the Authors do not correctly report depth values for the 1976 events, as indicated in the references (data in table 2 are not those in Slejko et al, 1999, as cited in the text). The claim that the May 6, 1976, hrs 19:59 foreshock, M=4.5, on a transpressive fault, induced the hrs 20:00 main event, M=6.4, on a reverse fault, is difficult to accept. The Authors could have presented stress transfer modeling to support their assertion (eg. Lin and Stein, 2004, King et al., 1994, Nostro et al., 1997).

The serious shortcoming of this work is that it is based on poor quality data (location and depth of events in the 1976 sequence) and assumptions that are difficult to prove (geometries of structures at depth). These assumptions are supported by self-citation. Furthermore, the work is completely devoid of theoretical arguments in support of the theses the Authors want to prove. Some references to international Authors could be useful (eg., Sibson, 1992, Scholz 1990, Dieterich 1994).

For these reasons, the manuscript in the present form can not be accepted for publication and it should be rejected.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

Review of the manuscript “Structural complexity and seismogenesis: the role of the transpressive structures in the 1976 Friuli earthquakes (Eastern Southern Alps, NE Italy” by G. Patricelli and M. E. Poli.

 

The manuscript presents a seismotectonic study of the Friuli region in the Eastern Southern Alps which was affected by a destructive seismic sequence in 1976. This area is characterized by high structural complexity due to the superposition of Miocene-Pleistocene Neoalpine ~NW-SE contraction on the Paleogene Dinaric structures formed under ~NE-SW contraction. The authors reconstructed the subsurface 3D geometry of the main frontal thrusts imaged in industrial seismic lines (reaching a depth of about 4 km) and projected the hypocentres of the earthquakes registered in the 1976 – 1977 sequence, and those between 1978 and 2019 (interseismic phase). Based on the relationships between hpocentres and inferred tectonic structures, a model for the seismogenic sources of the region is proposed.

The paper is well organized and is accompanied by several figures, which are useful and clear (some improvements are suggested below). Data and methodologies are appropriate and appropriately described. Despite its regional target, the topic and the methodology are of interest also for an international audience. I think that the manuscript should be published in Geosciences with minor revisions. I have just one main concern, detailed below. The other comments are mostly suggestions about the style and figures.

Main comment

The methodology applied for the interpretation of the seismic lines and for the analysis of the earthquake data are stated in detail in the methods section. In contrast, it is not clear which are the constraints used for extrapolating the structures to the seismogenic depth (i.e., the dashed red lines in Figs 8, 9 and 12). In particular, I suggest adding some details about the hypocentres interpolation for obtaining the (very interesting, indeed!) folded surface of the ST (Fig. 13), and the associated uncertainties, given the scattered distribution of the hypocentres locations.

Line by line comments

Lines 41- 44: please, break the sentence in two

Line 42: different maximum principal stress orientation?

Line 54: delete “Aiming at the stud of ….earthquakes,”

Line 57: gently>kindly

Line 108: Mw is correct?

Lines 118-120: Sentence is unclear, please rephrase.

Line 272: surrounding

Line 348 – Fig. 4: please, indicate the positions of the enlarged areas in Fig. 3.

Line 354: delete depth

Line 356: the dome and basins structures are not so obvious in Fig. 5. Maybe add a view from above, painting the surfaces with their aspect?

Lines 367-369: the sentence is confusing and can be shortened significantly

Line 401 – Fig. 7: the size of the symbols is larger in the legend than in the figures. I understand that the event numbers are listed in Table 2, however it would be easier if magnitude and date were reported close to the main shocks focal mechanisms.  

 Line 416: low-to-medium angle

Line 430: Dinarides?

Figs 8, 9 , 12: as above, it would be useful to highlight the date and magnitude of the main shocks in the figures, to help the reader following your reasoning.

Line 482: 1976

Line 548: in depth

Line 549: interseismic period

Lines 719 – 721:?

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Dear Authors,
although I am not entirely convinced of the arguments proposed, the work in the current form is more supported, and therefore it can be accepted to be published in the present form.

Regards

 

Author Response

Dear Reviewer,

we still thank you for your criticism which allowed us to valide our model by inserting the additional elaboration (Coulomb Stress Change). A further improvement of our proposed model would surely be achieved through a field structural survey.

Best Regards,

Giulia Patricelli