|In this study, a scheme is outlined for classifying the hazards of thin-skinned faults offshore Israel. As the authors outline, thin-skinned offshore faults may not necessarily be tectonic, but still pose hazards to offshore infrastructure. However, relatively few studies have been developed to characterize the hazards of these offshore faults. Hence this study clearly fits within the scope of NHESS.|
I do have some comments and suggestions for this manuscript, which will hopefully clarify some concepts and place the study in its wider context. However, these are generally quite minor comments, and mainly relate to the faults and earthquake section. Please note I am not readily familiar with the regional tectonic setting, and so have not commented on these aspects of the paper.
I hope the authors and editor find this review useful
Title: I would recommend changing the title to 'Marine thin-skinned faults.....' or something similar. Offshore faults can generate a variety of hazards that are not really considered in this study (e.g. ground shaking, tsunami, seiches), so it might be worth acknowledging this in the title
Lines 46-49: I disagree that the criteria for whether onshore faults are active or not is ‘well established.’ The Bryant and Hart (2007) reference used here outlines a Holocene criterion for fault activity, but this only applies to the specific case of mitigating surface rupture hazards in California. Elsewhere, the definition for fault activity may only require evidence of displacements over much larger timescales (from 125,000 to 5 million years; see Styron and Pagani 2021 compilation for examples). Indeed, even these criteria may not accurately identify all future onshore surface rupturing earthquakes (Nicol et al 2016), and many active fault databases now incorporate ‘potentially active’ or ‘capable’ faults, which have no chronostratigraphic evidence of recent activity but have other characteristics that suggest they may host future earthquakes (e.g., Maldonado et al 2021; Seebeck et al 2022; Williams et al 2022). In short, I suggest revise this sentence to highlight there are (ongoing) debates and challenges for how we define if faults are active, regardless of whether they’re onshore or offshore.
• Maldonado, V., Contreras, M., & Melnick, D. (2021). A comprehensive database of active and potentially-active continental faults in Chile at 1: 25,000 scale. Scientific data, 8(1), 1-13.
• Nicol, A., Van Dissen, R. J., Stirling, M. W., & Gerstenberger, M. C. (2016). Completeness of the paleoseismic active‐fault record in New Zealand. Seismological Research Letters, 87(6), 1299-1310.
• Seebeck, H., Van Dissen, R. J., Litchfield, N. J., Barnes, P. M., Nicol, A., Langridge, R. M., ... & Lee, J. M. (2022). New Zealand Community Fault Model–version 1.0.
• Styron, R., & Pagani, M. (2020). The GEM global active faults database. Earthquake Spectra, 36(1_suppl), 160-180.
• Williams, J. N., Wedmore, L. N. J., Scholz, C. A., Kolawole, F., Wright, L. J. M., Shillington, D. J., et al. (2022). The Malawi Active Fault Database: An onshore-offshore database for regional assessment of seismic hazard and tectonic evolution. Geochemistry, Geophysics, Geosystems, 23, e2022GC010425. https://doi.org/10.1029/2022GC010425
Lines 76-89: Related to my comment on the title, I suggest some statement is needed in these paragraphs to clarify that this case study is mainly concerned with fault displacement hazards, and not with other fault hazards
Figure 1a: Please outline what is the MC annotation? Presumably Mt Carmel?
Lines 242-55: As written, this section is concerned with measuring the height of sea-floor scarps using bathymetric data. I would therefore just clarify that these are measurements of ‘scarp height’ and ‘scarp displacement’ during the most recent fault movements (i.e., the displacements since the seabed sediments were deposited), and not the total fault displacement (as recorded by the older subsurface seismic reflectors and shown in Fig 3a).
Lines 265-277: Suggest adding some detail in this method section about how fault area and displacement on the 350 ky reflector were measured. Some of this information is included in the results (Section 4.2.1) but this content could be moved to the methods. In addition, more information could be provided about the uncertainties associated with the displacement measurements due to the vertical resolution of seismic data (typically about ¼ of the seismic data's wavelength; Widess 1973). Also, were the fault areas (e.g., as show in in Figs 7 and 8) mapped manually or using an automatic fault-picking routine in Kingdom?
• Widess, M. B.: How Thin Is a Thin Bed?, Geophysics, 38, 1176– 1180, https://doi.org/10.1190/1.1440403, 1973.
Lines 290-300: If measuring fault throw across the 350 ky reflector, then really the slip rate you obtain is a vertical displacement rate, not a fault displacement rate (which implies you’ve projected the throw measurement through fault dip) as indicated here.
Lines 548-549: It seems unclear why because the fault displacement rates and sedimentation rates are nearly the same, this precludes the possibility of fault creep. Is there a reason why creeping faults can’t produce sea floor scarps?
Line 552-554: I think it will help the reader visualize the relationship between faults and the earthquake locations if the hypocentres were plotted on one of the maps shown in this study, with hypocentres coloured by depth. This would help show in 3D the discrepancy between fault locations and focal depth.
Related to this point, given that the faults described in this study have moderate-gentle dips, then the question isn’t whether the earthquake hypocentres coincide with the fault’s seafloor trace; it’s whether the hypocentres coincide with the fault’s down-dip projection. I recognize there is much uncertainty with extrapolating faults and the hypocentre locations, but has this analysis been attempted?
Lines 564-568: Notwithstanding the challenges of applying empirical scaling from tectonic faults to thin-skinned faults, just because the rupture areas for recorded M 2-4 earthquakes do not match the areas of the thin-skinned faults, this does not mean these faults cannot be the earthquake source. Firstly, these faults may have hosted M5 earthquakes, but just not in within the (limited) period during which we have been instrumentally recording earthquakes. Furthermore, it might just be that these faults are hosting seismicity where the rupture areas comprise smaller sub-segments of the fault.
I would therefore just delete this these lines. It could, however, be worth adding that with areas of 10-20 km^2 these faults are unlikely to host earthquakes (M~5) that will produce damaging ground motions (unless of course some of the closely-spaced faults rupture simultaneously in ‘multifault’ earthquakes). Note too, there are more recent earthquake scaling laws that should be used instead of Wells and Coppersmith (1994), for example, Thingbaijam et al (2017), though admittedly the differences in estimated magnitude may be small.
• Thingbaijam, K. K. S., Mai, P. M., & Goda, K. (2017). New Empirical Earthquake Source‐Scaling Laws. Bulletin of the Seismological Society of America, 107(5), 2225-2246.
Finally, I (and possibly other readers) would be interested to know if other regions (e.g. Gulf of Mexico?) experience earthquakes due to active salt tectonics or whether the salt tectonics in these regions is accommodated by creep. This may help constrain whether the faults in this study are seismogenic or not.
Review to nhess-2021-393:
“Classifying offshore faults for hazard assessment: A new approach based on fault size and vertical displacement”, by Laor and Gvirtzman.
Laor and Gvirtzman propose innovative and elegant approach to cope with fault hazards in marine environments, a challenging territory to explore this geohazard. The suggested methodology is formulated and exercised in a case study along the continental slope offshore Israel, but can be applied elsewhere around the world in similar marine environments. This paper is well worth publishing in NHESS.
Hereby I propose several comments and suggestions that in my opinion will improve the manuscript, widen the scope of the discussion, and extend its role among the previously published faults, landslides and seismicity maps of the study area.
I suggest rephrasing the highlights to better speak in favor of the importance, finding and potential application of this work. For example, the first highlight (Mapping “active faults”…) is a general notion not specific to this study; the forth highlight (Large faults scarps…) seems to have already been attributed to Elfassi et al. (2019a) in lines 142-144?
You propose a new innovative approach and exemplify it on the specific case study of the Israeli continental slope. Why not wrapping up the abstract by proposing its implication and application to elsewhere similar marine environments, marine building codes, hazard assessment for submarine infrastructure facilities, etc?
Line 21: Please explain in short, what do you mean by ‘active faults’: are they capable of surface rupture, coseismic surface deformation, ground acceleration, and within a given time frame? See also the relevant comment above.
Line 28-29 (and 64-65): You write about three hazard levels but mention only two? What would be the role of the middle category?
Lines 45-46: Some of the works mentioned in the introduction did dealt with active faults (e.g. Armijo et al., 2005); also, there is very interesting work of Elias et al. (2007) regarding active historical seismogenic fault offshore Lebanon, I think it should be mentioned as well.
The Dor and Palmahim disturbances play major role in this study. There is a need to give some background about them.
Section 1.2 deals with the goal and the methodology of this work. Consider rephrasing the headline to ‘Goal and methodology’?
Chapter 2. Scientific background
Lines 144-147: I think this hypothesis needs to be verified by magnitude estimation. For example, as a thumb rule, M~6 crustal earthquakes are considered the minimum for generating surface rupture. What would be the estimated magnitude of the high (red) hazard class of faults for generating surface rupture - you have length, depth, area, and can assume vertical offset, say 1 meter?
Lines 157-161: “… it has been suggested that faulting was initiated by basinwards salt flow” - is this explanation relevant also to group II (Figure 9) that is located outside the salt area? Or also to group I of strike slip nature?
Lines 171-174: There is a need to present in short the nature of the 350ky horizon, it is the key for evaluating the recent activity of the study faults. Similarly, describe in short the lithology of units 3 and 4. Is it the contrast between the two that yields the 350 ky horizon? Unit 4 is the lithological environment that hosts the faults system studied in this work.
Section 3.2 Bathymetry data and Table 1
What are the uncertainties associated with these grids, mainly in the vertical dimension, which is the key parameter to define the total offset and rate of slip.
Section 4.4. Fault geometry and location
Lines 328-332: Looks to me also like a set of blocks rotated around horizontal axis?
Line 380 – The very high sedimentation rate could also be attributed to down slope transport of materials?
5.4. Assessing the hazard of surface rupture
466-470: Please note that modern approach for surface rupture hazard mitigation is being developed towards Probabilistic Fault Displacement Hazard Analysis (PFDHA), much like PSHA for ground shaking.
There are a few transversal (striking E-W) faults in the mapped region. They seem to be unique and deserve some attention.
Hidden faults: Do you mean blind faults?
Lines 243-249: Can you explain the reason for the increase of sedimentation rate from the deep basin towards the off shelf zone? If this area is also subject to slope failure, one would expect increase of sediment accumulation towards the basin?
Line 308-9: Please rephrase.
Line 310: Should be: “dashed red line in…”?
General: Please increase font size of coordinates, legend or any text where needed, e.g. Figures 3c, 4, 6, 9, etc.
Figure 1: I suggest increasing the area covered by the bottom left inset so as to allow orientation to readers who are not familiar with the research area, and please, add coastline.
Figure 3c: Please explain in the caption the areal extend of section 3c. Also, denote the location of Palmahim disturbance.
Figures 7, 8, 9: Please mark the location and extent of these maps on one of the previous figures.
Figure 10b: Please mark unit 1 on panel b.
References and sources of Information
Can you provide references or links to the sources of data mentioned in Table 1? Else, I believe you need to acknowledge these sources?
Reference to the Kingdom HIS platform?
References not mentioned in the text
Kafri, U. and Folkman, Y., 1981. Multiphase reverse vertical tectonic displacement across major faults in northern Israel. Earth and Planetary Science Letters. Volume 53, Issue 3, Pages 343-348
Katz, O. and Hamiel, Y., 2018. The nature of small to medium earthquakes along the Eastern Mediterranean passive continental margins, and their possible relationships to landslides and submarine salt-tectonic-related shallow faults. Geological Society, London, Special Publications, 477, 15-22, https://doi.org/10.1144/SP477.5
Shamir, G., Bartov, Y., Sneh, A., Fleischer, L., Arad, V. and Rosensaft, M., 2001, Preliminary seismic zonation in Israel. GSI Report No. GSI/12/2001, GII Report No. 550/95/01(1).