Classifying offshore faults for hazard assessment: A new approach based on fault size and vertical displacement
- 1Geological Survey of Israel, Yesha'yahu Leibowitz 32, Jerusalem, Israel
- 2Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel
- 1Geological Survey of Israel, Yesha'yahu Leibowitz 32, Jerusalem, Israel
- 2Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel
Abstract. For many countries, the methodology for offshore geohazards mitigation lags far behind the well-established onshore methodology. Particularly complicated is the mapping of active faults. One possibility is to follow the onshore practice, i.e., identifying a sub-seabed Holocene horizon and determining whether it displaces this horizon for each fault. In practice, such an analysis requires numerous coring and often ends without an answer.
Here we suggest a new approach aimed for master planning. Based on high-quality seismic data, we measure for each fault the amount of its recent (in our specific case 350 ky) displacement and the size of its plane. According to these two independently measured quantities, we classify the faults into three hazard levels, highlighting the “green” and “red” zone for planning.
Our case study is the Israeli continental slope, where numerous salt-related, thin-skinned, normal faults dissect the seabed, forming tens of meters high scarp, which are crossed by gas pipelines. A particular red zone is the upper slope south of the Dor disturbance, where a series of big listric faults rupture the seabed in an area where the sedimentation rate is four times faster than the displacement rate. We suggest that this indicates seismic rupture rather than creep.
May Laor and Zohar Gvirtzman
Status: final response (author comments only)
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RC1: 'Comment on nhess-2021-393', Anonymous Referee #1, 04 Mar 2022
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.
General Comments
- Fault hazard: The problematics of fault hazards should be explained already in the introduction so as to allow the reader a better background and understanding along the text and before arriving to section 5.4 and Figure 13. Please resolve this general term into its specific aspects: surface rupture, coseismic deformation and ground acceleration. Hidden/blind faults may produce coseismic deformation without surface break.
- Fault maps: Several fault maps have already been published in the past and besides citing them it is important to discuss, at least qualitatively, how the newly presented map relates to them. Furthermore, past researchers proposed hypotheses about specific faults, such as the one along the Israeli coastline, the Pelusium line (Neev et al,. 1973), transversal faults bordering the Palmahim disturbance (Garfunkel and Almagor, 1979), fault offshore the Carmel Coastal Plain (Kafri and Folkman, 1981), etc. I think it is important to place the present work and discuss its role among and along the history of research, at least in a quantitative manner.
- Seismicity: Studying active faults, there is a need to refer to the ongoing seismicity in the region (e.g. Katz and Hamiel, 2018) by discussing the finding of the present work in relation with the location, depth, magnitudes and mechanism of the continental slope seismicity, at least qualitatively.
- Seismogenic zone: The PGA map of the Israeli Building Code 413 is based on seismogenic zones defined by Shamir et al. (2001). How does the presented hazard map (e.g. Figure 13) relates to these zones? Should the continental slope be added as a new seismogenic zone to the database of the Israeli PGA map?
- Landslides: Same idea as above.
Specific comments
Highlights
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?
Abstract
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?
Introduction
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?
Discussion
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.
Technical comments
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…”?
Figures
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).
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RC2: 'Comment on nhess-2021-393', Stéphane Baize, 05 May 2022
The paper presents a large compilation and analysis of high-quality bathymetric and seismic-reflection data, with the aim of providing classification and mapping of subsea faults that are potentially hazardous for installations.
The region of concern is vast (120 x 40 km²) and corresponds to part of the margin of the eastern Mediterranean basin, off Israel. This margin is subjected to significant deformations of the most superficial layers (0 to 1-2 km) of Plio-Quaternary age. These deformations are largely due to the mobility of this sedimentary cover on the salt layer dating from the Messinian, amplified by the slope of the margin.The paper fits the objective of the journal, presents relevant products for further hazard analyses (surface displacement maps, deterministic hazard map) and then deserves to be published. However, I think there must be major revisions, including formal improvements, additional analysis, and moderation in some interpretation.
The major criticisms are as follows.
- Some figures are too small and labels and captions are barely visible.
- The authors should develop the data and methods section and provide more information on data availability and processing options (mainly seismic reflection).
- The part of Section 2 concerning ancient (pre-tertiary) geology could be lightened, and the part concerning elongated Neogene. In addition, a paragraph is missing which links (or not) the structures observed with the tectonic faults known on land.
- Section 4 (Results) should be restructured to be consistent with the maps and figures produced. Some figures are barely commented and deepened, which is a shame.
- Section 5 (Discussion) needs to be improved. Most of the current content is a summary of previous sections, not a discussion.
- The interpretation of seabed scarps, as evidence of coseismic ruptures, is in my opinion doubtful, even erroneous, if one considers the context of the development of structures on the slope of the margin and the occurrence of salt-related deformations. To demonstrate a relationship with earthquakes, the authors must provide more observations.
- There is a lack of analysis of the maps and displacement profiles produced (Figure 3 in particular), especially when comparing seabed scarp heights and Unit 4 throws at key locations, and/or for each fault identified.
- It would be useful that the authors suggest ways of using the deterministic hazard map provided. Is this to be used to exclude any installation on «red» faults, or is it a decision tool for the engagement of further studies? Which of studies could be made to assess the hazard (probability of displacement)? What about the existing installations on “red”, “yellow”, “green” faults?
- Finally, an important point is to state on the availability of original data (bathymetry, seismic profiles), developed tools (algorithm) and results (numerical files of fault maps, seabed scarps, displacement measurement points, etc.).
The details of the comments are available in the attached pdf file.
Stéphane Baize
May Laor and Zohar Gvirtzman
May Laor and Zohar Gvirtzman
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