the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
GIS-based characterization of fault zones in South-Korea using information on seismicity, in-situ stress and slip tendency – Evaluation of respect distances for nuclear waste disposal site screening
Abstract. Identification of seismically active fault zones and the definition of sufficiently large respect distances from these faults which enable avoiding the damaged rock zone surrounding the ruptured ground commonly are amongst the first steps to take in the geoscientific evaluation of sites suitable for nuclear waste disposal. In this work we present a GIS-based approach, using the earthquake-epicentre locations from the instrumental earthquake record of South-Korea to identify potentially active fault zones in the country, and compare different strategies for fault zone buffer creation as originally developed for site search in the high seismicity country Japan, and the low-to-moderate seismicity countries Germany and Sweden. In order to characterize the hazard potential of the Korean fault zones, we moreover conducted slip tendency analysis, here for the first time covering the fault zones of the entire Korean Peninsula. For our analyses we used the geo-spatial information from a new version of the Geological map of South-Korea, containing the outlines of 11 rock units, which we simplified to distinguish between 4 different rock types (granites, metamorphic rocks, sedimentary rocks and igneous rocks) and the surface traces of 1,528 fault zones and 6,654 lineaments identified through years of field work and data processing, a rich geo-dataset which we will publish along with this manuscript. Our approach for identification of active fault zones was developed without prior knowledge of already known seismically active fault zones, and as a proof of concept the results later were compared to a map containing already identified active fault zones. The comparison revealed that our approach identified 16 of the 21 known seismically active faults and added 472 previously unknown potentially active faults. The 5 seismically active fault zones which were not identified by our approach are located in the NE- and SW-sectors of the Korean Peninsula, which haven’t seen much recent seismic activity, and thus are not sufficiently well covered by the seismic record. The strike directions of fault zones identified as active are in good agreement with the orientation of the current stress field of the peninsula and slip tendency analysis provided first insights into subsurface geometry such as the dip angles of both active and inactive fault zones. The results of our work are of major importance for the early-stage seismic hazard assessment that has to be conducted in support of the nuclear waste disposal siting in South-Korea. Moreover, the GIS-based methods for identification of active fault zones and buffering of respect areas around fault zone traces presented here, are applicable also elsewhere.
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CC1: 'Comment on egusphere-2023-2674', Giacomo Medici, 22 Mar 2024
General comments
Good geophysical research on the management of faulted nuclear waste repositories. The manuscript can be improved following the specific comments below.
Specific comments
Lines 13-15. The first sentence of your abstract is too long. Please, split it in two parts.
Lines 99-140. Provide more detail on the fault zones. Either extensional or strike-slip faults? Which is the dominant type?
Lines 99-140. Can you provide more detail on the geometry of the major faults such as thickness of the damage zone? The reader can make a correlation between thickness of the damage zone and length (see Figure A1).
Lines 108-110. There are also well known examples of repositories in other countries with either crystalline or sedimentary rocks. See also my comments below.
Lines 390-440. If the methodology is divided in sub-paragraphs, the same should be for the data. There is no match between the Paragraphs 3 and 4 on this point.
Lines 524-527. Take into account references on migration of contaminants where highly permeable faults are mapped in the area of a proposed repository for nuclear waste. Sellafield (West Cumbria, NW England) with crystalline and sedimentary rocks, see references below:
- Gutmanis, J. C., Lanyon, G. W., Wynn, T. J., & Watson, C. R. (1998). Fluid flow in faults: a study of fault hydrogeology in Triassic sandstone and Ordovician volcaniclastic rocks at Sellafield, north-west England. Proceedings of the Yorkshire Geological Society, 52(2), 159-175.
- Medici, G., West, L. J. (2023). Reply to discussion on ‘Review of groundwater flow and contaminant transport modelling approaches for the Sherwood Sandstone aquifer, UK; insights from analogous successions worldwide’ by Medici and West (QJEGH, 55, qjegh2021-176). Quarterly Journal of Engineering Geology and Hydrogeology. 56 (1), qjegh2022–097. doi:10.1144/ qjegh2022-097.
Lines 760-938. Please, insert the relevant references suggested above on migration of contaminants in an area of a proposed nuclear waste repository cut by normal faults.
Figures and tables
Figure 1. Specify the number of faults in the rose diagrams that should be 457+163+128 to match figure 8.
Figure A1. There is a correlation between length of a fault and its maximum capacity in terms of magnitude of earthquakes. This point has not been fully discussed in your manuscript. Please, consider my point due to the fact that you have produced a very important figure.
Table 1a. The content under “Map Colour” in unclear. Please revise it or apply changes to the caption.
Citation: https://doi.org/10.5194/egusphere-2023-2674-CC1 -
AC1: 'Reply on CC1', Stefan Bredemeyer, 03 May 2024
Reply to CC1: 'Comment on egusphere-2023-2674' by Giacomo Medici, 22 Mar 2024, Citation: https://doi.org/10.5194/egusphere-2023-2674-CC1
Dear Giacomo Medici,
thank you for the appreciation of our work and for your helpful suggestions for the improvement of the manuscript. Sorry for letting you wait! We applied all changes as you suggested in the specific comments section of your comment. Applied changes are as follows:
Specific comments
Lines 13-15. The first sentence of your abstract is too long. Please, split it in two parts.
Reply: You are right, we split the sentence into two.
Changes in Line 13:
Identification of seismically active fault zones and the definition of sufficiently large respect distances from these faults commonly are amongst the first steps to take in the geoscientific evaluation of sites suitable for nuclear waste disposal. These steps are of great importance as they enable to avoid construction of a repository in the damaged rock zones surrounding existing faults, particularly if these exhibit long-lived activity and thus are prone to rupture further.
Similarly, we split the long sentence starting in Line 228:
However, only 38,217 (i.e., 95 %) of the 40,134 fault zone vertices in the dataset were considered as a consequence of that 5 % of the fault vertices are located outside the boundaries of the here considered geological map. That is due to the fact that a few faults and lineaments are situated offshore, and some faults are located in a buffer zone along the northern border of the country which is not covered by information on lithology.
Lines 99-140. Provide more detail on the fault zones. Either extensional or strike-slip faults? Which is the dominant type?
Reply: The dominant type is strike-slip because of the transpressional setting in and around the Korean Peninsula. We added three sentences with corresponding information in Line 111 and one sentence in Line 158.
Insertion in Line 111:
The neotectonic stress regime in and around the Korean Peninsula was initiated in early Pliocene (i.e. about 5-3.5 Mio years ago) and is dominated by transpressional stresses imposed by the mostly NW-directed subduction of the Pacific and Philippine plates beneath the Eurasian plate and the far-field stresses imposed by the collision of the northward moving Indian plate with the Eurasian plate (e.g. Chough et al., 2000; Chough et al., 2010; Cheon et al., 2023). As a result, the crust of the Korean Peninsula is affected by both E-W oriented compression and N-S oriented extension, as e.g. was shown through measurements of dilatational stress rates obtained from 4 years of continuous GPS recordings acquired between 2000 and 2004 (Jin & Park, 2006). Focal mechanisms of recent earthquakes moreover indicate that the predominant faulting type is strike-slip with minor thrust components and an ENE-WSW oriented maximum horizontal stress (Park et al., 2007; Jun & Jeon, 2010; Soh et al., 2017).
Insertion in Line 158:
Strain energy mapped through continuous GPS measurements furthermore indicates that the areas of highest strain and hence highest earthquake potential are located in the central and northern parts and along the eastern shoreline of the Korean Peninsula (Jin & Park 2007). For this reason, many of the fault zones along the southern margin of the Precambrian Gyeonggi Massif, in the central parts of the metasedimentary/metavolcanic Okcheon Fold Belt and the Precambrian Yeongnam Massif, and all across the Cretaceous sedimentary Gyeongsan Basin either may be active or are at high risk to become reactivated (Figure 1).
Corresponding references:
Cheon, Youngbeom, Chang‑Min Kim, Jin‑Hyuck Choi, Sangmin Ha, and Seongjun Lee, Taehyung Kim, Hee‑Cheol Kang and Moon Son (2023). Near‑surface termination of upward‑propagating strike‑slip ruptures on the Yangsan Fault, Korea. Nature Scientific Reports | (2023) 13:9869 | https://doi.org/10.1038/s41598-023-37055-7
Chough, S.K., S.-T. Kwon, J.-H. Ree, D.K. Choi (2000). Tectonic and sedimentary evolution of the Korean peninsula: a review and new view. Earth-Science Reviews 52 2000. 175–235. doi: 10.1016/S0012-8252(00)00029-5
Chough, S.K. and Y.K. Sohn (2010). Tectonic and sedimentary evolution of a Cretaceous continental arc–backarc system in the Korean peninsula: New view. Earth-Science Reviews 101 (2010) 225–249. doi:10.1016/j.earscirev.2010.05.004
Jin, Shuanggen and Pil-Ho Park (2006). Crustal stress and strain energy density rates in South Korea deduced from GPS observation. TAO, Vol. 17, No. 1, 169-178.
Park, Jong-Chan, Woohan Kim, Tae Woong Chung, Chang-Eob Baag and Jin-Han Ree (2007). Focal mechanisms of recent earthquakes in the Southern Korean 2006 – Peninsula. Geophys. J. Int. (2007) 169, 1103–1114. doi: 10.1111/j.1365-246X.2007.03321.x
Jun, Myung-Soon and Jeong Soo Jeon (2010). Focal Mechanism in and around the Korean Peninsula (Korean with English abstract). 지구물리와 물리탐사 Jigu-Mulli-wa-Mulli-Tamsa. Vol. 13, No. 3, 2010, p. 198~202
Soh, Inho, Chandong Chang, Junhyung Lee, Tae-Kyung Hong and Eui-Seob Park (2017). Tectonic stress orientations and magnitudes, and friction of faults, deduced from earthquake focal mechanism inversions over the Korean Peninsula. Geophys. J. Int. (2018) 213, 1360–1373. doi: 10.1093/gji/ggy061
Lines 99-140. Can you provide more detail on the geometry of the major faults such as thickness of the damage zone? The reader can make a correlation between thickness of the damage zone and length (see Figure A1).
Reply: Information in this respect is merely available for major faults, which have been active recently, but we can provide information on the Yangsan fault which represents the longest surface trace in our dataset as reference in Line 144.
Insertion in Line 144:
Detailed information on damage zone widths and fault core widths usually is rarely available because of the tremendous efforts and costs of obtaining such information. Also in Korea, such information primarily exists for the major fault systems which recently have been active and thus have been subject to systematic palaeo-seismic studies and field surveys. The long-lived Yangsan fault in SE-Korea e.g. is one of these major structures, which recently went into the focus of such research efforts, as a consequence of the M 5.5 Geyongju earthquake in 2016 (e.g. Cheon et al., 2023). The surface trace of the ENE-SWS trending Yangsan fault extends about 200 km across the Gyeongsang basin (inset map in Figure 1) and above-mentioned studies have shown that its fault core is up to 100 m wide, while the damage zone extends up to several kilometres (Cheon et al., 2023). Having such numbers at hand as reference is useful for the determination of sufficiently large respect distances for both, active and inactive fault systems.
Following the rationale of your request, we further added some information on the limitations of the dataset and future research requirements at the end of the discussion:
5.4 Limitations of the available data and requirements for future research
The fault zone buffering strategy selected here aims at far-field enveloping of unsuited areas based on available data. Currently available data however does yet allow to consider 1) subsurface geometry of fault planes at sufficient detail, 2) across-fault asymmetry and along-strike heterogeneities of corresponding damage zones, 3) differences in rock mechanical properties of different rock types affecting the width of damage zones, and 4) local anisotropies and heterogeneities of rock masses affecting their deformation and breaking behaviour. Furthermore, as mentioned in the data description above, the majority of minor faults in the length range <2 km seems to be unknown and thus hasn’t been mapped yet. Such information still requires being acquired and compiled, since this length range may comprise many of the splay faults defining the damage zones around larger fault strands. The buffer zones resulting from our analysis thus are intended to be refined by more detailed field investigations during a later stage of the site search, particularly where the fault zones cross areas deemed to be suited for repository construction.
Corresponding references:
Cheon, Youngbeom, Chang‑Min Kim, Jin‑Hyuck Choi, Sangmin Ha, and Seongjun Lee, Taehyung Kim, Hee‑Cheol Kang and Moon Son (2023). Near‑surface termination of upward‑propagating strike‑slip ruptures on the Yangsan Fault, Korea. Nature Scientific Reports | (2023) 13:9869 | https://doi.org/10.1038/s41598-023-37055-7
Kim, T., Choi, J.H., Cheon, Y., Lee, T.H., Kim, N., Lee, H., Kim, C.M., Choi, Y., Bae, H., Kim, Y.S., Ryoo, C.R., and Klinger, Y. (2023). Correlation of paleoearthquake records at multiple sites along the southern Yangsan Fault, Korea: Insights into rupture scenarios of intraplate strike-slip earthquakes. Tectonophysics, 854, p.229817. https://doi.org/10.1016/j.tecto.2023.229817
Lines 108-110. There are also well-known examples of repositories in other countries with either crystalline or sedimentary rocks. See also my comments below.
Reply: That is a good point. We added some more countries as examples to show that consideration of crystalline and sedimentary rocks as potential host rock actually is (and should be) a common approach where these are available and referred to literature reviewing the geoscientific approaches of different countries for further reading.
Insertion in Line 121:
South-Korea plans to start their site search with a blank white map, and choses to investigate crystalline and sedimentary rock types as host rock for the deep geological repository (e.g. Kim et al., 2000), as is done in many other countries, e.g. in Finland, France, Belgium, Germany, Switzerland, UK, and USA (Faybishenko et al., 2016; Hartley et al., 2017).
Corresponding references:
Faybishenko, Boris, Jens Birkholzer, David Sassani, Peter Swift (2016). International Approaches for Deep Geological Disposal of Nuclear Waste: Geological Challenges in Radioactive Waste Isolation - Fifth Worldwide Review. Prepared for: U.S. Department of Energy, by Lawrence Berkeley National Laboratory and Sandia National Laboratories. DOI: 10.2172/1353043
Hartley, Lee, David Roberts, Fiona Hunter, and Barbara Lambers (2017). Evidence - The early stages of implementing geological disposal: regulatory use of geoscientific information. Project Number: SC100011, Environment Agency - March 2017. ISBN: 978-1-84911-390-8
Lines 390-440. If the methodology is divided in sub-paragraphs, the same should be for the data. There is no match between the Paragraphs 3 and 4 on this point.
Reply: You are right, the description of the results of the earthquake cluster analysis rather is content for the results section and hence we moved it down to the results section 4.1 Earthquake clusters. The results of the fault zone classification are now in section 4.2 Potentially active and inactive fault zones.
Deletion of the paragraph starting in Line 385, because of redundancy w.r.t. contents in section 4.2 of the results section:
Comparing the extracted features from rules 1 and 2, there is a large intersection between the two groups, where faults fulfilled both rules 1 AND 2. 128 of the faults found by rule 1, additionally are within 1 km range of an earthquake epicentre and touch or intersect an earthquake cluster. Only 35 of faults picked by rule 2 do not touch or intersect pre-determined earthquake-epicentre clusters at the same time.
Insertion of method description in Line 385 as replacement for the above deleted passage:
The results of the feature extractions done by application of rules 1 and 2 were then compared by means of intersecting the two datasets in order to determine the overlap between the two groups of extracted fault segments, in order to find the fault zone traces which satisfied both rules 1 and 2, and those merely satisfying rule 2.
Insertion in section 4. Results, Line 389:
For this purpose, we first generated heatmaps of the earthquake epicenter locations in the catalogue which in turn were used to outline 1) earthquake clusters and 2) 2x2 km search kernels around each of the earthquake epicenters. The results of the earthquake cluster analysis are described in section 4.1. Both the outlines of the earthquake clusters and the search kernels were then used for extraction of potentially active fault zones. The results of the classification into potentially active fault zones and likely inactive fault zones are described in section 4.2.
Insertion in Line 394:
4.1 Earthquake clusters
Insertion and adjustments of the text passage on earthquake cluster analysis results in Line 395:
The outlining of earthquake clusters described in section 3.3 yielded outlines of 169 earthquake-clusters, 78 of which are located on South-Korean mainland (Figure 7). They cover (…)
Insertion and adjustments in Line 395:
4.2 Potentially active and inactive fault zones
The feature extraction described in section 3.4 yielded 2 largely overlapping sets of in total 488 fault surface traces, which in the following are to be classified as “potentially active in recent times”, and a remainder of 1,040 fault zones which, at least according to the earthquake catalogue, are not located in immediate proximity to regions of recent earthquake activity (Figure 7).
457 of the 1,528 known fault zones (…)
Lines 524-527. Take into account references on migration of contaminants where highly permeable faults are mapped in the area of a proposed repository for nuclear waste. Sellafield (West Cumbria, NW England) with crystalline and sedimentary rocks, see references below:
Gutmanis, J. C., Lanyon, G. W., Wynn, T. J., & Watson, C. R. (1998). Fluid flow in faults: a study of fault hydrogeology in Triassic sandstone and Ordovician volcaniclastic rocks at Sellafield, north-west England. Proceedings of the Yorkshire Geological Society, 52(2), 159-175.
Medici, G., West, L. J. (2023). Reply to discussion on ‘Review of groundwater flow and contaminant transport modelling approaches for the Sherwood Sandstone aquifer, UK; insights from analogous successions worldwide’ by Medici and West (QJEGH, 55, qjegh2021-176). Quarterly Journal of Engineering Geology and Hydrogeology. 56 (1), qjegh2022–097. doi:10.1144/ qjegh2022-097.
Reply: Thank you for hinting at these works. We read them with great interest and inserted them as reference at suggested instance in Line 527 and furthermore added a reference on fault damage zones. The sentence now reads as follows:
Apart of determining whether fault zones are active or not, safety distances have to be assigned around fault zones (e.g. He et al., 2022) and for this purpose the areas of corresponding damage zones (e.g. Kim et al., 2004) have to be estimated in order to ascertain that the confinement functions of a repository are not lost due to nearby fault activity or by perturbations such as increased water, gas and contaminant permeability due to fault displacement (e.g. Gutmanis et al 1998; Medici & West 2022, 2023).
Corresponding references additional to those proposed above:
Kim, Young-Seog, David C.P. Peacock, David J. Sanderson 2004. Fault damage zones. Journal of Structural Geology 26 (2004) 503–517. doi: 10.1016/j.jsg.2003.08.002
Medici, G and West, LJ (2022) Review of groundwater flow and contaminant transport modelling approaches for the Sherwood Sandstone aquifer, UK; insights from analogous successions worldwide. Quarterly Journal of Engineering Geology and Hydrogeology, 55 (4). qjegh2021. ISSN 1470-9236 https://doi.org/10.1144/qjegh2021-176
Lines 760-938. Please, insert the relevant references suggested above on migration of contaminants in an area of a proposed nuclear waste repository cut by normal faults.
Reply: Done as described in the reply to the previous comment.
Figures and tables
Figure 1. Specify the number of faults in the rose diagrams that should be 457+163+128 to match figure 8.
Reply: The rose diagrams in Figure 1 show the fault orientations of all 1,528 fault surface traces available for analysis (where 488 are active and 1,040 are currently inactive according to our results). We now indicate that number on the rose diagrams in Figure 1 as we did in Figure 8 for the subsets of potentially active faults.
Figure A1. There is a correlation between length of a fault and its maximum capacity in terms of magnitude of earthquakes. This point has not been fully discussed in your manuscript. Please, consider my point due to the fact that you have produced a very important figure.
Reply: You are right, this correlation previously was addressed in the data description, but we now re-located it to the discussion. More specifically, we now hint at the correlation in the data description, and moved magnitude and maximum displacement estimates for Korean fault zones from the data description to a new subsection (section 5.2) of the discussion. Furthermore, we deleted the average earthquake magnitude estimates which previously were indicated in Tables 2 and A1 for the different rock types, since it doesn’t add too much value to the manuscript to know that each rock type hosts fault zones which at average may produce magnitude 6 earthquakes. Also since the magnitude estimates are now discussed later in the manuscript, the Figure A2 showing the estimates has been moved down and now is Figure A6 (the numbers of Figures A3-A6 have been changed accordingly).
Insertion in Line 232:
Considering that fault length distributions typically follow a power law having most fractures in the short length range it furthermore attracts the attention that the length range <2 km only comprises 238 fault segments in comparison to the 2 – 4 km range described above which comprises 485 fault segments (Appendix Figure A1). This discrepancy hints at that our dataset probably is largely incomplete in the length range <2 km, and that we presumably lack several hundred fault surface traces in this length range which haven’t been noticed and mapped yet for the determination of damage zone extents. Moreover, note that some of the major and minor local fault segments may be part of larger fault strands, which must be taken into account when applying fault zone scaling laws to determine e.g. the magnitudes of earthquakes, the maximum displacement, and the damage zone widths these fault zones can produce, if a future earthquake event ruptures the entire fault strand (see sections 5.2 to 5.4). The main fault strand of the Yangsan fault zone in our dataset e.g. is represented by a 126 km continuous segment line and some shorter segments to the NE and SW, rather than extending along the previously mentioned 200 km. Estimates obtained from application of scaling laws to this data thus inevitably will be underestimates.
Text passage moved from the data description to section 5.2 starting in line 589:
5.2 Capability of faults to rupture a canister for high-level radioactive waste disposal
Earthquake magnitude commonly correlates with the geometric features of the rupture it produces, such as its rupture length, the average and maximum displacements along the fault plane and the slip rate (e.g. Wells and Coppersmith, 1994). Knowing the rupture length of a particular fault hence amongst others enables estimation of the magnitude it can produce. According to Wells and Coppersmiths (1994) empirically determined relationship between Moment Magnitude M and surface rupture length SRL of a fault (M = 5.08 + 1.16*log (SRL [km])), the fault zones investigated in this work, would be capable of producing earthquakes in the magnitude range 4.2-7.5 (Appendix Figure A6). The largest earthquake magnitude of M 7.5 was obtained for the 126 km long surface rupture trace of the Yangsan fault. Despite being an underestimate, since we only consider a fraction of the actually 200 km long fault strand, this estimate is quite a bit larger than the maximum magnitude of 6.8 estimated from fault displacement by Kyung et al. (2010) for the Yangsan fault zone. Modern canisters for disposal of spent nuclear fuel as e.g. used in Finland and Sweden, and as planned to be used also in Korea (Choi et al., 2013) consist of a copper-shell and a massive cast iron inset, which are designed to withstand an arbitrary shear of 0.05 m (SKB, 2010). Thus, the maximum displacement on a fault cutting through the disposal site shall not exceed 0.1 m according to the results of the safety assessments performed by SKB (SKB, 2010). Relating maximum displacement Dmax to the surface rupture lengths SRL of the faults in our dataset utilising the regression line (log(Dmax [m]) = -1.38 + 1.02*log(SRL [km])) of Wells and Coppersmiths (1994), all of the faults considered here would be capable to produce a maximum displacement which exceeds 0.1 m.
Table 1a. The content under “Map Colour” in unclear. Please revise it or apply changes to the caption.
Reply: We revised the caption and table heading to show more clearly that we provide the hexadecimal colour codes and the RGB colour codes of the colours used for rock type distinction.
Additional to the requested changes we made minor corrections.
Line 216: correction/harmonization of the spelling of the name of the Okcheon fold belt, previously “misspelled” in this instance as Ogchon belt
Line 325: The number of the fault zone segments picked by the circular 1-km search kernel is 142 and not 128. We corrected that number accordingly and the sentence now reads: “This slight enlargement of the search kernel area in comparison to a more circular area if using a smaller pixel size of 0.1 km, resulted in picking 163 instead of only 142 earthquake events in the vicinity of known faults in South-Korea.” This change does not cause any conflict with any other contents or statements in this work.
Line 340: We inserted “and objects” into “A contour map was then created from this heatmap raster image data (Figure 6), in order to enable working with the heatmap information as a tool for selecting defined areas and objects in other datasets such as our fault zone traces located within earthquake clusters”
Line 495: We moved the contents of the brackets one word further to the back to place it behind the word it refers to (yellow and red lines in Figure 7).
Line 497: We inserted “usually” into “…, which is consistent with the fact that fractures optimally oriented for slip usually are highly dipped for the strike-slip stress regime (Xie & Min, 2016).”
Line 640: We inserted another “potentially active” into the fault length+activity classification which we proposed at the end of the discussion of our manuscript and simplified the length description for the category of minor inactive faults from “(10m-1km)” to (<1 km). The sentence now reads as follows: “The authors of this work thus propose using 3-km-buffers applied both to potentially active supra-regional and regional fault zones (>10 km length), 1-km-buffers applied to potentially active major and minor fault zones (<10 km length) and further to apply 100-m-buffer zones to inactive regional faults (> 10 km length), 50-m-buffers to inactive major faults (1-10 km length), and 5-m-buffers to inactive minor faults (<1 km length).”
Citation: https://doi.org/10.5194/egusphere-2023-2674-AC1
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AC1: 'Reply on CC1', Stefan Bredemeyer, 03 May 2024
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CC2: 'Comment on egusphere-2023-2674', Ki-Bok Min, 30 Apr 2024
This is an interesting and useful work for the siting of geological repository for nuclear waste. Slip tendency analysis has not been actively used in the community of nuclear waste repository community and this work can contribute to paving the way for more wider application.
The merit of this work is that it can cover entire country by utilizing GIS based approach and this work can be a decent starting point for initializing site investigation.
One need to bear in mind that there are uncertainties associated with slip tendency as the input parameter such as friction coefficient and in situ boundary stresses often have uncertainties.
This work has never been used in Korea, and, overall, this is a nice contribution.
Citation: https://doi.org/10.5194/egusphere-2023-2674-CC2 -
RC1: 'Comment on egusphere-2023-2674', Tiegan Hobbs, 14 May 2024
I wish to thank the authors and editor for the chance to review this very engaging paper, however, I find that I must recommend that this paper not be published in its current format. The manuscript uses a 32-year seismic catalogue of under 900 earthquakes with average magnitude 2.6 and a national database of bedrock faults to try to identify active faults in South Korea, for application to selection of nuclear waste disposal sites. The method used to process the seismic data for comparison to fault locations is not aligned with industry best practice for smoothing seismicity catalogues for hazard assessment, and gives no consideration to 3D structure of these faults at depth. Additionally, and most critically, this catalogue is inherently far too short to draw any reasonable conclusion about the kind of long-term hazard (thousands of years) that must be understood in order to site facilities for nuclear waste storage at shallow depth in the earth’s crust. Finally, the discussion of buffer widths is interesting, but does not present enough information to helpfully allow a reader to understand what buffers may be most appropriate in South Korea. The paper concludes with what I would describe as a fairly subjective recommendation about a set of active faults and buffer widths to be used for nuclear storage site screening in North Korea, without sufficient analysis or discussion of hazard from unmapped faults.
I worry that policy makers or stakeholders may misinterpret this paper as an authoritative seismological screening of safe sites for nuclear waste storage, which would require at least a probabilistic assessment of seismic hazard over very long return intervals, further paleoseismic investigation of a region surrounding any site of interest, and 3D geological models. However, I applaud the intention of wanting to find a technique for creating a national inventory of active faults for public safety using widely available data.
For publication in future, I would want to see the analysis redone with all seismic data (going back hundreds of years), using a well-established seismicity smoothing method (or implementing a national seismic hazard model), and potentially including the impact of, say, age of deposition/emplacement/latest deformation for geological units and perhaps geodetic strain accumulation in determining which faults are likely to rupture in the current tectonic regime. I believe conclusions should be toned down in either case, presented alongside a full discussion of known unknowns when it comes to active fault inventories, and described only as a possible approach for very high-level screening with large uncertainties that should be dealt with at each site of interest.
I regret that this review is quite negative, but feel that nuclear storage represents a very high stakes application that warrants a high degree of scrutiny. If you have any questions, please don’t hesitate to reach out.
Kind regards,
Tiegan Hobbs
Comments, in addition to in-text revisions:
- MAJOR: The definition of ‘active’ fault already exists: a fault with evidence of earthquake rupture in the Holocene. There should be a fuller discussion of how and why the criteria for ‘activity’ were chosen for this paper, and I think it would be appropriate to use a different word than ‘active’ for the faults meeting this criteria. This is in particular because the criteria used in this paper are very relaxed and somewhat imprecise (e.g. an earthquake within 1km of a fault makes that fault active, without regard for the 3D structure of that fault or depth of the earthquake.
- MINOR: Please describe what ‘elaborate’ field work is, as this term is used repeatedly.
- MAJOR: How does the seismic hazard model of South Korea compare to the regions identified as potentially suitable for nuclear waste disposal? This analysis does not consider regions which may not have current/modern seismicity but which are capable of rupturing in the future, which would be very important for nuclear waste storage. For example: blind active faults (therefore not mapped), faults with very long recurrence which have minimal/no ongoing seismicity until the next major event.
- MINOR: Fix Figure 2 to make it “flow”. See suggestions in text.
- MAJOR: Relocation procedure and final spatial uncertainty of the KMA catalogue need to be stated explicitly, as it is different than the resolution of the data. Same for the older datasets that were excluded. You should also indicate the magnitude of completeness for each catalogue and show the seismic stations used in the relocation. If there are fewer stations in the north mainland it could explain the apparent paucity of events. I also think it would be critical to include all older data, in order to have as much information as possible about the long-term seismicity in South Korea, particularly for a nuclear waste application. There could have been a swarm of seismicity in the north that just hasn’t been active in the very short 32 year period of consideration (many fault lines have recurrence intervals of hundreds to thousands of years).
- MAJOR: The use of Kernel Density Estimation to create a seismicity model is out of line with current best practice from the Seismic Hazard Modelling community. They commonly use a smoothed seismicity approach (e.g. https://doi.org/10.1785/BSSA0860051372, https://doi.org/10.1785/0220180040, https://doi.org/10.1016/j.jafrearsci.2020.103894, https://doi.org/10.1177/87552930231215428). If the authors wish to break from the best practice, I think they should explain the physical reason for the change in approach. As it stands, the authors are using a very small/short-term seismic catalogue, with very few details given about its magnitude of completeness, relocation, declustering, etc., and applying a KDE method that ends up representing seismicity over a broad area in a misleadingly small pixel. These small pixels, representing large regions, are then used to identify which faults are potentially active with implications for nuclear waste storage. As the authors identify 457 fault segments based on this approach, with roughly 900 earthquakes of mostly small magnitude, they are therefore identifying faults with an average of 2 events per fault. This type of analysis should be handled by observational earthquake seismologists with experience in seismic hazard modelling, and preferably should be treated by consideration of a fully probabilistic seismic hazard model designed for assessment of long recurrence hazard sources. This will mean including seismicity from a much longer interval and likely consideration of seismic source zones as well as regions which may be capable of seismicity even if they have not had a modern earthquake recorded instrumentally.
- MAJOR: Figure 6 colormap is wrong, according to caption, and also shows light yellow areas where there is less than 1 but more than 0 earthquakes. This is impossible. See note in text.
- MINOR: Figure A3 needs to be part of the main text.
- MAJOR: The 1km analysis seems both (1) needlessly complicated and (2) insufficient to actually help identify causative faults of these earthquakes. If you want to find faults within 1km of an earthquake, you just need to draw a 1km buffer around each epicenter and intersect it with fault lines, not obscure it with a heatmap and then intersect that polygon. That analysis is different than finding faults within 1km of an earthquake. Secondly, this analysis doesn’t take into account three dimensional structure of the faults at depth – it may be that they actually are much more than 1km apart. What you are really doing here is seeing if the surface trace of a bedrock fault is within 1 km of the surface projection of earthquakes. This is not a meaningful assessment of whether a fault is seismically active. Finally, your average earthquake size is magnitude 2.6, which is very small. As a seismologist, I wouldn’t necessarily consider that every small event needs to occur on a major (capable of M5+) fault. Therefore, labelling a fault as active based, potentially, on a single small earthquake is not valid. I must therefore say that I don’t feel this analysis has sufficient scientific merit to be presented in a published paper as is.
- MAJOR: Line 403 states that faults which did not meet the criteria used herein (proximity to clusters of seismicity or a single event from a 32-year catalogue of less than 900 events) are “classified as not currently active”. This is potentially dangerously misleading, as the analysis of the potential seismic hazard from these faults has not been appropriately nuanced/sufficient to make this claim. But stakeholders such as nuclear energy operators, dam operators, etc. may cite this study to justify not completing further investigations. It may also lead stakeholders or the public to believe that only known faults are capable of hosting earthquakes in the future, which we know to be untrue. Therefore, I must object in very strong terms to this analysis and phrasing being published as is.
- MAJOR: The slip tendency analysis does not use any known dip angles, but instead calculates the slip tendency values at representative dips. The authors then conclude that because their faults show high slip tendency at high dip angle that their faults are indeed active, and that since they are active they are probably high angle (circular logic). I don’t find this particularly convincing without knowing the true dip angle of these faults. They then investigate the slip tendency value of the “not active” faults as well. It seems that this analysis just shows that almost any fault with high dip angle will have high slip tendency, rather than proving something about this particular subset of “active” faults.
- MAJOR: This work talks about “behavioural segments” for buffer width determination, but the current fault dataset may not be segmented in the same way. Usually behavioural segments are based on a detailed knowledge of the behaviour of the fault (e.g. creeping rate, seismicity rate, characteristic rupture extent, multi-event rupture segmentation, location of asperities), whereas in my experience, bedrock fault databases are segmented somewhat arbitrarily. Therefore, the authors should prove that this dataset has sufficient behavioural segmentation information for this method to be applied. They should also address the issue of faults which are capable of rupturing together, such as is seen in California (UCERF model), Alaska (Denali-Tintina Fault), etc.
- MINOR: Ensure all maps have lat/lon markings at the periphery.
- MAJOR: When establishing buffer zones, the authors have not discussed horizontal uncertainty in the location of mapped faults. Depending on when they were mapped (were handheld GPS units used to denote high-precision locations or were geologists triangulating off of nearby landmarks to get an approximate location?), what criteria was used to determine their presence (was a fault zone observed or was a change in lithology assumed to be due to a fault which was then placed somewhere between the last location of lithology A and first location of lithology B), and the attention paid by cartographers digitizing paper maps (how were overlapping lithology boundaries harmonized? What behaviour was assumed under water bodies that were not mapped?), there can be significant uncertainties in a faults mapped location — certainly larger than 100m. The suggested guidelines are therefore unrealistically precise given our knowledge of faults.
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RC2: 'Comment on egusphere-2023-2674', Ramon Arrowsmith, 07 Jun 2024
Review of
GIS-based characterization of fault zones in South-Korea using
information on seismicity, in-situ stress and slip tendency - Evaluation
of respect distances for nuclear waste disposal site screening
by
Bredemeyer et al
Reviewer
Ramon ArrowsmithSummary
Interesting paper presenting a methodological exploration largely geospatially-based to identify potentially active faults.
I appreciate the detailed work and the clear evidence of the basis on rich datasets. The readability can be significantly improved (and thus the paper made nore effective) if the writing were streamlined by trimming substantial extraneous detail from the main text and placing it accessibly in the appendix. This reads in places like a report to an agency and in that sense is thorough. However, many readers will struggle to find what are the important contributions if they are diluted.I am concerned about the definition of recently active faults as presented in the paper. What we learn is that there are faults, variably oriented relative to the current stress state, which are associated with instrumental seismicity. It is logical that they may have been active in the past, and will be again in the future, but just because a fault not producing seismicity over that last few decades, does not mean it is inactive. There are many cases of seismically quiet faults (locked) which have produced great earthquakes. This is particularly a concern in my view for the intraplate low deformation rate setting of the Korean peninsula. So, the semantics of active faults need to be addressed.
General comments
1) While I appreciate data rich papers--and this is one--there is too much detail in places making it hard to read and diluting the impact. For example there are places I have tried to highlight with unnecessary detail in the text. I have noted for example, that Tables 1a and 1b are not apparently needed for the main text. I think the text or caption should better make the case for the inclusion of such detail in the main text. Otherwise, put it in the appendix and refer to it. In other cases, we have long and dense enumeration of data that can be stated more simply for the reader. In another example, Why is knowing that the interquartile range of the fault surface trace segmentation important? How does it aid in understanding the processes and the hazard? If you cannot clearly demonstrate the relevance, then it should be cut. THis comment also refers to at least some of the seemingly trivial logical expressions used in QGIS.
2) I am not comfortable ascribing even a generic probability increase of rupture, given the significant emphasis on disciplined probabilistic analysis in geological repository safety assessment. See for example, "Updated Implementation Guidelines for SSHAC Hazard Studies" NUREG-2213. Where in that structured approach, might this analysis fit, and how indeed does it indicate a probability increase for rupture?
3) The discussion of the comparison of the buffering is interesting. However, why is it a problem to exclude some percentage of the territory? That is the point of the exercise. And, there is still at least 70% remaining. I think this point needs to be more carefully stated.
4) It is not clear what the slip tendency adds. I think it is a nice addition, but it needs to be integrated with the screening and the buffer analysis somehow, otherwise it appears like an unjustified addition to the paper.
Specific comments tied to the manuscript by element
Title--The title is ok but I don't like acronyms in titles and would think of restructuring it. Perhaps:
Characterization of fault zones in South-Korea using information on seismicity, in-situ stress and slip tendency - Evaluation of respect distances for nuclear waste disposal site screening
Abstract--Generally adequate abstract. However at lines 20-25, I would like to see a sentence or two more on the method details and how the results were integrated, and then compact or shorten other aspects of the abstract that are more general.
Line 40--Perhaps this is a bit pendantic, but GIS is really just a tool to do geospatial analysis. I think a bit more general tone about what it is about the study of the structures that is needed and their integration, which then requires the GIS toolkit is a bitter logic flow; rather than making it sound like the GIS is a hammer looking for a nail to pound.
Line 45--"which will be" what does that mean and when? Does it mean when the paper is published or some other time?
Lines 49-53--This is awkwardly stated. The active faults are those which might experience slip within the time period of concern for the repository. There activity in the (late) Quaternary is what we would like to use to anticipate their (relative) activity.
Lines 54-56--Important indeed to recognize the potential for reactivation of structures for the reasons noted; can also cite Townsend and Zoback; weak faults in strong crust?
Line 60--Could mention that the reason to do the field work and have the lidar is in part to characterise the landscape and surficial geology which may indicate a relationship between the faults and those features.
Lines 65-67--I get the idea of the association between the seismicity and well oriented faults, but I am not comfortable ascribing even a generic probability increase of rupture, given the significant emphasis on disciplined probabilistic analysis in geological repository safety assessment.
Line 70--I think it is important to acknowledge that the instrumental seismicity reflects a very short portion of the likely earthquake recurrence rate of an intraplate setting like Korea. There is a lot of literature on this point. Yes, you can use it because it is available and of high quality, but we know that it only gives an incomplete in time record of activity.
Lines 73-97--Good summary of the analyses to be done.
Line 99--Geology no capitalization
Line 105--data are plural
Line 135--Need to start a new paragraph here "A first concept..."
Tables 1a and 1b are not needed in the main text. The detail distracts from a straightforward summary of the work and the reader can get a sense of what the point is by review of figure 1.
Lines 165-175--This might be better as a small table.
Lines 191-195--I think this is too much detail. Why is knowing that the interquartile range of the fault surface trace segmentation important? How does it aid in understanding the processes and the hazard? If you cannot clearly demonstrate the relevance, then it should be cut.
Paragraph at line 205--again, why is this detail on lineament lengths important?
Line 233--amongst this detail, we find what is interesting that indeed the tectonic domains become evident.
Table 2--Ok it is useful to see the geometrically defined possible magnitude of about M6.
Table 3--I don't see the importance of this table for the main text. What is the point of it? It shows the thorough analysis but...
Table 4--ok but there is extraneous information. Why is the perimeter important? THe percentages are useful, but not to two significant figures.
Paragraph at line 290--Heatmaps are a standard product so some of the basic description of them could be trimmed. It is really just a visualizatoin of the analysis which is the spatial kernal density.
Paragraph at line 390--Here we get to the results. I would be very careful with the language here. We have a horizontal spatial association between fault traces and instrumental seismicity. Indeed that is a reasonable assumption that at least some of the earthquakes occurred along those faults. We don't know how the fault systems vary with depth and we know that the earthquakes are occurring in the upper crust so this 2D association is a first step. "Potentially active in recent times" is ok but recent here has different meanings. Based on the analysis, it is strictly within the time since the earthquakes occurred locally (or catalog age overall). It does not mean recent geologically unless you want explicitly state the assumption that activity in the instrumental period is consistent with longer time scale activity in the past (and the future).
Table 6--I am not convinced that this is very helpful for the main text. It is useful I suppose to have a sense of which rocks have more and less potentially active faults, but that information by itself, without other context could actually hinder an assessment of optimal siting regions if it is applied blindly.
Line 455--good result that they compare well but it is not so evident in figure 10.
Section 5.2--Slip tendency analysis is a useful complement to the other elements of the research presented here.
Line 483--Stress state reference ok but how about a quick reference to other data sets including the world stress map?
Line 490--I am concerned about the lack of knowledge on dip of the faults as analyzed here, given its importance in the slip tendency calculations.
Lines 497 and else where--"highly dipped" is not a standard nomenclature in my view; I would expect to read "steeply dipping"
Lines 515 and vicinity--This paragraph needs to be rewritten a bit more simply. There is some repetition and the long sentences make it hard to read. I would also write "low dip" and I think the deviatoric stress being a quantity should be referred to as "greater" or "smaller" or "lower"
Line 520--I don't think you should proceed here without summarizing briefly what we have learned with the slip tendency? It looks like maybe the steeper faults within the seismicity zones show a greater overall slip tendency. Can this be quantified? maybe a histogram of slip tendency to complement at least some of the panels in Figures 11 and 12?
Tables 7 and 8 and 12 and 13--The main result for the text narrative is that the polygon coverage in total is about 2.65% or 5.26% or 9.29% or 20.45% of the onshore portion of South Korea. I don't read a justification for the rest of the detail in this figure to be included in the main text.
Table 10--Significant figures are too many. We don't need to know to the 1000th of a percent how much the rock types are covered. There is various error in the computations that probably exceeds some of the sig figs.
Line 595 area--THe QGIS logical expressions are trivial and the reader would normally expect that this was how it was done. i would put a summary of these expressions in the appendix
Line 620 and area--Here we get something interesting appearing: the discussion of the comparison of the buffering. However, why is it a problem to exclude some percentage of the territory? That is the point of the exercise. And, there is still at least 70% remaining. I think this point needs ot be more carefully stated.
Appendix--This has useful information. More from the main text could be placed here to make the main text more readable and effective. I would like to see the appendix begin with a short overview as to what is contained within it.
Line 760--Strange formatting errors. Did we also lose the beginning of the references?
THe references would be much easier to read if they had a hanging indent.
Figures
Figure 1--This is not a geologic map but rather a lithologic map with overlain faults. It seems to be ok for the purposes of this study.
Line 115--I don't think it is necessary to provide the detail on the number of polygons, fault traces, and lineaments. It is distracting.
Line 121--Parenthetic statement about the data sources is appreciated, but is too much detail for this caption. I would have a brief reference to the appendices here and then in the appendix, provide this kind of information for the rare reader who might want to recreate the figure.Figure 2--I like the flow chart. I think some of the directionality at the lower right is not clear. I would think that the final output would be the discrimination of potentially active faults form all of the faults, followed by buffers around them. Thus, it should be more clear that the slip tendency is part of the screening and that the buffers are the final outputs.
Figure 3--nice looking figure.
Caption ending with detail on datasets may not be needed if that information can be presented in the appendix.Figure 4--ok; useful to see the seismicity along with the fault network.
FIgure 5--ok; but the color bar does not need to be nearly as large as the map. However, given that FIgure 6 shows similar, and is used in subsequent analyses, I would cut figure 5.
FIgure 6--ok; the color bar is way too large. I would cut all contours except the ones that are used for the intersection and then the reader can see the color map underneath and that will be sufficient. Delete figure 5
Figure 7--This is an important figure. Mask the offshore given that it is not part of the analysis.
Figure 8--lots of detail; could simplify for main text to highlight the main point?
Figure 9--Seems like a useful figure
FIgure 10--Good but hard to get the comparison between the datasets
Figure 11--THis is an interesting figure. Please only use one color bar scale (given that the range is the same for all) and make it about half the current size that will clean up the figure a bit.
Figure 12--THis is an interesting figure. Please only use one color bar scale (given that the range is the same for all) and make it about half the current size that will clean up the figure a bit.
Figures 13 and 14 and 16--This is ok but not that helpful. The map scale is such that the lineweight seems to obscure the buffer polygons and so it does not actually add anything. We get slightly more out of the /50 case. Can this be combined with a reference to earlier fault maps and maybe then here provide some zooms to interesting regions? If you must keep these figures, you could probably make them two or three panels of a single figure given the large amount of repeated information?
Figure 15--This is a helpful zoom. But, it needs some lat longs and indication box in a prior figure so we know where it is.
Figure 16--At least here the buffers are large enough that they can be seen at this scale. But again, maybe combine this somehow into a multipanel figure with Figures 13, 14, and 16? Provide similar zooms as FIgure 15?
Review questions:
Does the paper address relevant scientific and/or technical questions within the scope of NHESS? YES
Does the paper present new data and/or novel concepts, ideas, tools, methods or results? Marginally. I think that the GIS analysis of the rich datasets is interesting and correct but I don't see significant novelty.
Are these up to international standards? YES
Are the scientific methods and assumptions valid and outlined clearly? Mostly. See my concerns about definition of active faults.
Are the results sufficient to support the interpretations and the conclusions? YES
Does the author reach substantial conclusions? YES
Is the description of the data used, the methods used, the experiments and calculations made, and the results obtained sufficiently complete and accurate to allow their reproduction by fellow scientists (traceability of results)? YES there are but are perhaps documented with obfuscating detail.
Does the title clearly and unambiguously reflect the contents of the paper? YES
Does the abstract provide a concise, complete and unambiguous summary of the work done and the results obtained? NO; see comments above
Are the title and the abstract pertinent, and easy to understand to a wide and diversified audience? YES
Are mathematical formulae, symbols, abbreviations and units correctly defined and used? If the formulae, symbols or abbreviations are numerous, are there tables or appendixes listing them? YES
Is the size, quality and readability of each figure adequate to the type and quantity of data presented? NO
Does the author give proper credit to previous and/or related work, and does he/she indicate clearly his/her own contribution? YES
Are the number and quality of the references appropriate? YES
Are the references accessible by fellow scientists? YES
Is the overall presentation well structured, clear and easy to understand by a wide and general audience? NO
Is the length of the paper adequate, too long or too short? Far too long for the result.
Is there any part of the paper (title, abstract, main text, formulae, symbols, figures and their captions, tables, list of references, appendixes) that needs to be clarified, reduced, added, combined, or eliminated? YES; see my comments above
Is the technical language precise and understandable by fellow scientists? YES
Is the English language of good quality, fluent, simple and easy to read and understand by a wide and diversified audience? YES except for a few places I have highlighted.
Is the amount and quality of supplementary material (if any) appropriate? YES, but some may in the end not be totally relevant. The authors should justify to the reader why the data are included, either in the main text or the appendix. THe reader gets the sense that the data and the analysis of high quality but there is a lot of repetition and detail that may be extraneous to the results.Citation: https://doi.org/10.5194/egusphere-2023-2674-RC2
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