Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbaatar (Mongolia)

Al-Ashkar, Abeer; Schlupp, Antoine; Ferry, Matthieu; Munkhuu, Ulziibat

doi:https://doi.org/10.5194/se-13-761-2022

Articles | Volume 13, issue 3

https://doi.org/10.5194/se-13-761-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-13-761-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 13, issue 3

Research article

|

31 Mar 2022

Research article |

| 31 Mar 2022

Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbaatar (Mongolia)

Abeer Al-Ashkar, Antoine Schlupp, Matthieu Ferry, and Ulziibat Munkhuu

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Final revised paper (published on 31 Mar 2022)
Supplement to the final revised paper
Preprint (discussion started on 30 Aug 2021)

Interactive discussion

Status: closed

RC1:
'Comment on se-2021-91', Daniela Pantosti, 12 Oct 2021
The paper by Al-Ashkar et al. entitled Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbatar (Mongolia), describes and characterize, from a seismogenic point of view, an active fault located south Ulaanbataar, the capital city of Mongolia. Large earthquakes are well known in the western part of Mongolia but recent papers have shown that, even if of smaller Magnitude, earthquakes may hit the capital area and be deadly.

Therefore any contribution to the understanding of the seismic potential of this part of Mongolia where more than half of the country population is concentrated, represents a valuable information. The knowledge of faults that may generate an earthquake is a critical ingredient to estimate the seismic hazard of a region and to prepare scenarios of occurrence to direct prevention and preparedness actions. So this paper is a scientific contribution to science and society. The paper is based on tectonic geomorphology and paleoseismology approaches including up to date techniques. The presentation, both text and figures can be improved to make it more immediate and friendly for the reader. The calculation of the interevent time needs some revision and all the other calculations/estimates (e.g., slip rate, coseismic slip) needs full explanation of the data used.

I have annotated the MS and summarize below the general comments.

English language needs some improvements.

At the beginning of 2021 a paper from Suzuki et al was published on SRL and shows the presence of a fault located very close to the capital city named Ulaanbaatar fault. On the basis of tectonic geomorphology and trenching the authors show that this is an active fault threatening this part of Mongolia. The Ulaanbataar fault is not discussed in your paper nor reported in your maps. However, I strongly recommend to integrate the Suzuki et al data and discuss convergences or divergences between the results and interpretation to provide a more complete view of the hazard for the capital city and surroundings.

Since a few years the scientific community as adopted the use of Common Era – CE/BCE to replace AD and BC. Please change accordingly.

I cannot reproduce the interevent interval 2080+-470 from the ages of MRE and PE. Maybe I have lost something. MRE can have occurred anytime between 775CE and today, or better the year in the Mongolian history from which an earthquake along this fault would have been reported in the historical documentation. In Europe this can be 1600-1700 CE. What about Mongolia? Can 1800 be the oldest age for an earthquake to be recorded in historical documentation in Ulaanbataar? It is true that the town was established there in 1778? Can we assume that any earthquake after that date should appear in some historical documentation? If this is correct the MRE can have occurred anytime between 775 CE and 1778 CE. Thus, this is the range to be used for interevent calculation along with 1605-835BCE for the PE. On this basis the interevent can be as long as 3383 and as short as 1610 years. Your interval appears much smaller: 1610-2550 yr All the calculation derived from the interevent estimate should be refreshed.

The morphologic evidence of the fault sections should be better explained, what are the geomorphic elements that support the reconstruction of the fault trace? These should be discussed and also highlighted in figures (as for example in fig 11).

Figures are too small and resolution increased

Figures 2 and 3 should converge in one single figure with geology draping the topography

Figure 4 should show the fault without covering it with a black line. I would recommend to remove the line and use arrows to point to the fault. As it is now it is impossible to recognize the fault geomorphic expression. A better version of the fault trace is presented in figure 13. I would suggest that these two figures are merged to make a single one composed of two panels: 1) a good DEM highlighting the geomorphic elements used to recognize the fault trace (using arrows and symbols not covering the fault) and 2) the fault trace with all possible details always on the DEM. Consider that this figure should be at the beginning (eg fig 4) because it is critical to the descriprion of the fault sections.

Figures 5-10+12 are nice reconstructions of offset streams to measure offset. I think that part of these should go as supplementary material to leave space to field photos showing the fault, its geomorphic evidence and setting. I would also recommend to extend the summary table 1 including site name and coordinates, measured offset and type/age of sediments recording the offset. Moreover, the estimate of uncertainties appear well too small. You should consider first the resolution of the images, then max and min measure of offset with their own uncertainty (that should be calculated by correlating stream axis with different trends especially when streams are not perpendicular to the fault and have a windy geometry). Therefore, all these uncertainties sum up in the cumulative offset evaluation.

Figure 14 contains some details on the complexity of the fault trace I was asking to show in the chapters describing the fault. However, the scale is still not adequate for these fine complexities (ex.: I cannot see the changes in strike), some field remote sensing evidence should be shown as in fig 11.

Figure 15 g. Add units names in the log
Citation: https://doi.org/10.5194/se-2021-91-RC1
- AC1:
  'Reply on RC1', Antoine Schlupp, 04 Jan 2022
  Dear Daniela Pantosti,
  We start with our best wishes for 2022 and good health.
  All the co-authors thanks you very much for your detailed review of our submitted paper about Sharkhai fault, the propositions of correction and the comments that allowed us to clarify, correct and improve our first submission.
  All the correction you proposed in the PDF have been included in the new version. Thank you for this detailed review.
  Here under, you will find our answers to each of your general comment and to two specific comments in the PDF. There is no opposition and we agree with them.
  
  Answers to general comments.
  Comment D. Pantosti: English language needs some improvements.
  
  Answer: all the suggestions have been included. A complete review on the English language has been done as well.
  Comment D. Pantosti: At the beginning of 2021 a paper from Suzuki et al was published on SRL and shows the presence of a fault located very close to the capital city named Ulaanbaatar fault. On the basis of tectonic geomorphology and trenching the authors show that this is an active fault threatening this part of Mongolia. The Ulaanbaatar fault is not discussed in your paper nor reported in your maps. However, I strongly recommend to integrate the Suzuki et al data and discuss convergences or divergences between the results and interpretation to provide a more complete view of the hazard for the capital city and surroundings.
  
  Answer: We include in the corrected version of our paper the recent work of Suzuki et al. 2020. This “Ulaanbaatar fault” is discussed in the “introduction and context” part with other known faults. We also added it to the new Figure 2 (results of the merging of initial figure 2 and 3 as proposed by D. Pantosti)
  Comment D. Pantosti: Since a few years the scientific community as adopted the use of Common Era – CE/BCE to replace AD and BC. Please change accordingly.
  
  Answer: All the dates have been changed to the common Era- CE/BCE.
  Comment D. Pantosti : I cannot reproduce the interevent interval 2080+-470 from the ages of MRE and PE. Maybe I have lost something. MRE can have occurred anytime between 775CE and today, or better the year in the Mongolian history from which an earthquake along this fault would have been reported in the historical documentation. In Europe this can be 1600-1700 CE. What about Mongolia? Can 1800 be the oldest age for an earthquake to be recorded in historical documentation in Ulaanbataar? It is true that the town was established there in 1778? Can we assume that any earthquake after that date should appear in some historical documentation? If this is correct the MRE can have occurred anytime between 775 CE and 1778 CE. Thus, this is the range to be used for interevent calculation along with 1605-835BCE for the PE. On this basis the interevent can be as long as 3383 and as short as 1610 years. Your interval appears much smaller: 1610-2550 yr All the calculation derived from the interevent estimate should be refreshed.
  
  Answer: The comment is right. We checked the oldest age for an earthquake to be recorded in historical documentation. We can consider 1778 CE as the oldest age for an earthquake in the region of Ulaanbaator. The calculations of interval time are corrected, the minimum is 1610 yr. and the maximum is 3383 yr., thus yields an average interval time of 2496 ± 887 years. The induced values of slip rate estimates are also corrected (paragraphe 4.2 Magnitude, co-seismic displacement and slip rates).
  Comment D. Pantosti: The morphologic evidence of the fault sections should be better explained, what are the geomorphic elements that support the reconstruction of the fault trace? These should be discussed and also highlighted in figures (as for example in fig 11).
  
  Answer: Drainage offsets are the main geomorphological features observed along the Sharkhai fault, in addition to small scarps of about 50 cm height along the northern section ( trench site Figure 14). Between them, we can follow more or less the fault traces in the HR images and on the field a “lineament” at surface, related to the eroded fault trace, locally associated to smoothed scarp or a change in the slope (see new figure 4). Outside HR images or field observations, the fault is very difficult to follow or even detect. (see complements in text).
  Comment D. Pantosti: Figures are too small and resolution increased.
  
  Answer: The figures in PDF have a reduced resolution. All the figures will be provided with the highest resolution for the final version.
  Comment D. Pantosti: Figures 2 and 3 should converge in one single figure with geology draping the topography
  
  Answer: We merged initial figures 2 and 3 (the topography and the geological context of The Ulaanbaatar region and the active faults) in the new Figure 2. All the other figure numbers have been corrected
  Comment D. Pantosti: Figure 4 should show the fault without covering it with a black line. I would recommend to remove the line and use arrows to point to the fault. As it is now it is impossible to recognize the fault geomorphic expression. A better version of the fault trace is presented in figure 13. I would suggest that these two figures are merged to make a single one composed of two panels: 1) a good DEM highlighting the geomorphic elements used to recognize the fault trace (using arrows and symbols not covering the fault) and 2) the fault trace with all possible details always on the DEM. Consider that this figure should be at the beginning (eg fig 4) because it is critical to the description of the fault sections.
  
  Answer: The figure 4 is modified as requested, the new one (called now figure 3) is composed of two panels: one with a good DEM without fault trace and the second with the details.
  Comment D. Pantosti: Figures 5-10+12 are nice reconstructions of offset streams to measure offset. I think that part of these should go as supplementary material to leave space to field photos showing the fault, its geomorphic evidence and setting. I would also recommend to extend the summary table 1 including site name and coordinates, measured offset and type/age of sediments recording the offset. Moreover, the estimate of uncertainties appear well too small. You should consider first the resolution of the images, then max and min measure of offset with their own uncertainty (that should be calculated by correlating stream axis with different trends especially when streams are not perpendicular to the fault and have a windy geometry). Therefore, all these uncertainties sum up in the cumulative offset evaluation.
  
  Answer: Some figures are displaced in supplementary material. The table 1 has been extended with the information requested except for the type/age of sediments recording the offset that has not been studied in this work. The uncertainties are re-estimated considering the resolution of the images (see details in text and legend of figures).
  Comment D. Pantosti: Figure 14 contains some details on the complexity of the fault trace I was asking to show in the chapters describing the fault. However, the scale is still not adequate for these fine complexities (ex.: I cannot see the changes in strike), some field remote sensing evidence should be shown as in fig 11.
  
  Answer: These details in the mapping have been removed from figure 14 as they are very local complexity of fault traces (at metric scale) and they do not bring fundamental information for our results and the mapping of the fault.
  Comment D. Pantosti: Figure 15 g. Add units names in the log
  
  Answer: The unit names have been added.
  
  Answers to specific comments included in the PDF version of the reviewed paper.
  Comment D. Pantosti line 12: Part of it was already reported in Suzuki et al as Saharai fault? Is correct? see fig 1
  
  Answer: The Sharkhai fault reported by Suzuki et al. (fig 1 of their paper) at very small scale is based directly on our work and preliminary results (Al-Ashkar et al., 2013 and Schlupp et al., 2013) on Avdar and Sharkhai faults. Our work is not cited in the legend of their figure 1 but well cited in their text.
  Comment D. Pantosti line 256: this age is meaningless, any age younger than the age of the industrial time cannot be considered. Just for your information the living roots are removed in the lab by acids during the preparation of the sample. Just do not consider this measure as reliable and use sample
  
  Answer: It seems it was a confusion. The age was BP and not AD, therefore it was not associated with industrial time but to 45 ± 80 CE. We clarified in the text the part related to the roots which was not the idea understood.
  
  Thank you again for the review.
  With kind regards,
  The co-authors
  
  Citation: https://doi.org/10.5194/se-2021-91-AC1
RC2:
'Comment on se-2021-91', Laurent Bollinger, 22 Oct 2021
« General comments »

This article by Al-Ashkar et al -Tectonic geomorphology and paleoseismology of the Sharkai fault- documents a newly discovered active fault in the close vicinity of Ulaanbaatar, Mongolia’s capital city.

The authors mapped the surface trace of the fault using Hi-res pleiades images and estimate cumulative tectonic offsets of geomorphic features along strike. They excavated the fault, documenting quaternary sedimentary units affected by deformations, and describe the faulting associated to the occurrence of two paleoearthquakes. The geological units deformed by these two earthquakes are dated by radiocarbon dating of bulk sediments, and bracket the time of occurrence of the two earthquakes. The dating indicates that the last earthquake happened after 860+/-85 AD and that the penultimate occured between 1515+/-90 and 945+/-110 BC.

The article exposes implications of these results in term of earthquake potential realisation along the fault / hazards. These implications fall within a large bracket of values –associated with substantial uncertainties - that mainly depend upon strong hypothesis but are clearly exposed by the authors.

I find this work relevant, the authors convincingly show:

That some quaternary geomorphic features are offset along a >40 km-long fault

That two earthquakes affected the young sediments exposed within the excavation

The inter-event time deduced from the bulk sediment radiocarbon ages is a rare estimate for a fault in central Mongolia ; most of the faults being not associated with multi-earthquake records.

Overall, the manuscript is well written and the paleoseismological results clearly exposed. The claims are supported by data and illustration. The figures illustrating the morphotectonic part of the article and the trench logs are clear. The discussion and conclusions present implications at the right level, evaluating the consistency of the results (associated with substantial uncertainties) to previous ones in the vicinity (partially discarding some of the most extreme scenarios when inconsistent with the regional knowledge). I have only a few specific remarks requiring clarifications.

I therefore accept this manuscript after what I would regard as minor revisions.

Here follows a list of specific comments which need clarifications and technical corrections

%%%%%%%%%%%“Specific comments ”%%%%%%%%%%%%

Part 1 Introduction and context :

I find the introduction not well structured. Indeed, in my opinion, the succession of paragraphs from line 50 to the end of the introduction go too much back and forth (e.g. from paragraph line 50 : Ulanbaatar exposure to earthquakes, then active faults, then geology, the Ulanbaatar exposure and risk, then the targetted faults

I suggest either :

-to separate the general introduction from the local seismotectonic setting.

-or to change the order of the paragraphs of the present introduction so that the succession go less back and forth

Table 1 :

You estimated 7 cumulative offsets of geomorphic features along the fault, and found that two fall around 57 meters and three around 35 meters. I think that a discussion regarding whether the geomorphic features offset by these quantities could be related (or not) with a specific climatic events would enrich your work.

Line 289-290 and Figure 15g

The geological units logged in the trench are well described in the text. However, a point remains unclear to me : several faults are mapped as possibly affecting the base of Unit U11 (red dashed line on figure 15g). In the meantime the text mentions that « Between 0 and 3m and between 5 and 7.5m all ruptures terminate at teh top of U30 and are truncated by the upper erosion surface« (see line 289-291). This is either not coherent, or the text do not sufficiently expose the observations and their uncertainties.

%%%%%%%%%%%%minor blemishes and technical corrections %%%%%%%%%%%%%%%

Introduction :

Line 32 : why opposing strike slip and uplift ? structure and motion ? replace by « numerous strike slip structures and minor thrust or normal faults … «

Paragraph around line 55 : Figure 2 should be referred in that paragraph, before introducing the local fault names.

Figure 2 : Gunj fault is missing on this figure. The kinematics associated to Avdar and Sharkai fault are missing

Paragraph 85 : about 1.5 million in the capital of 3.2 million : unclear, rephrase

Line 86 : insert ‘airport’ in « replacing the actual ‘airport’ too short and now too close to the city »

Line 91 : suppress « , which shows clear evidence for a major seismic activity » or replace it with something less strong. It is still the introduction and you already mention your results are clear and strong. The reader will make is own opinion.

Line 97 : with 2 earthquakes, it is difficult to speak about a « recurrence time ». An « inter-event time between the penultimate and latest earthquake » is more appropriate.

Line 102 : you mention the « very well-expressed geology » but I missed a description of the lithological nature of the bedrock along strike. Could be helpfull to understand the along strike variations of morphology of the two compartments of the fault.

Line 124 : replace « the « large extensional step-over, by « a » large….

Line 148 « limiting possible records of displacement » : rephrase

Table 1 : Specify the GPS location (in geographic) of P1 to P7 , as well as the location of the trench

Table 2 (and the text) We miss an information about where the samples were sent (which laboratory ?), what was their lab number (this table could also present their Delta 13C, and recall that the radiocarbon age were determined on bulk sediments)etc .

Line 220 : Could you be more specific about the nature of the massive Carboniferous bedrock ?

Indeed, in your introduction, you mention that the lithologies exposed in the region’s carboniferous rocks comprise sandstone, mudstone, conglomerates … but I found no mention to what was found in the field along strike Sharkai fault and in the trench.

Line 257 : suppress « both »

Line 279-280 : suppress the parenthesis after 15 b-e , replace 14g by 15g

%Data and ressources

I suggest adding information on the radiocarbon dating (which lab ? ) within that section.

I suggest listing the date of acquisition of the images that were used in the study (at least the date of acquisition of the images illustrating Figures 5-7-8-9-10-11-12.

%References :

I noticed that some recent references (post 2018) on faults in UB vicinity and central Mongolia are missing probably because your study took place years before the most recent works published was undertook. Updating the bibliography with recent references will make your paper more exhaustive and up-to-date.

Line 512 : Replace Reimer, P : by Reimer, P et al., …

Laurent Bollinger

Arpajon , 22/10/2021
Citation: https://doi.org/10.5194/se-2021-91-RC2
- AC2: 'Reply on RC2', Antoine Schlupp, 04 Jan 2022
  
  Dear Laurent Bollinger,
  We start with our best wishes for 2022 and good health.
  All the co-authors thanks you very much for your detailed review of our submitted paper about Sharkhai fault, the propositions of correction and the comments that allowed us to improve our first submission.
  Here under, you will find our answers to each of your comment. There is no opposition and we agree with them.
  Comments
  1 - Part 1 Introduction and context :
  I find the introduction not well structured. Indeed, in my opinion, the succession of paragraphs from line 50 to the end of the introduction go too much back and forth (e.g. from paragraph line 50 : Ulanbaatar exposure to earthquakes, then active faults, then geology, the Ulanbaatar exposure and risk, then the targetted faults
  I suggest either :
  -to separate the general introduction from the local seismotectonic setting.
  -or to change the order of the paragraphs of the present introduction so that the succession go less back and forth
  Answer: The introduction has been reconstructed to be more linear and clearer.
  
  2 - Table 1 :
  You estimated 7 cumulative offsets of geomorphic features along the fault, and found that two fall around 57 meters and three around 35 meters. I think that a discussion regarding whether the geomorphic features offset by these quantities could be related (or not) with a specific climatic events would enrich your work.
  Answer: The rates of deformation are first estimates associated with too large uncertainties to be correlated with climatic events. We nevertheless added a complement in the text for the two “groups” of cumulative offsets.
  
  3 - Line 289-290 and Figure 15g
  The geological units logged in the trench are well described in the text. However, a point remains unclear to me : several faults are mapped as possibly affecting the base of Unit U11 (red dashed line on figure 15g). In the meantime the text mentions that « Between 0 and 3m and between 5 and 7.5m all ruptures terminate at the top of U30 and are truncated by the upper erosion surface« (see line 289-291). This is either not coherent, or the text do not sufficiently expose the observations and their uncertainties.
  Answer: The phrases are corrected, Between 0 and 3m the ruptures truncated actually the unit U11, So they are considered MRE.
  
  minor blemishes and technical corrections
  Introduction :
  4 - Line 32 : why opposing strike slip and uplift ? structure and motion ? replace by « numerous strike slip structures and minor thrust or normal faults … «
  Answer: We agree and include your proposition.
  5 - Paragraph around line 55 : Figure 2 should be referred in that paragraph, before introducing the local fault names.
  Answer: We agree and include your proposition
  
  6 - Figure 2 : Gunj fault is missing on this figure. The kinematics associated to Avdar and Sharkai fault are missing
  Answer: We agree and include your proposition
  
  7 - Paragraph 85 : about 1.5 million in the capital of 3.2 million : unclear, rephrase Line 86 : insert ‘airport’ in « replacing the actual ‘airport’ too short and now too close to the city »
  Answer: We agree and include your proposition
  
  8 - Line 91 : suppress « , which shows clear evidence for a major seismic activity » or replace it with something less strong. It is still the introduction and you already mention your results are clear and strong. The reader will make is own opinion.
  Answer: We agree and include your proposition
  
  9 - Line 97 : with 2 earthquakes, it is difficult to speak about a « recurrence time ». An « interevent time between the penultimate and latest earthquake » is more appropriate.
  Answer: We agree and include your proposition
  
  10 - Line 102 : you mention the « very well-expressed geology » but I missed a description of the lithological nature of the bedrock along strike. Could be helpfull to understand the along strike variations of morphology of the two compartments of the fault.
  Answer: The lithological nature of the bedrock along strike has not been analyzed in this work dedicated to the activity of the fault and not to geological context. We did not add such a work despite the fact that we agree with the reviewer that it could be a complementary work for the description of the area.
  
  11 - Line 124 : replace « the « large extensional step-over, by « a » large….
  Answer: We agree and include your proposition
  
  12 - Line 148 « limiting possible records of displacement » : rephrase
  Answer: We agree and include your proposition
  
  13 - Table 1 : Specify the GPS location (in geographic) of P1 to P7 , as well as the location of the trench
  Answer: We agree and include your proposition
  
  14 - Table 2 (and the text) We miss an information about where the samples were sent (which laboratory ?), what was their lab number (this table could also present their Delta 13C, and recall that the radiocarbon age were determined on bulk sediments)etc .
  Answer: We agree and include your proposition
  
  15 - Line 220 : Could you be more specific about the nature of the massive Carboniferous
  bedrock ?
  Answer: We agree and include your proposition
  
  16 - Indeed, in your introduction, you mention that the lithologies exposed in the region’s carboniferous rocks comprise sandstone, mudstone, conglomerates … but I found no mention to what was found in the field along strike Sharkai fault and in the trench.
  Answer: Same answer as for the similar question at line 102: The lithological nature of the bedrock along strike has not been analyzed in this work dedicated to the activity of the fault and not to geological context. We did not add such a work despite the fact that we agree with the reviewer that it could be a complementary work for the description of the area.
  
  17 - Line 257 : suppress « both »
  Answer: We agree and include your proposition
  
  18 - Line 279-280 : suppress the parenthesis after 15 b-e , replace 14g by 15g
  Answer: We agree and include your proposition
  
  Data and ressources
  19 - I suggest adding information on the radiocarbon dating (which lab ? ) within that section.
  I suggest listing the date of acquisition of the images that were used in the study (at least the date of acquisition of the images illustrating Figures 5-7-8-9-10-11-12.
  Answer: We agree and include your proposition
  
  %References :
  20 - I noticed that some recent references (post 2018) on faults in UB vicinity and central Mongolia are missing probably because your study took place years before the most recent works published was undertook. Updating the bibliography with recent references will make your paper more exhaustive and up-to-date.
  Answer: We agree and include your proposition
  
  21 - Line 512 : Replace Reimer, P : by Reimer, P et al., …
  Answer: We agree and include your proposition
  
  Thank you again for the review.
  With kind regards
  Co-authors
  
  Citation: https://doi.org/10.5194/se-2021-91-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Antoine Schlupp on behalf of the Authors (05 Jan 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (06 Jan 2022) by Federico Rossetti

RR by Daniela Pantosti (28 Jan 2022)

ED: Publish subject to minor revisions (review by editor) (03 Feb 2022) by Federico Rossetti

AR by Antoine Schlupp on behalf of the Authors (09 Feb 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (13 Feb 2022) by Federico Rossetti

ED: Publish as is (13 Feb 2022) by Federico Rossetti (Executive editor)

AR by Antoine Schlupp on behalf of the Authors (14 Feb 2022)

Short summary

We present first constraints for the Sharkhai active fault near the capital city of Mongolia. The fault length is ~ 40 km, and the last earthquake occurred between 775 CE and 1778 CE and the previous between 1605 BCE and 835 BCE. We propose two possible scenarios with likely magnitudes of 6.7 ± 0.2 or 7.1 ± 0.7 and derive preliminary estimates of long-term slip rates. The Sharkhai fault should be considered in the seismic hazard assessment for Ulaanbaatar.