Using Ground-Penetrating Radar (GPR) to Investigate the Exceptionally Thick Deposits from the Storegga Tsunami in Northeastern Scotland
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this study, focusing on the Storegga tsunami, GPR was used to detect and interpret the tsunami deposits. Tsunami deposits are important evidence for the elucidation of giant paleo-tsunamis, and the results of a non-destructive GPR survey will greatly contribute to future studies of tsunami deposits. Please respond to the main comments below.
Creich's detailed GPR survey and sand bed interpretation may be somewhat problematic. The site is located at the land edge with a predominant axial length of the bay. In such a bay, tsunami inundation and very large sediment transport at the back of the bay are not expected unless the tsunami has a very long wavelength, as in the 1960 Chilean tsunami. A brief discussion of hydrodynamic motion seems necessary.
Figure 3 shows that borehole geological surveys were also conducted on the GPR survey line. Although the GPR visualization and the borehole geological survey are compared, why is the dating of the layers corresponding to the tsunami deposits not performed? As the authors claim, it is necessary to obtain more certainty that the sand layers detected by GPR are tsunami deposits. If you have such results from previous studies, it would be helpful to include them as references for the reader's understanding.
Minor comments are as follows,
The location of the submarine landslide surfaces of the Storegga tsunami and the locations surveyed in this study are unclear. Although there are many previous studies, please add a figure so that we can understand it in the article.
Figure 1 shows the measurement site. Please try to find out which figure (map) it corresponds to.
Figure 1 shows the measurement sites discussed in this article. It would be helpful for readers without knowledge of Nordic geography to understand the site better if the correspondence between Figure 1 and the following figures is described. For example, Milton Farm in Figure 1b corresponds to Figure 2.
In Figures 2, 3, 4, and 5, the same symbols are used for the survey lines. It may be better to use different symbols for different regions.
Author Response
We thank R1 for the detailed response and suggested minor edits which have helped to improve this manuscript.
In response to the question about the hydrodynamics of the tsunami within the Dornoch Firth at Creich, numerical modelling of the Storegga tsunami indicates a wavelength of 600-800 km (Vasskog et al 2013). In the discussion, we suggest that sand could be mobilised by a bore after the tsunami wave has broken (lines 514-521 in the revised manuscript).
With regard to the question on dating we agree that additional dating of organic matter would help to constrain the age of the deposits. We made an application for radiocarbon dating but we have not received a positive response.
Response to Minor Comments
Figure 1 has been modified to show the location of the Storegga slide.
An additional sentence has been added to the caption for figure 1. ‘Detailed images of the survey sites at Milton Farm are shown in figure 2, Creich is shown in figure 3, Ardmore in figure 4 and Dounie in Figure 5.’
R1 suggests that different symbols could be used to show the survey lines at different locations. We disagree with this suggestion because it is better to be consistent with the use of symbols within the paper.
On Figure 6 the borehole symbol has a white solid fill so that it shows up better.
Reference
Vasskog, K. Waldmann, N. Bondevik, S. Nesje, A., Chapron, E., Ariztegui, D. 2013. Evidence of tsunami run-up at the head of Nordfjord, western Norway. J. Quat. Sci. 24, 391-402
Reviewer 2 Report
Comments and Suggestions for AuthorsDear authors,
In general, the paper is well structured and written. The subject covered contributes to improving and updating the knowledge of tsunami deposits in general and, in particular, for 5 sites on the east coast of Scotland, based on the interpretation of profiles obtained by the indirect method (GPR).
The text contains many references that require formatting, as well as the units next to the numbers (e.g. 5m or 5m).
The ages indicated in the text must use the same units, namely cal BP. The attached document contains some suggested changes. Most of them are related to the aspects mentioned above.
At the beginning of section 5 you mention that the GPR results are better than expected and explain that this may be due to erosion of the rock alteration soils during glaciations and therefore the available sediments are immature. However, in sections 2 and 4 you say that there is clay material (silty clay) above or below the tsunami deposit. Is it really clay (mineral) or just clay-sized materials and organic matter? Is there any X-ray analysis of these sediments? Because silty clay means that there is more clay than silt... which doesn't support your hypothesis.
If it's not clay minerals, but just fine materials with organic matter, then your hypothesis seems plausible. I think the opposite situation needs another justification, namely low water content. If the composition is in fact clay minerals, but they are dry, they have low electrical conductivity and therefore do not attenuate the GPR signal.
Comments for author File: 
Comments.pdf
The text is well written in English and easy to read, with only minor changes required.
Author Response
We thank Reviewer 2 for the constructive comments.
The comments from R1 and R2 have helped us to revise the citations in the text. A space has been inserted between numeral and units where this was missing.
All of the ages quoted in the text are derived from the literature and in reporting the ages we have used the values and units that were originally reported with updates where these are available, e.g. the review of Smith et al., 2004.
With regard to clay minerals we have checked the relevant text (Lines 485-490 in the revised text), and confirm that we state that it is clay minerals that are missing not clay sized particles. To clarify this distinction we have added an extra sentence to the results section stating 'Lithological descriptions of sediments in boreholes are based on visual assessment of the sediment grain size. The term clay refers to clay sized particles rather than clay minerals unless explicitly stated.'
The alternative hypothesis suggested by R2, 'low water content' and dry sediments is not applicable in Scotland. While the East coast of Scotland is the dryer side of the country rainfall can still be expected on 170 days in a year and the agricultural land surveyed at Creich and Dounie have land drains to prevent the soil from being water logged. At Wick the survey site is very wet with boggy conditions on the river floodplain. The only site that is well drained is coarse grained shingle beach ridges at Ardmore.
R2 indicated that coordinates and scales should be added to the graphic images (Figures 1, 2, 3, 4 and 5). The Figures: 1, 2, 3, 4 and 5 all have scales in km, and north arrows showing the orientation. The coordinates are not shown on the figures but coordinates for each location are included in the text.
Reviewer 3 Report
Comments and Suggestions for AuthorsI recommend the authors to expand parts of the GPR sections in which the reflectors related to sediments related to tsunami events stand out. I suggest adapting the bibliographic references with those of Remote Sensing. I would appreciate that the authors present photos of the sediments related to the tsunamis, because there are no photos that evidence these statements.
Comments for author File: Comments.pdf
Author Response
Review 3 makes three points.
- This paper describes and discusses the use of GPR in imaging tsunami deposits. There is no unique property of the reflections or reflectors that would provide unequivocal identification of tsunami deposits from GPR surveys alone.
- The references are formatted for Remote sensing. Some changes to the citations in the text have been highlighted by Review 1 and 2 and these have been corrected.
- There are no outcrops of the tsunami deposits in the areas surveyed so field photographs are not possible. In this study we have not dug any trenches either so photographs of the deposits are not available.
Reviewer 4 Report
Comments and Suggestions for AuthorsYou've provided a wealth of data and interpretation that was, for the most part, a pleasure to review. There are a number of minor issues, which, after addressing will help general readers. Two very important improvements would be to change elevation to depth in the figures or change the depth references in the text to elevation so the reader could compare what you've written to what is displayed in the figures.
Secondly, you have a duplicate figure. You need to replace Figure 11 with the real data from Dounie.
Some other edits and comments are in the attached PDF file.
Comments for author File: Comments.pdf
No issues with the English language.
Author Response
We thank Reviewer 4 for their compliments.
Instead of changing the text from depth to elevation we have added a depth scale to each of the figures alongside the elevation scale. Hopefully this will aid readers in identifying features by depth.
Thank you for noting that figure 11 was not correctly uploaded. This has been changed in the revised manuscript.
We have revised the caption on Figure 6 to include better descriptions of panels d and e.
Figure 8 has been modified with the addition of north arrows as well as larger font for the coloured scale bars.
We checked the velocity of the hyperbolic reflection using a curve fitting toll in EKKO view 2 and it fits a velocity of 0.1 m/ns. This velocity suggests that it is from a buried object rather than an airwave.
