Tsunamis boulders on the rocky shores of Minorca (Balearic Islands)

Roig-Munar, Francesc Xavier; Vilaplana, Joan Manuel; Rodríguez-Perea, Antoni; Martín-Prieto, José Ángel; Gelabert, Bernadí

doi:https://doi.org/10.5194/nhess-18-1985-2018

Articles | Volume 18, issue 7

https://doi.org/10.5194/nhess-18-1985-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/nhess-18-1985-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 7

Research article

|

17 Jul 2018

Research article |

| 17 Jul 2018

Tsunamis boulders on the rocky shores of Minorca (Balearic Islands)

Francesc Xavier Roig-Munar, Joan Manuel Vilaplana, Antoni Rodríguez-Perea, José Ángel Martín-Prieto, and Bernadí Gelabert

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Final revised paper (published on 17 Jul 2018)
Preprint (discussion started on 12 Sep 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review', Anonymous Referee #1, 08 Oct 2017
- AC1: 'Author comment', Bernadi Gelabert, 04 Dec 2017
RC2: 'comments on the manuscript', Anonymous Referee #2, 18 Oct 2017
- AC1: 'Author comment', Bernadi Gelabert, 04 Dec 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (07 Dec 2017) by Mauricio Gonzalez

AR by Bernadi Gelabert on behalf of the Authors (17 Jan 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Jan 2018) by Mauricio Gonzalez

RR by Anonymous Referee #1 (27 Jan 2018)

RR by Anonymous Referee #2 (30 Jan 2018)

Suggestions for revision or reasons for rejection

General comments
Authors start affirming that most boulders are deposited by tsunamis, but this is the aim of the paper that should be demonstrated.
Usually when writing a paper, some rules that help readers to understand are listed as follow:
1) In the introduction, what is already known and then what we want to demonstrate (and not the conclusion we want).
2) General geological framework with information on seismic sources and tsunamigenic earthquakes and observed tsunamis
2.3) Study site
3) Method
4) Results
5) Discussion
6) Conclusion
Therefore, I suggest reordering the chapters

Abstract “The age of the boulders in most of the studied localities show a good correlation with historical tsunamis. Age of the boulders, direction of imbrication and estimation of run-up necessary for their placement, indicate dislodging and transport by North African tsunami waves that hit the coastline of Minorca”.

You have not shown a good correlation with historical tsunamis, age is either recent or post some date; furthermore, you did not estimated true possible run-up, but run-up that need to move boulders.

The map with western Mediterranean area showing earthquake distribution would help to understand the location of tsunamigenic areas around Balearic Islands. This map is the last figure, but it should be the first.
Moreover, why do you exclude Iberian earthquakes from tsunamigenic sources, if the 1756 tsunami was observed at Balearic Islands?
In the first version of the manuscript, this was clearly reported and you deleted it.

Information on wave regime, and tectonic setting (did your sites undergo to uplift or subsidence?) is not fully reported.
In the new paragraph maritime climate, you affirm that the maximum wave height of 10 m was observed. However, where is the maximum wave height of 10 m measured? At the buoy or in the coast? Are these values in deep water, or breaking wave heights?
If values are at buoy, you must consider breaking waves, which may be considerably higher than the waves in deep water (see e.g. Sunamura and Horikawa,1974).

Again, data on the maximum wave heights related to historical tsunamis that hit the Minorca Island are maximum 3 m both observed and computed. If this value is in deep water, you must consider breaking wave, which may be considerably higher. Please try to compute them.

Authors show in Table 1 tsunamis observed in the Balearic Islands and their surroundings in the last four centuries, while in Figure 2 tsunami from northern Algeria affecting Balearic Islands modelled by Roger and Hebert (2008). Both show maximum wave heights of 2 m and the May 21, 2003 tsunami was 3 m high; they were the highest tsunami waves recorded in recent years in the Balearic Islands. With reference to the studied boulders, Authors affirm that ”Our findings along the higher cliffs of the W coastline, requires tsunamis run-ups 13 m high and / or storm run-ups of 18.6 m”.
The observed maximum tsunami wave is 3 m high and it is 10 m less than the run-up that need to dislodge the boulders (13 m), whereas the 18.6 m storm run-up is only 7 m more than the 11 m observed!
On the other hand, it is possible that storms had higher waves than you observed or that the deposition of boulders by tsunami/s occurred before your historical observation period.

The last remark regards your dating methods. You report different values for 14C through the text and you do not show your dating results with calibration and error. How did you date with 14C boulders 1964 AD and 1856 AD? Usually the last three centuries are uncertain in 14C dating. These boulders seem to have been recently emplaced because are among the “five of the analysed boulders showing marine fauna”. Furthermore at Page 5 lines 10-14, you said “A boulder from Son Ganxo (SE of Minorca, Fig. 6) is a fragment of shoreline notch (wave-cut notch); located 2.5 m above sea level, at a distance of 18.4 m from the cliff edge, with a weight of 4.75 t. Radiocarbon dating determined an age younger than 1964 AD. Another boulder in Sant Esteve (SE of Minorca, Fig. 6) is situated about 19 meters from the waterfront and 1 m above sea level, with a weight of 43.15 t, and 14C dating determined an age younger than 1856 AD”. Therefore, they are likely storm boulders.
Also dating with post-depositional dissolution pans (Fig. 4b) is not useful; the range of dispersion of calculated ages makes the values overlapping. Therefore, they just show that boulders where recently deposited.

Reference
Sunamura, T., Horikawa, K., 1974. Two-dimensional beach transformation due to waves.
Proceedings 14th Coastal Engineering Conference. American Society of Civilian
Engineers, pp. 920–938.

Specific comments
Page 1. Lines 23: “Some are positioned well above the maximum stand of any recorded storm wave.” This is a conclusion. You must demonstrate it.
Page 1. Line 30: “In fact, in many areas of the Western Mediterranean, metric size boulders have been interpreted as remnants of the tsunamis occurred in the last centuries (Pignatelly = Pignatelli et al., 2009). Yes, but other authors showed that these are storms deposited (again you should demonstrate)
Page 2. Lines 1-26: Move all this part in the chapter where you describe the setting of your region and the maritime climate.
Page 2 line 20. “There are also historical records reporting a flooding event with a run-in up to 2 km inland on the east coast of Majorca (the largest of the Balearic Islands) in 1756 (Fontsere, 1918).
Reformulate this sentence: There are also historical tsunami records reporting a flooding up to 2 km inland on the east coast of Majorca (Yes but in Minorca?)
Page 2, lines 7-8: “The last seismic event recorded that affected Minorca Island was the Zemmouri (Algeria) earthquake that took place on May 21, 2003, with a magnitude of 6.9 Mw”.
Was it the earthquake that affected Minorca or rather Minorca was affected by the following tsunami?
Page 2. Line 25. “Thus, in the last 60 years the maximum extremal wave height detected is of 11 m at the 2001 medicane (Jansà, 2013)”. See general comments, was it observed at buoy?
At Page 4 line 20: in the last 50 years by a maximum wave height of 10 m (use always 60 years or 50 years)
Page 5. Line 20 blocs = blocks
Page 7. Line 29-30 “storm run-ups of 14 m are needed to dislodge the boulders, while tsunamis run-ups of only 8 and 13 m would explain their position”.
The observed maximum tsunami wave is 3 m and it is 10 m less than the run-up that need to dislodge the boulders (13 m), whereas the 14 m storm run-up is only 3 m more than the 11 m observed!
Page 8 lines 2-4: “tsunamis run-ups 13 m high and/or storm run-ups of 18.6 m. ….and require storm run-ups of more than 21 m that are not plausible, while the height of a tsunami run-up required to position the boulders is only 9 meters.” Are you sure? 13 m vs 18.6 m and 9 m vs 21 m, some calculation was wrong.
Page 8, lines 12-14: “For these reasons we think than run-ups heights on Minorca would have been several times higher than tsunami wave heights. On the contrary, as they shoals, wave heights increase its run-up heights in a much lesser way and thus, it is impossible to reach the run-up values obtained from the hydrodynamic equations”.
Very confusing sentence. Furthermore, you must not think but demonstrate, for example computing run-up at coast using values at deep water.
Page 8 lines 15-27 move in the study area chapter
Page 8 lines 28-30 “Regarding the dating of the boulders, although only two blocks with embedded marine fauna have been radiocarbon dated, such dates serve as a reference to the second dating method used. Our C14 results show than in one case a block was moved after 1720 AD,
Sure? Your dating was 1856 AD. Was not it?
Page 9, lines 25-26. “have been dislodged and positioned by the action of tsunami waves, although some of these boulders have also been reworked by storm waves”.
I do not understand. Why can storms rework boulders but cannot deposit them?

Finally, a revision of written English would be welcomed.

Figures should be quoted from the first (Figure 1) to the last (Figure 10) and not without order.
The figures and figure captions have several locality names reported in different way.
Please make attention to the points and comma
1000 in English is 1,000 and not 1.000; 14.4 m and not 14,4 etc.
Mean weight of these boulders is 4,6 t, with a maximum of 21,9 t. Average cliff height is 12 m,=
Mean weight of these boulders is 4.6 t, with a maximum of 21.9 t. Average cliff height is 12 t,
In the figures 6-7-8 Agrupated = grouped (clustered)
Table 1 – It is not clear if the affected area is referred to earthquake or tsunami
It is better to show: earthquake tsunami source and the affected area by the tsunami

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ED: Reconsider after major revisions (further review by editor and referees) (15 Feb 2018) by Mauricio Gonzalez

AR by Bernadi Gelabert on behalf of the Authors (27 Mar 2018) Author's response Manuscript

ED: Reconsider after major revisions (further review by editor and referees) (06 Apr 2018) by Mauricio Gonzalez

AR by Bernadi Gelabert on behalf of the Authors (16 May 2018) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (30 May 2018) by Mauricio Gonzalez

AR by Bernadi Gelabert on behalf of the Authors (06 Jun 2018) Author's response Manuscript

ED: Publish as is (13 Jun 2018) by Mauricio Gonzalez

AR by Bernadi Gelabert on behalf of the Authors (13 Jun 2018)

Short summary

Large boulders have been found on platforms that comprise the rocky coastline of Minorca (Balearic Islands), several tens of meters from the edge of the cliff, up to 15 m above the sea level, and kilometers away from any inland escarpment. The age of the boulders, the direction of imbrication and the estimation of run-up necessary for their placement indicate dislodging and transport by northern Africa tsunami waves that hit the coastline of Minorca.