Reanalysis of the 1761 transatlantic tsunami

Wronna, Martin; Baptista, Maria Ana; Miranda, Jorge Miguel

doi:https://doi.org/10.5194/nhess-19-337-2019

Articles | Volume 19, issue 2

https://doi.org/10.5194/nhess-19-337-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/nhess-19-337-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 2

Research article

|

13 Feb 2019

Research article |

| 13 Feb 2019

Reanalysis of the 1761 transatlantic tsunami

Martin Wronna, Maria Ana Baptista, and Jorge Miguel Miranda

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Final revised paper (published on 13 Feb 2019)
Preprint (discussion started on 19 Mar 2018)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of reanalysis of the 1761 transatlantic tsunami, by Wronna et al. for NHESS', Uri S. ten Brink, 29 May 2018
- RC2: 'Review of the manuscript "Reanalysis of the 1761 transatlantic tusnami"', Ceren Ozer Sozdinler, 09 Jul 2018
  - AC2: 'Response to reviewer Ceren Sozdinler', Martin Wronna, 08 Aug 2018
- AC1: 'Answers to comments U. S. ten Brink', Martin Wronna, 08 Aug 2018

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (12 Aug 2018) by Ira Didenkulova

AR by Martin Wronna on behalf of the Authors (20 Aug 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Aug 2018) by Ira Didenkulova

RR by Ceren Ozer Sozdinler (27 Aug 2018)

RR by Sara Martínez-Loriente (15 Oct 2018)

Suggestions for revision or reasons for rejection

Prior consideration:
Tsunami modelling is not my field of research, so I think I should not evaluate that part of the work.

Major comment:
Given that the innovation of this work with respect Baptista et al. (2006) is that here they propose a geological source for the 1761 earthquake (EQ.), the reader expects to find a strong geological background in this paper supporting their proposal ... and this is not the case. Some important references of the study area should be added to the references list, especially the works focused on the study of tectonic structures mentioned throughout the work (and others missing!). The main problem that I detect in this work is that the proposed geological source for the 1761 EQ does not exist.

Minor comments:
- Is it really necessary to provide a verbal description of all observations? They appear summarized in tables and figures, and Baptista et al. (2006) can be referenced.
Pag.1
Line 28: The SWIM is not only dominated by NE-SW trending structures. It would be important to mention the WNW-ESE strike-slip SWIM faults and explain a little about them (Rosas et al., 2009; Terrinha et al., 2009; Zitellini et al., 2009; Bartolome et al., 2012; Martínez-Loriente et al., 2013)
Lines 28-30: Please, add the references of the main works carried out in the region focused on each structure. For example: “The SWIM is dominated by large NE-SW trending structures limiting the Horseshoe Abyssal Plain (HAP). The large NE-SW striking structures are the Coral Patch fault (CPF) (Martínez-Loriente et al., 2013), the Gorringe Bank fault (GBF) (Zitellini et al., 2009; Jiménez-Munt et al., 2010; Sallarès et al., 2013; Martínez-Loriente et al., 2014), the Horseshoe fault (HSF) (Gràcia et al., 2003a; Zitellini et al., 2004; Martínez-Loriente et al., 2018) and the Marques de Pombal fault (MPF) (Gràcia et al., 2003a; Terrinha et al., 2003; Zitellini et al., 2004)”; or you can add all the references together at the end (Gràcia et al., 2003a; Terrniha et al., 2003; Zitellini et al., 2004, 2009; Jiménez-Munt et al., 2010; Sallarès et al., 2013; Martínez-Loriente et al., 2013, 2014, 2018, … ).
Important NE-SW trending structures are missing: the Sao Vicente Fault (SVF) (Gràcia et al., 2003a; Zitellini et al., 2004), the Horseshoe Abyssal Plain Thrust (HAT) (Martínez-Loriente et al., 2014), and the Seine Hills (SH) (Martínez-Loriente et al., 2013).
Pag.2
Figure 1: The figure caption needs to be improved. For example: authors must specify that the red stars refer to source locations; they should explain the difference between green and yellow tsunami observation points; the authors should add other important tectonic structures of the study area such as the SVF, HAT, SH or SWIM faults. The bathymetry used in Figure 1 of Baptista et al. (2016) is much better than the one used in this work to make figures 1, 2, and 3. The authors may consider using the same bathymetry as in the previous work. In addition, the cartography of the Horseshoe and Coral Patch fault traces is very inaccurate, and they appear much longer than they really are. Review the references suggested above to improve them.
Pag.3
Line 5: To compute inundation a “real” high-resolution bathymetry is needed. The minimum resolution required is 10 m.
Line 11-12: “with a strong magnitude event on 25th November 1941 …” Specify the magnitude of the seismic event
Line 18: Same as in Pag.1. Some important NE-SW thrust faults are missing and references should be added.
Line 20: “the aseismic SWIM-Lineaments …”. Are you sure they are aseismic? Please, review the following references: Geissler et al., 2010, Bartolomé et al., 2012, Grevemeyer et al., 2016, Silva et al., 2017, and references therein.
Line 23-24: “the Coral Patch ridge (…), with a predominating flower structure geometry”. This statement is not true. Review Martínez-Loriente et al., 2013.
Line 25: “the CPF is closest to the area suggest by Baptista et al. (2006).” There is a distance of about 200 km.
Pag.4
Figure 2: Define “TTT”
Line 7: “we consider a possible fault as an extension of the CPF closest to the area presented by Baptista et al. (2006).” What does this mean? The authors cannot invent faults … Their assumptions have to stick to reality and be based on the structures known in the area.
Line 11: “we test different earthquake fault parameters (…)” Specify which parameters have been tested
Pag. 6
Line 2: “documents report …” What documents? Add references.
Line 8: Please, provide all measures in international system units.
Pag. 7
Table 1: All the abbreviations that appear in a figure or Table must be defined in the corresponding figure or table caption. In this case, “TTT”.
Pag 8
Line 8-9: I am in contact with colleagues who are experts in inundation modelling, and they told me that the minimum resolution acceptable to compute inundation is 10 m. Do the authors trust the results obtained with an inadequate bathymetry?
Pag 9
Line 2-6: There are like 600 km between the 1941 and 1975 EQ epicentres and the proposed for the 1761 event in Figure 2. I do not understand why the authors have tested the compatibility between these events. It does not make sense to me … In addition, the authors should consider adding the epicentres of these EQ in Figure 2 since they are mentioned in the text.
Line 21: “(…) the mean dip angle of 40 degrees suggested by Martinez-Loriente et al. (2013)”. In the aforementioned reference, a dip of 30° ±5 is suggested for the CPR fault, which is a different structure than the one proposed by the authors as the source of the 1761 EQ. Martinez-Loriente et al., suggest that the CPR does not continue to the west.
Hypothesis A-MS: Based on what geological knowledge of the area do the authors propose this solution? Do the authors know any structure in the area that is subdivided into 4 segments each of 50 km long?
Pag 10
Table 2: What is the rationale for using these values? For example, the depth for “Hyp. A” is 10 km … On the basis of what information do the authors choose this value? or a width of 50 km (????)
Hypothesis B: I do not know any strike-slip fault with NE-SW orientation and 280 km long (!!!) at the proposed source location for the 1761 EQ.
Pag 18
Line 27: “It is possible to find a geological source compatible with the source area deduced from TTTs and with macro-seismic intensity data” Really? What is the “real” geological source proposed by the authors? In Figure 3, the proposed fault plane does not correspond to any real geological structure.

Reference list:
Baptista, M. A., Miranda, J. M., and Luis, J. F.: In search of the 31 March 1761 earthquake and tsunami source, Bulletin of the Seismological Society of America, 96(2), 713-7211, doi:10.1785/0120050111, 2006.
Baptista, M. A., Miranda, J. M., Batlló, J., Lisboa, F., Luis, J., and Maciá, R.: New study on the 1941 Gloria Fault earthquake and tsunami, Nat. Hazards Earth Syst. Sci., 16, 1967-1977, https://doi.org/10.5194/nhess-16-1967-2016, 2016.
BARTOLOME, R., GRACIA, E., STICH, D., MARTINEZ-LORIENTE, S., KLAESCHEN, D., MANCILLA, F.L., LO IACONO, C., DA~NOBEITIA, J.J. & ZITELLINI, N. (2012) Evidence for active strike-slip faulting along the Eurasia-Africa convergence zone: Implications for seismic hazard in the SW Iberian Margin. Geology, 40(6), 495–498.
GEISSLER, W.H., MATIAS, L., STICH, D., CARILLHO, F., JOKAT, W., MONNA, S., IBENBRAHIM, A., MANCILLA, F., GUTSCHER, M.A., SALLARES, V.&ZITELLINI, N. (2010) Focal mechanisms for sub-crustal earthquakes in the Gulf of Cadiz from dense OBS deployment. Geophys. Res. Lett., 37, L18309.
GRACIA, E., DAñOBEITIA, J.J., VERGÉS, J. & PARSIFAL Team (2003a) Mapping active faults offshore Portugal (36N– 38N): Implications for seismic hazard assessment along the southwest Iberian margin. Geology, 31, 83–86.
Grevemeyer, I., Matias, L., Silva, S., (2016). Mantle earthquakes beneath the South Iberia continental margin and Gulf of Cadiz – constraints from an onshoreoffshore seismological network. Journal of Geodynamics, 99, 39-50, http://dx.doi.org/10.1016/j.jog.2016.06.001
Jiménez-Munt, I., M. Fernández, J. Vergés, J. C. Afonso, D. Garcia-Castellanos, and J. Fullea (2010), Lithospheric structure of the Gorringe Bank: Insights into its origin and tectonic evolution, Tectonics, 29, TC5019, doi:10.1029/2009TC002458.
MARTINEZ-LORIENTE, S., GRACIA, E., BARTOLOME, R., SALLARES, V., CONNORS, C., PEREA, H., LO IACONO, C., KLAESCHEN, D., TERRINHA, P., DA~NOBEITIA, J.J. & ZITELLINI, N. (2013) Active deformation in old oceanic lithosphere and significance for earthquake hazard: Seismic imaging of the Coral Patch Ridge area and neighboring abyssal plains (SW Iberian Margin). Geochem. Geophys. Geosyst., 14, 2206–2231.
MARTINEZ-LORIENTE, S., SALLARES, V., GRACIA, E., BARTOLOME, R., DA~NOBEITIA, J.J. & ZITELLINI, N. (2014) Seismic and gravity constraints on the nature of the basement in the Africa-Eurasia plate boundary: New insights for the geodynamic evolution of the SW Iberian margin. J. Geophys. Res. Solid Earth, 119, 127–149.
Martínez‐Loriente, S. , Gràcia, E. , Bartolome, R. , Perea, H. , Klaeschen, D. , Dañobeitia, J. J., Zitellini, N. , Wynn, R. B. and Masson, D. G. (2018), Morphostructure, tectono‐sedimentary evolution and seismic potential of the Horseshoe Fault, SW Iberian Margin. Basin Res, 30: 382-400. doi:10.1111/bre.12225
SALLARES, V., MARTINEZ-LORIENTE, S., PRADA, M., GRACIA, E., RANERO, C.R., GUTSCHER, M.A., BARTOLOME, R., GAILLER, A., DA~NOBEITIA, J.J. & ZITELLINI, N. (2013) Seismic evidence of exhumed mantle rock basement at the Gorringe Bank and the adjacent Horseshoe and Tagus abyssal plains (SW Iberia). Earth Planet. Sci. Lett., 365, 120–131
Sónia Silva, Pedro Terrinha, Luis Matias, João C. Duarte, Cristina Roque, César R. Ranero, Wolfram H. Geissler, Nevio Zitellini, (2017). Micro-seismicity in the Gulf of Cadiz: Is there a link between micro-seismicity, high magnitude earthquakes and active faults? Tectonophysics, 226-241 (717). doi: 10.1016/j.tecto.2017.07.026
TERRINHA, P., PINHEIRO, L.M., HENRIET, J.P., MATIAS, L., IVANOV, M.K., MONTEIRO, J.H., AKHMETZHANOV, A., VOLKONSKAYA, A., CUNHA, T., SHASKIN, P. & ROVERE, M. (2003) Tsunamigenic-seismogenic structures, neotectonics, sedimentary processes and slope instability on the southwest Portuguese Margin. Mar. Geol., 195(1–4), 55–73.
TERRINHA, P.,MATIAS, L., VICENTE, J., DUARTE, J., LUIS, J., PINHEIRO, L., LOURENCO, N., DIEZ, S., ROSAS, F.,MAGALHAES, V., VALADARES, V.,ZITELLINI, N.,ROQUE, C.,MENDES VICTOR, L. & MATESPRO Team (2009) Morphotectonics and strain partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data. Mar. Geol., 267, 156–174.
ZITELLINI, N., ROVERE, M., TERRINHA, P., CHIERICI, F., MATIAS, L. & BIGSETS Team (2004) Neogene through Quaternary tectonic reactivation of SW Iberian Passive Margin. Pure Appl. Geophys., 161, 565–587.
ZITELLINI, N., GRACIA, E., MATIAS, L., TERRINHA, P., ABREU, M.A., DEALTERIIS, G., HENRIET, J.P., DA~NOBEITIA, J.J., MASSON, D.G., MULDER, T., RAMELLA, R., SOMOZA, L. & DIEZ, S. (2009) The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar. Earth Planet. Sci. Lett., 280, 13–50

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ED: Reconsider after major revisions (further review by editor and referees) (16 Oct 2018) by Ira Didenkulova

AR by Martin Wronna on behalf of the Authors (04 Dec 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 Dec 2018) by Ira Didenkulova

RR by Anonymous Referee #4 (10 Jan 2019)

Suggestions for revision or reasons for rejection

This paper reanalyses the 31 March 1761 tsunami that occured west of the Straits of Gibraltar. After a determination of the approximate location, comparisons of modelling results and observations are made to descriminate between a compressive fault source model and an alternative model with a greater strike-slip component.

There are many challenges to identifying the source of an event this far in the past, for which there is no modern instrumental data. These challenges are compounded by a shortage of geophysical survey data in the source region. My opinion is that the authors have made a reasonable attempt to identify the source of this tsunami which is worthy of publication after some modification, but which still falls short of being a definitive paper on this event source due to some weaknesses in the methodology.

Main concerns and suggested changes:

[As I am not very familiar with the geology of this area, my comments are generally focussed on the tsunami modelling aspects]

My primary concerns are to do with limitations in the source identification methodology, which I would like to see more clearly acknowledged.

The authors set out to distinguish between source models which vary according to the extent of strike-slip motion included. Their more-compressive model A and a more-strike-slip model B. As the original model A produced wave periods inconsistent with historical observations they implemented a four-segment model 'A-MS' with varying slip of 7/15/15/8 metres on each segment. I think the reader should be told how this ditribution of slip was chosen? If various combinations of slip were tried in order to obtain good results, the reader should know this, or if the choice was made on a geological/geophysical basis we should also be told. As it stands the authors could be accused of biasing the results towards the hypothesis that they favour, since they did not also experiment with alternative slip models for hypothesis B.

Tsunami heights at a coastline can vary considerably depending on fine-scale bathymetric features close to the coast, so the application of Green's Law using an offshore coarse-grid wave amplitude to make comparisons with historical observations introduces a lot of uncertainty. I would like to see this better acknowledged. I think the reader should also be told how 5m was chosen for the depth at the shore used in Green's Law, since this seems rather arbitrary, and a different choice could lead to different conclusions.

Other points:

There are a few minor grammatical errors, that should be picked up in copy-editing, so I will not list these here.

Figure 1 might be better presented as two full-width figures (maybe Figure 1a and 1b?). The problem is that the inset in Figure 1 is currently too small and hard to read.

The analysis of the combined effects of tide and tsunami amplitude in the discussion is a bit weak, as the maximum water level does not necessarily occur at the time of maximum tsunami amplitude (a different comination of tide + tsunami could be higher). I doubt this will affect the final conclusions (and would not insist that the authors do this), but in an ideal paper it would be better to show the modelled tsunami signal superimposed on top of a simple model for the tide.

Minor points:
p1, line 8, replace 'set' with 'setting'
p1, line 15, should say '.. 1st November 1755 earthquake ...'
p2, line 4, if the inset to figure 1 was expanded it might be nice to identify the HAP on it.
p9, line 15, replace 'ancient' with 'historical'
p19, line 20, '... source A is ...' should probably be '... source A-MS is ...'?

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ED: Publish subject to minor revisions (review by editor) (10 Jan 2019) by Ira Didenkulova

AR by Martin Wronna on behalf of the Authors (18 Jan 2019) Author's response Manuscript

ED: Publish as is (21 Jan 2019) by Ira Didenkulova

AR by Martin Wronna on behalf of the Authors (22 Jan 2019)

Short summary

We investigate the source of the 1761 earthquake and tsunami. The reanalysis of the tsunami travel times agrees with an earlier suggested source area. We check the geodynamic setting of the area and place a fault as an extension to an identified fault and use numerical modelling to distinguish between two candidate sources. One of our theories is compatible with the geodynamic setting and reproduces well the observed tsunami parameters.