the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2024
Surficial sediment remobilization by shear between sediment and water above tsunamigenic megathrust ruptures: experimental study
Abstract. Large megathrust earthquakes that rupture the shallow part of the interface can cause unusually large co-seismic displacements and tsunamis. The long duration of the seismic source and high upper-plate compliance contribute to large and protracted long-period motions. The resulting shear stress at the sediment/water interface in, for example, the Mw9.0 2011 Tohoku-Oki earthquake, could account for the surficial sediment remobilization identified on the outer margin. Through physical tank experiments, we test this hypothesis by exploring shear between sediment and water, interactions between high and low frequency seismic waves, and sediment properties (chemistry, grain size, water content and salinity). Our results show that low-frequency motion during a 2011-like earthquake can entrain several centimeters of surficial sediment and that entrainment can be enhanced by high-frequency vertical oscillations. These experiments validate a new mechanism of co-seismic sediment entrainment in deep-water environments.
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RC1: 'Comment on egusphere-2024-2011', Joan Gomberg, 30 Sep 2024
This paper presents results of a novel experiment that is well thought out and interesting. It should set the precedent for subsequent studies, which is what good science should do! The rationale for the work and the design of the experiment is particularly well explained, and over all the paper is well written. I believe that with some minor clarifications the paper should be publishable.
I suggest that in addition to the emphasis on the ‘tsunamigenic’ aspects of subduction thrust earthquakes (e.g., as in line 19 stating “Despite their high tsunamigenic risk”) the authors also note the hazards associated with the shaking and consequent damage, which lead to significant risks. This is particularly true given the long duration and low-frequencies of the shaking that are unique to these large megathrust earthquakes and impact large major structures (e.g., tall buildings, long bridges, submarine cables) and ground failure events (e.g., liquefaction, landsliding).
A more complete test of the hypotheses tested in this study and their significance would include demonstration that only waves with characteristics unique to large earthquakes mobilize surficial sediments; i.e., in addition to showing that long period oscillations can mobilize surficial sediments it would be useful to know that shorter period oscillations do NOT do so! While the experiments were not designed to test a range of frequencies, even just noting more clearly what published studies show would be useful. That is, are there observations of traditional turbidites from M<9 earthquakes that did NOT mobilize surficial sediments?
The paragraph between lines 71-84 would benefit from some revision, to make it more concise and clearer. I have tried to make some suggestions in the annotated text.
Please describe what the physical rationale is for assuming that high-frequency vertical acceleration enhances entrainment, as suggested in lines 93-4?
Perhaps it would be clear to a sedimentologist, but for a non-sedimentologist like me the description of the sediment water compositions in lines 111-115 needs clarification (and should be clear without having to look at the Supplement for Table S1). There seem to be two fine sand contents (percentages are fractions relative to what?), and 50% to 20% sediment content. How do these percentages relate to the fine sand percentages? The phrase “with the remainder composed equally of silt and clay” is unclear; what is this the remainder/leftover of, does “equally” mean the same amounts each of silt and clay or something else, and how is amount measured? How do all these things relate to just two mixtures, Mix 1 and Mix 2?
I would also suggest calling the three mixtures by names that are more descriptive than just numbers, so the reader doesn’t have to remember the characteristics implied by the numbers. For example, instead of Mix 3 it could be called the ‘saline XXX mix’, and Mix 2 the “fresh XXX mix” with XXX noting the distinguishing feature of Mix 2.
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CC1: 'Comment on egusphere-2024-2011', Michael Clare, 08 Oct 2024
This is an interesting, well-written and well-illustrated study that performed experiments to understand the response of sediments to disturbance by large magnitude mega-thrust earthquakes. It provides an improved mechanistic understanding of a previously observed thin remobilisation of surficial seafloor sediments following large quakes in several sites. It is therefore a novel and widely-relevant study.
Scaling – some discussion on scaling issues/limitations of the experiment should be added. Including the issues of only considering one type of clay (bentonite). What are the key assumptions or uncertainities in scaling this up to the real world?
The Introduction should also address other mechanisms that can mobilise sediments during/after earthquakes as this is not the only mechanism that has potential to occur.
The fidelity of palaeoseimology is painted as being complete in the introduction, so I think it is necessary to recognise that this is not always so straightforward and will not apply everywhere. Not much needs to be added, but just a recognition that there are places where there may not always be a direct link between large earthquake and resultant deposit.
Add to the conclusions/discussion the wider application of this study. Where is it most likely to be applicable? And where will it not be? This would be useful addition information for the reader.
Line 26 – I would caveat this to add some caution around the fact that this is not always possible and records can have varying degrees of fidelity. As-written this reads as if turbidite paleoseismology is always valid so would suggest some balance.
Howarth, J.D., Orpin, A.R., Kaneko, Y., Strachan, L.J., Nodder, S.D., Mountjoy, J.J., Barnes, P.M., Bostock, H.C., Holden, C., Jones, K. and Cağatay, M.N., 2021. Calibrating the marine turbidite palaeoseismometer using the 2016 Kaikōura earthquake. Nature Geoscience, 14(3), pp.161-167.
Bernhardt, A., Melnick, D., Hebbeln, D., Lückge, A. and Strecker, M.R., 2015. Turbidite paleoseismology along the active continental margin of Chile–Feasible or not?. Quaternary Science Reviews, 120, pp.71-92.
Nieminski, N.M., Sylvester, Z., Covault, J.A., Gomberg, J., Staisch, L. and McBrearty, I.W., 2024. Turbidite correlation for paleoseismology. Geological Society of America Bulletin.
Atwater, B.F., Carson, B., Griggs, G.B., Johnson, H.P. and Salmi, M.S., 2014. Rethinking turbidite paleoseismology along the Cascadia subduction zone. Geology, 42(9), pp.827-830.
Maier, K.L., Strachan, L.J., Tickle, S., Orpin, A.R., Nodder, S.D. and Howarth, J., 2024. Testing turbidite conceptual models with the Kaikōura Earthquake co-seismic event bed, Aotearoa New Zealand. Journal of Sedimentary Research.
Line 31 - Understanding event deposits starts with sediment entrainment.
I would suggest that that it is not just entrainment but also remobilisation. I think the study should introduce early on the potential mechanisms that can involve mobilisation of sediments during or after an earthquake to set the scene effectively, as it currently reads as if there is only one possible mechanism.
For example, direct ground motion mobilising thin surficial fluid-rich sediments, cyclic loading that destabilises continental slope sediments that generate a mass movement (landslide, debris flow etc), generation of sediment density flow due to disturbance of sediment into the water column, as well as secondary events such as earthquake-triggered tsunami that disturbs seafloor sediments. This would be useful context up front. The mechanism proposed by the authors is valid but is not the only mechanism that gives rise to deposits that may potentially be linked to earthquakes.
General question on the experiment setup: To what extent does bentonite represent real-world conditions? There is quite a range of cohesivity across different types of clays so some explanation or justification of bentonite would be useful.
Line 125 – these two mechanisms are interesting. I think setting up the different mechanisms that have been proposed by past studies for this remobilisation would be useful to introduce in the Introduction. The reader really needs to know what the current state of knowledge/understanding is regarding these mechanisms to determine whether the results are confirmatory or really novel.
Figure 4 – I would find it useful if the details of the mix, as well as the name of the Mix could be added to the legend so it is really clear to the reader and the figures can stand-alone.
Line 167 – this confirms previous hypotheses from a number of studies in Japan and elsewhere (e.g. Moernaut et al. 2017) - but this new study provides a mechanism to explain this. I would suggest broadening the study reach and also referencing some of the studies from elsewhere too.
Moernaut, J., Van Daele, M., Strasser, M., Clare, M.A., Heirman, K., Viel, M., Cardenas, J., Kilian, R., de Guevara, B.L., Pino, M. and Urrutia, R., 2017. Lacustrine turbidites produced by surficial slope sediment remobilization: a mechanism for continuous and sensitive turbidite paleoseismic records. Marine Geology, 384, pp.159-176.
Citation: https://doi.org/10.5194/egusphere-2024-2011-CC1 -
RC2: 'Comment on egusphere-2024-2011', Michael Clare, 14 Oct 2024
This is an interesting, well-written and illustrated study that performed experiments to understand the response of sediments to disturbance by large magnitude mega-thrust earthquakes. It provides a new mechanistic understanding of a previously observed thin remobilisation of surficial seafloor sediments following large quakes in several sites. It is therefore a novel and widely-relevant study.
Scaling – some discussion on scaling issues/limitations of the experiment should be added. Including the issues of only considering one type of clay (bentonite). What are the key assumptions or uncertainities in scaling this up to the real world?
The Introduction should also address other mechanisms that can mobilise sediments during/after earthquakes as this is not the only mechanism that has potential to occur.
The fidelity of palaeoseimology is painted as being complete in the introduction, so I think it is necessary to recognise that this is not always so straightforward and will not apply everywhere. Not much needs to be added, but just a recognition that there are places where there may not always be a direct link between large earthquake and resultant deposit.
Add to the conclusions/discussion the wider application of this study. Where is it most likely to be applicable? And where will it not be? This would be useful addition information for the reader.
Line 26 – I would caveat this to add some caution around the fact that this is not always possible and records can have varying degrees of fidelity. As-written this reads as if turbidite paleoseismology is always valid so would suggest some balance.
Howarth, J.D., Orpin, A.R., Kaneko, Y., Strachan, L.J., Nodder, S.D., Mountjoy, J.J., Barnes, P.M., Bostock, H.C., Holden, C., Jones, K. and Cağatay, M.N., 2021. Calibrating the marine turbidite palaeoseismometer using the 2016 Kaikōura earthquake. Nature Geoscience, 14(3), pp.161-167.
Bernhardt, A., Melnick, D., Hebbeln, D., Lückge, A. and Strecker, M.R., 2015. Turbidite paleoseismology along the active continental margin of Chile–Feasible or not?. Quaternary Science Reviews, 120, pp.71-92.
Nieminski, N.M., Sylvester, Z., Covault, J.A., Gomberg, J., Staisch, L. and McBrearty, I.W., 2024. Turbidite correlation for paleoseismology. Geological Society of America Bulletin.
Atwater, B.F., Carson, B., Griggs, G.B., Johnson, H.P. and Salmi, M.S., 2014. Rethinking turbidite paleoseismology along the Cascadia subduction zone. Geology, 42(9), pp.827-830.
Maier, K.L., Strachan, L.J., Tickle, S., Orpin, A.R., Nodder, S.D. and Howarth, J., 2024. Testing turbidite conceptual models with the Kaikōura Earthquake co-seismic event bed, Aotearoa New Zealand. Journal of Sedimentary Research.
Line 31 - Understanding event deposits starts with sediment entrainment.
I would suggest that that it is not just entrainment but also remobilisation. I think the study should introduce early on the potential mechanisms that can involve mobilisation of sediments during or after an earthquake to set the scene effectively, as it currently reads as if there is only one possible mechanism.
For example, direct ground motion mobilising thin surficial fluid-rich sediments, cyclic loading that destabilises continental slope sediments that generate a mass movement (landslide, debris flow etc), generation of sediment density flow due to disturbance of sediment into the water column, as well as secondary events such as earthquake-triggered tsunami that disturbs seafloor sediments. This would be useful context up front. The mechanism proposed by the authors is valid but is not the only mechanism that gives rise to deposits that may potentially be linked to earthquakes.
General question on the experiment setup: To what extent does bentonite represent real-world conditions? There is quite a range of cohesivity across different types of clays so some explanation or justification of bentonite would be useful.
Line 125 – these two mechanisms are interesting. I think setting up the different mechanisms that have been proposed by past studies for this remobilisation would be useful to introduce in the Introduction. The reader really needs to know what the current state of knowledge/understanding is regarding these mechanisms to determine whether the results are confirmatory or really novel.
Figure 4 – I would find it useful if the details of the mix, as well as the name of the Mix could be added to the legend so it is really clear to the reader and the figures can stand-alone.
Line 167 – this confirms previous hypotheses from a number of studies in Japan and elsewhere (e.g. Moernaut et al. 2017) - but this new study provides a mechanism to explain this. I would suggest broadening the study reach and also referencing some of the studies from elsewhere too.
Moernaut, J., Van Daele, M., Strasser, M., Clare, M.A., Heirman, K., Viel, M., Cardenas, J., Kilian, R., de Guevara, B.L., Pino, M. and Urrutia, R., 2017. Lacustrine turbidites produced by surficial slope sediment remobilization: a mechanism for continuous and sensitive turbidite paleoseismic records. Marine Geology, 384, pp.159-176.
Citation: https://doi.org/10.5194/egusphere-2024-2011-RC2 -
RC3: 'Comment on egusphere-2024-2011', Valerie Sahakian, 23 Oct 2024
This is a really interesting and cool study that is an important contribution to understanding the physical processes behind our observations of paleo event deposits. I am excited to see it published, and have a handful of comments below that I think could strengthen the work, and links to interpretations of deposits moving forward.
Main comments:
- Because the motivation/interpretation that entrainment is related to long period motions, and because this relies on assumptions of the likely frequencies that will be amplified in different portions of the wedge, I think it is important to expand your description of how you attain the frequencies in Figures 1 and S1; perhaps move the text from the caption of Figure S1 to its own supplementary section, and expand on the equations used to go from 2.1km depth -> 6s fundamental mode; it is not entirely clear to me from the text how this was obtained. A table might support this, with average Vp or Vs velocities and depths
- Given that these fundamental modes are likely amplification of horizontal motions, it would be good to discuss in the manuscript how this is accommodated with your experimental setup, and how that contributes to your interpretation that it is long period motion that generates entrainment. i.e., is it from the water flow? If so, how do you relate flow velocity to amplification at those specific periods? The vertical motions in the experiment are higher frequency than the oscillatory modes predicted, so I think this component is important to link the horizontal component of your setup to static or dynamic deformation/ground motions, and how your experiments translate to existing observations (or how the could/should be modified moving forward to do so).
- Lastly, I think it would be good/important to show a Figure 4, but plotting all runs, and add in the supplement, to represent all data and how they contribute to the interpretation (or why certain runs were selected for the final interpretations but not all)
Minor comments:
- Suggest changing Mw to a bold M (M); Mw is technically work magnitude and is used because of a misinterpretation of some Hanks and Kanamori publications (Tom Hanks was clear that moment magnitude is supposed to be a bold M).
- It would be useful to have references to support this statement on line 31: “The premise in sediment dynamics has been that the bed is fixed, and the fluid moves relative to it, creating shear stress that entrains sediment. ”
- It would be useful in the Introduction to specify what “low frequency/long-period” is (i.e., <1Hz, <0.1Hz, etc.). e.g., lines 48-55
- Would be good to clarify this statement on lines 71-72, that it is specific to Tohoku: “In an area ~50 km wide from the trench on the upper plate (Fig. 1B) and ~100 km along it, the static horizontal displacements at the seafloor (Fujiwara et al., 2011) and at the megathrust (Yue & Lay, 2011, 2013) are both about ~50 m”
- Line 73: An appropriate reference in place of Ekstrom personal communication would be Ma 2012, GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L11310, doi:10.1029/2012GL051854, 2012 .
- Clarify line 85: “Based on available seismic velocities” – clarify Vp, Vs, etc.
Citation: https://doi.org/10.5194/egusphere-2024-2011-RC3
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