Peer review of: Stable isotopes show that earthquakes enhance permeability and release water from mountains (2020)



RR2
 Reviewers' comments:
Reviewer #1 (Remarks to the Author):

  Second review by Alasdair Skelton of Earthquakes release water from mountains submitted to
 Nature Communications by Takahiro Hosono and co-authors.
 I am fully satisfied that the authors have responded adequately to all of my comments and I
 recommend publication of this manuscript in its present form.
  
Reviewer #2 (Remarks to the Author):
 I think this is a useful paper which attempts to answer an important question - whether there is
 any evidence for a change in recharge to an aquifer system after a major EQ. Other papers have
 suggested this as a possibility, but the large number of isotope samples provides possibly more
 compelling evidence for this.
 However, overall, I still find the paper confusing and numerous things need to be clarified. Some
 of this is confusion about the geographic area and some of the terminology used – but then there
 should be no assumption that the reader will be familiar with the area, so the paper should be
 clear.
 I also feel that the paper puts forward some statements that, to me, are still hypotheses. None of
 this can be proved - what the paper is doing is putting forward evidence that suggest there is
 something happening, so to me it should be framed more as interpreting data to test a hypothesis,
 and less as a certain outcome.
 I find the order of parts of the paper leads to confusion – there are some basic background
 observations half way through the paper that would help the reader understand the situation if
 explained earlier on.
 The main point is that groundwater appears to have moved closer to mountain foot spring water in
 terms of composition. However, before the EQ, there was already considerable overlap between
 groundwater and mountain spring water. Post-EQ, it appears that the composition of some
 groundwater samples has moved towards that of mountain spring water, but the majority were
 already pretty similar. I don’t think this point is brought out. Effectively there is a subtle but
 observable shift away from the soil porewater composition, towards the mountain foot spring
 composition. You can’t say that post-EQ the distributions are identical, though they become more
 similar.
 General comments:
 I got confused throughout about “high water”, “higher water” and “lower water” seasons. At 127
 you say the “higher water season” is Oct – Dec, on the page before you say the “high water
 season” is Apr-Sept. It would help many people to understand what you mean by these
 descriptions (is there a very strong seasonal signal that is greater than any longer-term
 variability?), and when the different seasons are, as there seems to be some inconsistency.
 Personally I have not come across groundwater-focussed papers using this terminology before.
 I find the order of the sections a bit odd – but this may be the preferred order? That is Method and
 Samples after discussion.
 Specific comments:
 Line 64 – I would say that this is still a hypothesis
 Lines 73-76 – needs re-wording – the sentence just doesn’t read well and I am not clear about the

 point being made.
 Lines 84-85 – be clear that you are talking about a possible increase in permeability in the
 recharge areas and GWL increase in the down-gradient regional GW flow system
 Line 113 – you need to be clear here, and throughout the paper where you are referring to. When
 you say the study area, which bit of the study area do you mean?
 line 141 - where are the stagnant groundwaters on the figure?
 Line 148 – I can only see 2 groups on Fig 3a, not 4 – this may be down to the size of the figure
 Line 161 – they don’t compare samples from different seasons – the point of these figures is to
 compare before and after the EQ
 Line 171 – this is hypothesised to have caused post-seismic water levels to rise
 Line 177 – they have become more similar – they are not identical
 Line 181 onwards – I would put this paragraph possibly after line 220, and start it with “in contrast
 to the previous evidence....”. You have been putting forward one hypothesis, then this is in
 disagreement with that hypothesis, so you need to be clear that this doesn’t fit with your
 hypothesis. Why do you say that this effect is localised? Is it that the evidence doesn’t support it in
 a widespread area – or is there just not enough data?
 Line 218 – similar to, not identical
 Line 239 – it doesn’t demonstrate, it suggests
 Line 248/9 – you introduce “groundwater pools” for the first time. These need some explanation.
 249-250 – this doesn’t follow on from the previous sentence – why could you add a large volume?
 Line 252-253 – but the GW pools are in the recharge area
 Line 257 – this point doesn’t appear to be “in addition”
 Lines 257-260 – this doesn’t make sense to me. This is the first mention of preferential pathways,
 and if you are going to talk about them, then there should be some introduction much earlier
 Lines 262-263 – I am not sure what preferential pathways surface waters would travel along. A
 surface water itself obviously travels along a preferential pathway, but are you talking about
 recharge to groundwater?
 Line 269 – you don’t need to have liquefaction to get a co-seismic increase in pore pressure
 Line 270 – the results support the previously proposed scenario – of what?
 Line2 272 onwards – some of this needs to be introductory, somewhere around Line 62 – that
 would make some of the introductory information much more understandable
 Line 282 – 283 – if this paper supports this isotope work, then it is really important to make that
 clear.
 Line 283 – I am getting quite confused about which water levels
 Lines 285 – 286 – does the modelling talked about just previously, provide any evidence that you
 would get a long-term rise in GWLs as a result of permeability increase in the mountain aquifers? I
 would have thought that the system would return to normal – there is no change in recharge
 presumably, so if it resulted in a long term increase in groundwater levels down-gradient, then
 some water must have gone elsewhere
 Line 287-288 – you say earlier in the paper that some of the lines of evidence basically disagree
 with the isotope results, and put the contradictions down to localised effects.
 Line 289 – it shows how earthquakes may alter regional systems
 Line 290 – first time there is mention of high storage. In many volcanics, storage is low due to
 lack of connected porosity. Permeability relies on interconnected fractures/joints.
 Line 315 – say May and June 2016 to make it clear
 Fig 1 and Supplementary Fig 8 seem to have different areas identified as the recharge/significant
 GWL rise areas
 Supplementary Fig 2 – I cannot read the key at all
 Supplementary Fig 3 – the groundwater level change is in relation to what? The instantaneous
 water level at some point before the EQ?
 Reviewer #3 (Remarks to the Author):

 Re. Hosono et al.
Sincerely Christoff Andermann
 The authors I have done a great job revising the manuscript. I see that all my comments and
 concerns have been answered and especially the figures are much more clear now. I have no
 further comments and recommend the manuscript for publication in Nature communications.