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

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Review by Alasdair Skelton of Earthquakes release water from mountains submitted to Nature 
Communications by Takahiro Hosono and co-authors. 
 
 
What are the major claims of the paper? 
 
The submitted manuscript builds from the published work of Hosono et al. (2019) in which 
changes of groundwater levels are shown to have occurred in response to the 2016 Mw 7.0 
Kumamoto earthquake. In the submitted manuscript, the authors use an extensive stable isotope 
dataset for oxygen and hydrogen (n = 1150) to show that these changes were caused by a release 
of water from mountain aquifers in response to co-seismic rupturing. 
 
Hosono, T., Yamada, C., Shibata, T., Tawara, Y., Wang, C. -Y., Manga, M., Rahman, A. T. M. S. & 
Shimada, J. Coseismic groundwater drawdown along crustal ruptures during the 2016 Mw 7.0 
Kumamoto earthquake. Water Resources Research 55(7), 5891–5903 (2019). 
doi.org/10.1029/2019WR024871 
 
 
Are they novel and will they be of interest to others in the community and the wider field? 
 
This claim is novel for the reason given by the authors “because of the lack of datasets to describe 
water provenances before and after earthquakes”. The claim is of interest not only for researchers 
studying earthquake hydrology but also with respect to water security, a key issue for earthquake 
hazard mitigation. 
 
 
Is the work convincing, and if not, what further evidence would be required to strengthen the 
conclusions? 
 
The work is convincing and the conclusions are largely supported by the data. There are, however, 
some areas of ambiguity where improvements are necessary, as well as some issues related to the 
disposition of the manuscript, both of which will be discussed below. 
 
 
On a more subjective note, do you feel that the paper will influence thinking in the field? 
 
The manuscript will influence thinking in the field in that it elegantly demonstrates how stable 
isotopes can be used as a powerful and widely applicable tool for understanding hydrological and 
hydrochemical changes coupled with earthquakes. 
 
 
Major comments 
 
I have two major comments, one concerning interpretation of the data and one concerning the 
disposition of the manuscript: 
 
Concerning interpretation of the data: Data in Fig. 2 are used to argue that “the isotope ratios of 
groundwaters changed … from their original compositions towards compositional fields of mountain 
spring waters …” (lines 125-128). What is actually observed on the figure is a clustering of isotopic 
data towards the compositional field of mountain foot spring waters. Prior to the earthquake a 
good proportion of groundwater data overlapped mountain spring data. This difference does not 
undermine the conclusions of the study: the wider spread of isotopic values prior to the 
earthquake probably affects mixing of mountain spring water, soil pore waters and surface river 
waters. After the earthquake, mountain foot spring water clearly dominates the system, 
supporting the authors’ conclusions. 
 
Concerning the disposition of the manuscript: Lines 55-80 are not results of this study: they are 
results of the study published in Water Resources Research by Hosono et al. (2019). These 
“results” should therefore form part of the previous section which should be entitled “Introduction” 
or similar. The results for the manuscript are described in lines 82-123, so this section should be 
entitled “Results”. These new results are discussed in lines 125-194, so this section should be 
entitled “Discussion”. The conclusion and wider significance of the study are presented in lines 
197-216, so this section should also be entitled accordingly. 
 
 
Minor comments 
 
Lines 85-86: I suggest rewording “plot along near the local meteoric line during high water season 
(April to September including rainy season) but to the right to this line” to “plot slightly to the right 
of the local meteoric water line for the high-water season (April to September including rainy 
season).” 
 
Lines 88 and 91: Here you refer to isotopic shifts due to partial evaporation and recharge 
elevation. This would be easier for the reader to follow by adding corresponding arrows to show 
the directions of these shifts in Fig. 2. 
 
Line 89: I suggest replacing “to the local meteoric water” with “of this”. 
 
Line 94: I suggest replacing “during” with “for the”. 
 
Line 97: I suggest replacing “are” with “could be”. It is clear that that “groundwaters in the study 
area are recharged by precipitation during April to September” but unclear that mixing of all three 
of soil porewaters, mountain aquifer waters and the Shira River waters must have occurred. 
 
Lines 98-99: You state that the isotopic signatures of water are the same in unconfined and 
confined aquifers. I am not sure how the reader is expected to see this from the data as it is 
presented. 
 
Line 104: Please replace “mountain spring waters” with “mountain foot spring waters” 
 
Line 139: Please replace “Fig. 4a” with “Supplementary Fig. 4a”. 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
Key results: 
The paper attempts to use a new approach (isotope analysis) to determine the source of water 
that has resulted in a groundwater level rise post-earthquake. The large number of samples 
available means that this is quite a novel approach. 
 
Validity: 
In general I consider that, in an attempt to keep the paper brief, there is a lot of background 
information left out, which makes interpretation of the paper difficult. The structure also makes it 
difficult to follow. As it is, I would say that the information presented is not sufficient to justify the 
proposed conclusions, but it would be interesting if the data could support the conclusions. There 
are also a number of specific issues: 
The hypothesis is that an increase in permeability has resulted in a groundwater level rise. 
Increases in permeability cause a decrease in groundwater level (Darcy’s Law). If the hypothesis is 
that there is enough additional input to overcome increased output, or is it is that permeability has 
increased in some areas and not in others, this needs to be clearly explained. 
 
The figures are unclear about what they are showing. References to the figures say that they show 
something, but it is difficult to understand the point that is being made. For example, Fig 1 is said 
to cross-cut groundwater flow systems (Line 42), but it is difficult to see this in Fig 1. Fig 2 – again 
unclear as to what the point is that the authors are making – the pink arrows do not always show 
higher delta D for a given delta 18O – are the pink arrows general direction of movement, or 
movement of individual samples? There are so many points on the figure, that it is difficult to see 
what is happening – perhaps put all of the points on one figure, and then just the points that you 
say are moving post-EQ on another? 
 
It is unclear what actually happened with regards to the measured groundwater levels – the 
description in the introduction is confusing. The first figure in the supplementary material helped 
me to understand, and should be within the main paper. But I would also like to see some 
example hydrographs. One of the main issues that is not mentioned or argued is that groundwater 
level fluctuate in response to many drivers – if the authors are arguing that groundwater levels are 
still elevated after a year, how have they excluded other reasons? Rutter et al (2017) using simple 
analytical modelling to show that the rise in GWLs was not being driver by climate or abstraction – 
I would have expected some similar evidence. The manuscript really needs to justify why any GWL 
changes were due to EQs – especially after a year – there are many other drivers that need to be 
accounted for. 
 
There are contradictions or apparent contradictions throughout the paper. Because of the lack of 
clarity in lines 43-52, it appears that there is a contradiction as to whether water levels increased 
or decreased. Line 56 there is talk of crustal extension, then Line 59 talks about contraction. In 
Line 105 the authors say the shift is smaller for unconfined aquifers, then in line 126 say that the 
change is especially changed for the unconfined aquifers. 
 
Line 55-58 – I got lost as to the point being made 
 
Line 66, the authors introduce hot springs, without stating their relevance. 
 
Line 68 – what second hypothesis? The manuscript needs to be clear rather than the reader having 
to go back and work out what it is talking about. 
 
Lines 86-88 – it is difficult to follow the reasoning. 
At various point there is mention of unconfined vs confined aquifers, but the figures do not 
differentiate between the two. 
 
Lines 139-140 does not make sense 
 
Line 184 the authors introduce ‘stagnant’ flow areas with no other introduction. 
 
There are many other part where there is further explanation required. 
 
Originality and significance: 
It is unclear whether the conclusions are original as I was not able to follow enough of the 
reasoning. 
 
Data & methodology: 
The approach was theoretically sound, but the arguments were difficult to follow. 

Appropriate use of statistics and treatment of uncertainties: 
There is no mention of uncertainties. 
 
Conclusions: 
There were no conclusions presented. It was difficult to follow the reasoning, there were too many 
contradictions, and too little justification of statements to be able to justify the discussion. 
 
Suggested improvements: 
Much better explanation throughout, including a more robust introduction, and better presentation 
of the results would help. 
 
Clarity and context: 
The first half of the abstract was good – then I lost the reasoning and explanation of water level 
changes in the final part. There was no introduction, and no conclusions, and the reasoning 
throughout was difficult to follow. 
 
 
 

Reviewer #3 (Remarks to the Author): 
 
Re.: review: “Earthquakes release water from mountains”, by Hosono et al. 
 
This manuscript by Hosono et al. discusses the groundwater hydrology response to a large 
earthquake in Japan. The findings of the manuscript are based on an unprecedented large stable 
water isotope database covering all major units of the hydrological system from before and after 
the 7 Mw Kumamoto earthquake. Based on the analysis of the dataset the authors can 
demonstrate a major change of groundwater system for the entire area. I find in particular 
interesting the demonstration of changes between the recharge and discharge area that have 
major implications for our understanding of subsurface flow pathways on the landscape scale, from 
mountains to lowlands. The findings are well suited to be published in Nature Communications. I 
like to congratulate the authors for this very unique and insightful work. 
 
I have three minor to major points and several editorial comments. 
 
1) Although the authors try to rule out in detail the potential seasonality effect on the isotopic 
signature, I am not yet entirely convinced by the arguments and documentation. The main 
problem is that all pre-earthquake samples represent the low water season and a good share of 
the post-earthquake samples do cover the high-water season (July and August) and the moths 
after (September and October). Especially in the end of the rainy season, groundwater should 
reflect the purging of the aquifer and might potentially inherit seasonality signatures. This 
naturally asks the question of seasonality effects on the observations. I agree with the authors 
that groundwater should be well mixed and have an overall constant signature, however, I find 
this should be demonstrated more solid. In the reference 35 that was cited to underline the 
absence of seasonality in groundwater I could not find any isotopic data, maybe the authors 
include referenced data in the suppl. materials. I suggest to plot all groundwater data (incl. 
springs) in the dD vs 18O space as in figure 2 but color coded by the day of the year of sampling. 
Doing this should clearly demonstrate that the observed shifts towards mountain water signatures 
with less evaporation signals are real. 
 
2) In order to underline the physics behind the interpretations, the authors should provide some 
back of the envelope estimations of how fast the waters have to travel over distance in order to 
explain the changes of the isotopic signatures? I find this is important to first demonstrate the 
physical plausibility and second might even provide more evidence about the subsurface flow 
pathways. E.g. underpin the interpretation of predominant flow along fractures, cracks or any 
other large conduits. 
 
3) My last point refers to the groundwater increase in the recharge areas. I find the argumentation 
by the authors quit convincing but was wondering where the extra water is coming from or let’s 
say is missing? I understand that increased infiltration capacity in the recharge areas leads to a 
more effective groundwater recharge, leading to a rising of the groundwater table. However, if a 
substantial amount of the water is additionally infiltrated, or displaced, to the groundwater after 
the earthquake some other part of the hydrological system must receive less water. Was in the 
mountain areas any drop in river discharge and groundwater observed that might explain the 
overall water budget? I find confusing to not refer to the mountains as recharge area. What you 
demonstrate is that the recharge from the mountains to the lowlands has actually increased. The 
zone you term as recharge area is more a storage area, recharge to the groundwater is clearly 
higher from the mountains. 
 
Comments: 
• Line 61 to 63: I find this is a valid argument but this would imply that the deep water is fast 
upwelling. What about a more dynamic response where deeper water is pushing first the overlying 
shallower water out? 
• Line 68-69: The hypotheses need to be better outlined. It is not really clear what the second 
hypothesis is. 
• Line 74: To avoid confusion with be precise on the water level rise, “ground water level rise”. 
• Line 112: No isotope data is reported in the reference. 
• Line 139: Fig. 4a should be suppl. material? There is no figure 4 in main text. 
• Line 204-205: I am not entirely sure that I understand this argument. The recharge area and 
elevated areas (mountains) should be the same? This seems to be a problem of definition. Maybe 
the separation between High mountain areas and recharge area is confusing. 
• Figure 1: The Figure 1 a-b is borrowed wholesale from a different publication in WRR (eventually 
a reprint permission is needed?). The topographic information of a and c is redundant and I am 
not sure the volumetric strain changes (in b) are needed here. I would suggest to underline one 
map with an spatial interpolation of the water level change, e.g. after 45 days. One topographic 
map could be replaced by an isoscape map that actually illustrates the spatial distribution of 
isotopic signatures. 
• Figure 2: I find it visually very difficult to understand the differences between c, b, and d. I 
suggest to focus only on the compartments that have changed and keep the others in a bigger 
overview plot. I don’t understand the black arrows with the hot spring samples. Can the authors 
add the global meteoric water line for reference?