Zandvakili et al.
Progress in Earth and Planetary Science(2024) 11:32
https://doi.org/10.1186/s40645-024-00635-w

PAPER WITH FULL DATA ATTACHED

Progress in Earth andPlanetary Science

Open AccessGeofluid behavior prior to the 2018
Hokkaido Eastern Iburi earthquake: insightsfrom groundwater geochemistry
Zahra Zandvakili1* , Yoshiro Nishio1* and Yuji Sano1

Abstract
A notable decrease of groundwater Na/K ratios was observed several months before the 2018 Hokkaido Eastern
Iburi earthquake (M6.7) at a site approximately 20 km west of the earthquake’s epicenter. To investigate the causeof the decreased Na/K ratios, we analyzed groundwater samples (commercially bottled drinking water) to assess
the contribution of deep-derived fluids using Li and Sr isotopic ratios, which are reliable indicators of deep fluid
contributions. No pronounced changes in the 7Li/6Li and 87Sr/86Sr ratios were observed; thus, the pre-earthquake
decrease of groundwater Na/K ratios did not result from the input of deep-derived fluids. The pre-earthquake
decrease in the Na/K ratio observed in this study may instead be related to ­CO2 influx into the aquifer. The ­CO2
may have leaked from carbon dioxide capture and storage (CCS), because CCS was in operation near the epicenterof the 2018 Hokkaido Eastern Iburi earthquake. Decreases of the Na/K ratios and increases of the ­CO2 concentrations
in groundwater have been reported before other large earthquakes; thus, C­ O2 influx into groundwater may be a common phenomenon preceding earthquakes.
Keywords Na/K ratio, Hokkaido Eastern Iburi earthquake, Lithium isotope, Earthquake precursor, Carbon dioxide*Correspondence:
Zahra Zandvakili
b22d6c01@s.kochi-u.ac.jp
Yoshiro Nishio
yoshiro@kochi-u.ac.jp
1
Marine Core Research Institute, Kochi University, Monobe B200,
Nankoku, Kochi 783‑8502, Japan© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

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1 Introduction
Geofluids may play an important role in the occurrenceof large earthquakes, but the mechanism for this is poorly
understood (e.g., Zhao et al. 1996). The term “geofluids,”
as elucidated by Czauner et al. (2022), refers to groundwater, geothermal and hydrothermal fluids, and related
geo-resources. Geophysical methods using informationsuch as the seismic wave velocity structure and electrical conductivity are extremely useful for understanding
the distribution of geofluids over a wide area (e.g., Zhao
et al. 1996) and are the primary methods used in geofluidstudies. However, small volumes of geofluids cannot be
detected by geophysical methods, and thus geochemical
methods are powerful tools for obtaining information
the origins and behaviors of such geofluids (Nishio 2013).
Accordingly, it may be possible to detect the mixing of
fluids of different origins before a major earthquake (Liuet al. 2023). Geophysical and geochemical methods arethus complimentary, and their combined application can
improve our understanding of the behavior of geofluids
prior to major earthquakes.
A variety of elements and elemental ratios are considered when studying geofluids: Two of the useful ones
are lithium and strontium. The two stable isotopes of Li
are 6Li and 7Li, and their relative abundances are about
7.5% and 92.5%, respectively. The 7Li/6Li ratio between
the fluid and solid phases changes with reaction temperature (Marschall et al. 2007; Wunder et al. 2006); consequently, the 7Li/6Li ratio is a useful tool for constraining
geofluid reaction temperatures. Additionally, according
to You et al. (1996) and James et al. (2003), the concentration of lithium in the fluid reaches its maximum athigh temperatures. Thus, geofluid Li concentration thathas experienced high temperature is significantly higher
than that in surface water that has not experienced hightemperature. By the time that a geofluid is emitted at
the surface, surface water contamination will have been
unavoidable. Li, for which the concentration difference
between surface and non-surface waters is extremely
large relative to other elements, is less susceptible to surface water contamination. In a case study of the Arimaand Kii Peninsula region in southwest Japan, the reported
Li concentration in surface water from the Kushida River
within the Kii Peninsula area was 0.00034 mg/L (Umamet al. 2022); in contrast, non-surface water sources in
the Arima area (Arima hot spring) contained a notably
higher lithium concentration of around 40 mg/L (Umamet al. 2022). In addition, the Cl/Li ratio can be used toquantify the extent to which the observed δ7Li values are
influenced by surface water (Nishio et al. 2010). Data on
87
Sr/86Sr ratios are also useful in elucidating the origins
of geofluids. The 87Sr/86Sr ratio in groundwater provides

information about the rocks with which a geofluid hasinteracted at high temperatures.
Using commercially available bottled drinking water,
Tsunogai and Wakita (1995) reported an increase inchloride ion concentrations in groundwater before the1995 M7.2 Kobe earthquake. Using a similar approach,
Sano and their group focused on the isotopic compositions of the elements constituting volatile elements (i.e.,hydrogen, oxygen, carbon, etc.) in groundwater prior toseveral major earthquakes (Onda et al. 2018; Sano et al.
2020a, 2020b). Sano et al. (2020a) attributed a decreasein δ13C values and an increase in total dissolved inorganic carbon (TDIC) observed a few months before the2018 Hokkaido Eastern Iburi earthquake (M6.7) to ­CO2
influx into the aquifer. To understand the nature of the­CO2 influx into the aquifer prior to that earthquake, weinvestigated whether deep-derived fluids are involved or
not in this influx. In this study, we analyzed commerciallyavailable bottled drinking water samples that preservepre-earthquake groundwater compositions to assess
temporal changes in the concentrations of non-volatileelements and the Li and Sr isotopic compositions of thegroundwater.

2 Methods
The water samples used in this research were obtained
from commercially available bottled mineral water thatwas collected between 2016 and 2019 at two locations:Uenae, 23 km west of the epicenter of the M6.7 Hokkaido
Eastern Iburi earthquake; and Eniwa, 40 km northwestof the epicenter (Fig. 1). The hydrogen and oxygen isotopic ratios in the water were previously published bySano et al. (2020a). At both sites, water is extracted fromwells with a depth of approximately 100 m, located in the
volcanic–sedimentary aquifer of the Shikotsu pyroclastic
flow of Late Pleistocene age (Nakagawa et al. 2018). The
Uenae site is closer to the Ishikari–Teichi–Toen active
fault zone (ITTFZ) than is the Eniwa site.
The water from the borehole was placed into plastic
bottles after filtration. In this study, bottled water samples were processed using the protocol described by
Nishio et al. (2015). After the water samples had been
passed through a 0.2 μm PTFE syringe filter, all analytical procedures were performed under a filtered airflow(cleanliness level better than class 1000) and using 18.2
MΩ-grade water prepared by an ultrapure water system(RFU663EA, Advantec). The clean laboratory and all analytical equipment used in this study were at Kochi Core
Center, a joint research facility managed by Kochi University and JAMSTEC (Japan Agency for Marine-Earth
Science and Technology).

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The concentrations of major elements such as K, Ca,
Mg, Na, and Cl were determined with an ion chromatograph (ICS-2000, Thermo Fisher Sci.), and those of traceelements (Li, Rb, Cs and Sr) were determined with an
inductively coupled plasma mass spectrometer (ICPMS;
iCAP Qc, Thermo Fisher Sci.) using diluted sample solutions containing an internal standard of indium (withuncertainty better than ± 3%, as estimated from thereproducibility (2RSD) of standard solutions with saltcontents higher than those of the analyzed samples). To
determine isotopic ratios, Li and Sr were separated fromthe sample solutions using a column filled with AG50W
cation exchange resin (Bio-Rad Lab., USA). Then, Li
and Sr were further purified using columns filled with
AG50W cation exchange resin and Sr resin (Eichrom
Tech., USA), respectively. After the two-step column separation, Li isotopic ratios were measured with a multicollector ICPMS (MC-ICPMS; Neptune, Thermo Fisher
Sci.) and Sr isotopic ratios with a thermal ionization mass
spectrometer (TIMS; TRITON, Thermo Fisher Sci.). A
Li standard NIST L-SVEC solution was measured before
and after the sample analyses to correct instrumental
mass bias. The measured 7Li/6Li ratios are expressedas permil deviations from the NIST L-SVEC standard,
based on the formula δ7Li = ­[[7Li/6Li]sample/[7Li/6Li]L-SVEC

7
standard − 1] × 1000. Our δ Li value of the Institute for Reference Materials and Measurements BCR-403 seawater
standard was + 31.3‰, in good agreement with the previously reported value of + 31.0‰ (Millot et al. (2004)
and within our uncertainty of less than ± 0.3‰ estimatedfrom the long-term reproducibility. The 87Sr/86Sr ratiosof the samples were acquired using a TRITON TIMS
by loading purified Sr fractions onto single tungsten
filaments together with a tantalum activator solution.
The uncertainty on 87Sr/86Sr measurements was better
than ± 0.000007, as estimated from the long-term reproducibility. The average 87Sr/86Sr ratio that we obtained for
the JB2 rock standards was 0.703679 ± 0.000007 (2 SD,
n = 4), as previously reported by Nishio et al. (2010).

3 Results
The geochemical and isotopic data for groundwater collected at the Uenae and Eniwa sites (Fig. 1) are provided
in Table 1. The samples analyzed in this study are aliquotsof samples previously studied by Sano et al. (2020a), whoreported data for elements constituting volatile elementsin the samples, such as the δ13C values of C
­ O2. The samples from the Uenae and Eniwa sites analyzed in thisstudy were bottled between June 2015 and May 2019 andbetween June 2016 and October 2018, respectively. These

Fig. 1 Locations of the Uenae and Eniwa sampling sites. The yellow stars indicate the epicenter locations of the 2018 M6.7 Hokkaido Eastern Iburi
earthquake, the M5.1 earthquake on 1 July 2017, and the M5.8 aftershock on 21 February 2019. The red lines on the map represent active faultsof the Ishikari–Teichi–Toen fault zone. The inset map shows the location of the study area within Japan

2016/06/21

2016/11/04

2017/07/01

2017/10/22

2018/04/22

2018/10/04

2015/06/28

2016/09/19

2018/02/13

2018/04/19

2018/07/03

2018/07/12

2018/08/07

2018/09/13

2018/10/18

2018/12/06

2018/12/25

2019/05/28

Eniwa

Eniwa

Eniwa

Eniwa

Eniwa

Eniwa

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

Uenae

2.00

2.06

2.19

2.40

2.02

1.97

2.00

1.98

1.85

1.90

1.85

1.78

6.97

6.98

7.12

6.95

7.02

6.94

Sample dateLi(year/month/day) (μg/L)

Site

10.1

10.7

11.5

12.8

11.0

10.6

11.3

11.0

10.5

11.1

10.9

9.7

1.04

1.04

1.03

1.03

1.07

7.72

8.40

8.91

10.3

8.65

8.54

9.05

9.09

8.19

8.67

8.90

7.74

0.31

0.33

0.31

0.32

0.32

0.34

(μg/L) × ­10(μg/L)
1.14

Cs

Rb
2

112.9

120.9

141.2

193.3

120.0

112.4

121.2

117.3

99.5

105.7

93.3

88.2

47.9

47.3

49.5

47.8

47.9

49.1

(μg/L)

Sr

25.5

27.2

28.7

31.4

24.3

22.8

24.0

23.4

22.0

23.7

24.0

19.2

9.70

9.63

9.76

9.67

9.71

9.82

(mg/L)

Na

6.06

6.43

6.84

7.91

6.25

5.90

6.07

5.97

5.31

5.44

5.06

4.45

1.77

1.74

1.77

1.73

1.75

1.79

(mg/L)

K

4.73

4.96

5.34

6.34

4.97

4.76

4.95

4.92

4.76

4.86

4.37

5.15

3.23

3.23

3.25

3.22

3.22

3.22

(mg/L)

Mg

24.4

25.7

28.0

34.3

25.5

24.4

25.6

25.4

24.3

25.0

22.3

26.1

10.7

10.7

10.7

10.6

10.7

10.7

(mg/L)

Ca

37.1

42.5

48.2

71.1

41.1

35.3

40.9

38.3

32.3

36.7

29.5

29.4

7.15

6.83

8.03

7.55

7.80

7.39

(mg/L)

Cl

4.22

4.22

4.20

3.97

3.88

3.87

3.96

3.91

4.14

4.35

4.74

4.31

5.50

5.55

5.50

5.59

5.56

5.49

(w/w)

Na/K

Table 1 Chemical compositions, elemental ratios, δ7Li values, and 87Sr/86Sr ratios of water samples from the Uenae and Eniwa sites
3

18.6

20.6

22.0

29.6

20.4

17.9

20.5

19.4

17.4

19.3

15.9

16.5

1.03

0.98

1.13

1.09

1.11

1.06

(w/w, × ­10 )

Cl/Li

328

352

341

368

343

314

337

326

325

347

316

333

149

145

162

158

163

151

(w/w)

Cl/Sr

12.02

11.95

12.18

12.13

12.01

12.06

4.72

4.87

4.77

4.68

(‰)

δ7Li

0.70476

0.70476

0.70474

0.70476

0.70476

0.704770.705540.705510.705540.70553

Sr/86Sr

87

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Page 5 of 12collections include five and seven samples obtained fromthe Eniwa and Uenae sites, respectively, before September 6, 2018, the date of the M6.7 Hokkaido Eastern Iburi
earthquake. Additionally, our samples include one postearthquake sample from the Eniwa site and five from the
Uenae site.

Groundwater Cl/Li ratios at the Eniwa site, whichranged from 979 to 1,130, were markedly lower than surface water values (> 20,000; Nishio et al. (2010)), whereas
groundwater Cl/Li ratios at the Uenae site (15,900–
29,600) were notably higher than at the Eniwa site andcomparable to surface water values. The Cl/Sr ratios for
the Uenae site (314–368) were notably higher than thoseof the Eniwa site (145–163).
The δ7Li values of groundwater at the Eniwa site(+ 4.7‰ to + 4.9‰) were markedly lower than theδ7Li values of groundwater at the Uenae site (+ 12.0‰
to + 12.2‰; Fig. 4a, b). The δ7Li values of river waters
worldwide range from + 6.0‰ to + 37.5‰ (Huh et al.
1998). At the Uenae site, the δ7Li values of groundwater
were comparable to those of river water, whereas at the
Eniwa site, they were notably lower.
The 87Sr/86Sr ratios in the Uenae and Eniwa groundwater during the observation period were 0.70474–0.70477
and 0.70551–0.70554, respectively (Table 1). The basement rocks at both sites are mainly Quaternary volcanic
rocks. The 87Sr/86Sr ratios of groundwater collected at
the study sites are similar to the 87Sr/86Sr ratio of 0.70485
for Quaternary volcanic rocks reported by Notsu et al.
(1991). Marked temporal changes in the 87Sr/86Sr ratiosof samples were not observed during the study period ateither site (Fig. 4c, d).

3.1 Time‑series changes in groundwater geochemistry

The most notable result of this study is that the Na/K
ratio in the groundwater at the Uenae site decreased
markedly before the M6.7 Hokkaido Eastern Iburi earthquake (Fig. 2a). The Na/K ratio at the Uenae site began
to decrease around April 2018, about 5 months before
the earthquake, reached a minimum value in September 2018, when the earthquake occurred, then increased
until around December 2018, approximately 3 months
after the earthquake. In contrast, Na/K ratios in groundwater at the Eniwa site were consistent before and after
the earthquake (Fig. 2b). With the exception of the Na/K
ratios at the Uenae site, no preseismic signals weredetected in the analyses in this study, including in the7
Li/6Li and 87Sr/86Sr ratios (Figs. 2–4).
The concentrations of Li, Rb, Cs, Sr, Na, K, Mg, Ca, andCl, and the Cl/Li ratios, increased at the Uenae site about
one month after the earthquake (Fig. 3a–j). In contrast,
no obvious post-earthquake changes were observed ingroundwater at the Eniwa site (Fig. 3k–t).

4 Discussion

3.2 Comparison of groundwater geochemistryat the Uenae and Eniwa sites

4.1 Origin of groundwater collected from the Uenae
and Eniwa sites

The Li concentrations at the Eniwa site (6.9–7.1 µg/L;
Table 1) were markedly higher than those at the Uenae site
(1.8–2.4 µg/L). At both sites, groundwater Li concentrations were notably higher than the average Li concentrationin river water (0.57 µg/L; Nishio et al. (2010)). The concentrations of Na, K, Mg, Ca, Cl, Rb, Cs, and Sr in groundwater
were higher at the Uenae site than at the Eniwa site.

The Cl/Li ratio is a useful indicator for quantitatively
understanding the extent to which δ7Li values haveshifted as a result of surface-fluid contamination (e.g.,
Nishio et al. 2015, 2010). Surface fluids have never experienced high temperatures, whereas non-surface fluids
such as volcanic and deep-derived fluids may have experienced such temperatures. The observed δ7Li values ofFig. 2 Na/K ratio in groundwater at a the Uenae site and b the Eniwa site before and after the M6.7 Hokkaido Eastern Iburi earthquakeof September 2018

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Fig. 3 Temporal variations in the concentrations of Li, Rb, Cs, Sr, Na, K, Mg, Ca, and Cl and the Cl/Li ratio in groundwater for the Uenae and Eniwa
sites before and after the M6.7 Hokkaido Eastern Iburi earthquake of September 2018

groundwater are the result of binary mixing between
surface and non-surface fluids. Previous analyses ofwater samples from southwest Japan and Ontake volcano
spring waters yielded Cl/Li ratios of volcanic fluids and/or deep-derived fluids of less than 1100 (Nishio et al.
(2010) and 1000 (Kazahaya et al. 2014), whereas measured Cl/Li ratios of surface water were markedly higher,e.g., 20,000 for a water sample collected from the Shirakawa River in the Ontake area (Nishio et al. (2010)).
Therefore, water samples with Cl/Li values between
1100 and 20,000 are considered to be mixtures of surface water and deep-derived fluids. For the Uenae

groundwater samples, the Cl/Li ratios varied widelyfrom 15,900 to 29,600 but the δ7Li values were uniform, ranging from + 12.0 to + 12.2‰ (Fig. 5; Table 1).
As shown in the δ7Li–Cl/Li diagram (Fig. 5), the Uenae
site samples have almost constant δ7Li values while the
Cl/Li ratio varies significantly. If this is the result ofbinary components mixing of surface water (high δ7Li
values and Cl/Li ratios) and deep-derived fluids (low
δ7Li values and Cl/Li ratios), the δ7Li values should
increase with Cl/Li ratio. In the Uenae region, the δ7Li
values of both the surface water component (high Cl/
Li ratio) and the non-surface water component (low Cl/

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Fig. 4 Variations in δ7Li values and 87Sr/86Sr ratios before and after the M6.7 Hokkaido Eastern Iburi earthquake of September 2018 for a, c
the Uenae site and b, d the Eniwa site

(James et al. 2003). This relationship can be expressed as(Wunder et al. 2006):

�7 Lifluid - solid = 4.61 × (1000/T ) − 2.48

Fig. 5 Cross-plot of Cl/Li weight ratio and δ7Li for the Eniwa site(orange circles) and the Uenae site (blue circles)

Li ratio) are around + 12‰, indicating that the groundwater samples at the Uenae site had been affected bymixing with surface water during the sample collection period. However, the samples from the Eniwa site,with an average Cl/Li ratio of ~ 1000 and δ7Li values of
ca. + 4.7‰, did not vary during the sample collection
period.
In most minerals, Li is found in sixfold coordination,
whereas in fluids it occurs in fourfold coordination.
Thus, because of the preference of the lighter isotope forthe higher coordination site, the δ7Li value of the fluid
phase is higher than that of the solid phase (Oi et al.
1989; Wunder et al. 2006). This Li isotopic fractionation
between liquid and solid phases (Δ7Lifluid-solid) is temperature-dependent, decreasing with increasing temperature(1)

where T represents the reaction temperature associatedwith the water–rock interaction. The δ7Li values of the
volcanic rocks in the study area are unknown; however,
fresh volcanic rocks erupted in nearby areas of the subduction zone are expected to have the same δ7Li value, so
we calculated the reaction temperature by using the average value for volcanic rocks in Northeast Japan (+ 2.1‰
to + 4.4‰) reported by Moriguti et al. (2004), which are
subducted with the Pacific Plate as were the volcanics inour study. From this calculation, we infer that Li in the
non-surface water component of the Uenae groundwater
experienced temperatures above 120 °C.
From data on spring water from sites around Ontake
volcano, Nishio et al. (2010) pointed out that the δ7Li
values of samples with Cl/Li < 1100 were not affected bysurface water or seawater mixing. On this basis, the δ7Li
values (+ 4.7 to + 4.9‰) of groundwater at the Eniwa site
(Cl/Li = 980–1100) can be regarded as unaffected by surface water components. From Eq. (1), it can be inferredthat the Li in the groundwater at Eniwa experienced atemperature of 780 °C. Thus, the Li in the non-surface
water component of the groundwater at the Eniwa siteexperienced higher reaction temperatures than the
groundwater at the Uenae site.

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As mentioned in Sect. 3, no marked temporal changes
were observed in the 87Sr/86Sr ratios of the samples collected and analyzed during the study period at eithersite (Fig. 4c, d). This is consistent with the fact that the87
Sr/86Sr ratios of waters in Quaternary volcanic regions
tend to correlate with those of the surrounding volcanic
rocks (Notsu et al. 1991).
4.2 Pre‑earthquake decrease in Na/K ratio at the Uenae
site
4.2.1 Mixing model

The water samples from the Uenae and Eniwa sites can be
regarded as mixtures of a deep-derived fluid component
and surface water, as discussed in Sect. 4.1. The Na/K
ratios and δ7Li values of possible fluid components in theregion are illustrated in Fig. 6. One of the components issurface water in which the meteoric water has not experienced high temperature. The Na/K ratios and δ7Li values for the surface water component are 5 and + 23‰,
respectively. The Na/K ratio of the surface water is theaverage value of river waters of western Hokkaido with
low total dissolved solids as reported by Shigino (2011)(Appendix 1). The δ7Li value for surface water is theglobal average value reported by Huh et al. (1998). Due tothe the presence of Tarumae volcano in the area, volcanic
fluid resulting from the reaction of hot magma and meteoric water is also plotted in Fig. 6. As there are no available data on hot spring waters around Tarumae, the δ7Li
values of volcanic fluids obtained from hot spring waters
around Ontake volcano (Nishio et al. (2010) are shown.
Because the average δ7Li value of Ontake volcanic rocks(+ 2.7‰; Nishio et al. (2010)) is very close to the average
δ7Li value of volcanic rocks in Northeast Japan (+ 3.0‰;
Moriguti et al. 2004), in this study, the δ7Li value of the
volcanic fluids from Ontake volcano can be regarded as
representative of those of volcanic fluids from Tarumae
volcano. The Na/K ratio and δ7Li values for the volcanic
fluid component are 7 and + 3‰, respectively. As Na/K
ratios and δ7Li values for volcanic fluids, we used theaverage values of hot spring waters from two sites within
5 km of the crater of Ontake volcano reported by Nishio
et al. (2010).
In this study, we used data from Arima hot spring waterto represent deep-derived fluids in the forearc region.
The hydrogen and oxygen isotopic ratios of hot springwaters occurring in several forearc regions, including
Arima, are different from those of meteoric waters, suggesting that Arima-type deep fluids may be derived from
subducted slabs (Matsubaya et al. 1973). The Na/K ratioof 8 and δ7Li value of + 1‰ used for the deep-sourced
fluid are the average values of previously reported data
for Arima hot spring waters (Kusuda et al. 2014; Umam
et al. 2022). For groundwater relatively enriched inFig. 6 Mixing lines between surface water and deep-derived
fluids. Surface water data are for river waters in western Hokkaido(Shigino 2011) and global surface waters (Huh et al. 1998); datafor deep-derived fluids are for Arima deep fluid (Kusuda et al. 2014;
Umam et al. 2022) and Ontake volcanic fluid (Nishio et al. 2010).
Also shown are data for groundwater from the Eniwa site (orange
circles) and the Uenae site (blue circles). The Uenae groundwater
samples exhibit movement along the vertical axis (red dashed arrow)and not along the gray mixing lines; thus, the mixing of upwelling
deep-derived fluids with surface water cannot explain the observed
pre-earthquake decrease in the Na/K ratio. The red dashed arrowin the figure represents the movement of water samples along thisdirection, indicating the potential reaction with ­CO2

surface water components, such as at the Uenae site, theδ7Li value would be expected to decrease with a constant Na/K ratio when the contribution of fluids that haveexperienced high temperatures, such as volcanic fluids
and deep source fluids, increases (Fig. 6). Therefore, thedecreasing Na/K ratio with constant δ7Li values observedin the groundwater at the Uenae site before the Hokkaido
earthquake did not result from a change in the contribution of fluids that had experienced high temperatures,such as volcanic fluids or deep fluids.
4.2.2 CO2 influx model

A decrease in δ13C ­(CO2) and an increase in total dissolved inorganic carbon (TDIC) were reported ingroundwater at the Uenae site before the Hokkaido
Eastern Iburi earthquake in 2018 (Sano et al. 2020a). In
addition to changes in carbon isotopic ratios, Sano et al.
(2020a) reported variations in 14C activity, δ18O, andδD values prior to the earthquake. The beginning of thepre-earthquake decrease in the Na/K ratio at the Uenae
site documented in this study coincides well with the
beginnings of the changes in δ13C, 14C, and TDIC values reported by Sano et al. (2020a) (Fig. 7). Based on theδ13C, 14C, and TDIC results in groundwater, Sano et al.
(2020a) suggested that ­CO2 injected into the Tomakomai
subsurface for carbon dioxide capture and storage (CCS)Zandvakili et al. Progress in Earth and Planetary Science(2024) 11:32

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Fig. 7 Na/K ratios (blue circles) compared with a δ13C, b 14C activity, and c total dissolved inorganic carbon (TDIC) (Sano et al. 2020a) at the Uenae
site before and after the M6.7 Hokkaido Eastern Iburi earthquake of September 2018. The light blue and green arrows from February 2018
to the time of the earthquake represent changes in the Na/K ratio and TDIC of Uenae groundwater samples, respectively; notably, these changes
are negatively correlated during this period

may have entered the aquifer at the Uenae site a few
months before the 2018 earthquake.
Using a bottled water method similar to that applied
herein, Onda et al. (2018) proposed that significant
changes in groundwater oxygen isotopic ratios observed
prior to the 2016 M6.6 Tottori earthquake were directly
related to changes in volumetric strain. However, similar to our results, the changes in oxygen isotopic ratios
prior to the 2016 M6.6 Tottori earthquake reported byOnda et al. (2018) may be explained by oxygen isotopic
exchange with carbon dioxide entering the groundwateraquifer.

The concentration of each element in solution would be
expected to change as the pH decreases as a result of theinflux of ­CO2 into the aquifer. The contrasting behaviorof potassium (K) compared to other elements, possibly
related to the precipitation of K-bearing minerals, which
would be affected by water–rock interaction to a greater
degree than other minerals in the groundwater source
region, has been previously demonstrated by Skelton
et al. (2014) in groundwater in northern Iceland before
an earthquake there. However, the mechanism by which
only the Na/K ratio decreased markedly, while the otherelement ratios did not, remains unresolved.

Zandvakili et al. Progress in Earth and Planetary Science(2024) 11:32

The δ7Li data suggest that the pre-earthquake decreasein the Na/K ratio at the Uenae site was not caused by thecontribution of volcanic fluids and/or deep-derived fluids, as discussed in Sect. 4.2.1. The coincidence with thedecrease in δ13C values indicates that it is very likely thatthe decrease in the Na/K ratio was also due to the ­CO2
injection into the aquifer proposed by Sano et al. (2020a).
4.2.3 Low Na/K ratio in groundwater before a large
earthquake

A decrease in the Na/K ratio associated with an increasein the dissolved ­CO2 concentration of groundwater
before a major earthquake has been reported previously
at Slanic Moldova, Romania, before a M6.0 earthquake in2004 (Fig. 8; Mitrofan et al. 2008). The first assessmentof geological storage capacity in Romania was conducted
between 2006 and 2008 by EU GeoCapacity (2009).
Therefore, since no CCS had been carried out in Romania prior to 2004, the C­ O2 influx into the aquifer responsible for the negative correlation between dissolved ­CO2
concentration and Na/K ratio in groundwater observed
before that earthquake was of natural origin.
In contrast, no change in the Na/K ratio was observedin groundwater at the Eniwa site (Fig. 2b), despite theobserved decrease in δ13C ­(CO2) before the M6.7 Hokkaido Eastern Iburi earthquake reported by Sano et al.
(2020a). A possible explanation for the absence of a Na/K
change at the Eniwa site before the earthquake is a timelag, resulting from the different distances of the Eniwa
and Uenae sites from the C­ O2 source that produced thedecrease in the Na/K ratio.
The Uenae site, where the pre-earthquake Na/K change
was observed, is located near the ITTFZ (Fig. 1). Because
the fault zone provides a pathway for fluids rising from
depth, it is possible that the groundwater near the surface
at Uenae may have been affected quickly by subsurface
events such as the effects of ­CO2 inflow.

Fig. 8 Dissolved ­CO2 concentrations (orange circles) and Na/K ratios(blue circles) before the M6.0 Romania earthquake of 2004 (datafrom Mitrofan et al. (2008))

Page 10 of 12

4.3 Post‑earthquake changes in chemical compositions
and elemental ratios

Changes in chemical composition after an earthquake
may occur in response to fracturing of a hydrologicalbarrier between aquifers or as a result of a change in therelative pressures of connected aquifers (Thomas 1988).
Because the Uenae site is close to the ITTFZ fault zone
and the earthquake epicenter, observed changes in thechemical composition and elemental ratios of groundwater caused by the earthquake can be explained as a resultof changes in the stress–strain state of the fracture zone.
Such changes can alter aquifer properties and/or cause
local hydrostatic releases, mixing of water from different aquifers, or switching between different groundwater
sources along the faults (Rosen et al. 2018).
In the Unae site samples, an increase was observed in
the Cl/Li ratios (Fig. 3j) and in the concentrations of Li,
Rb, Cs, Sr, Na, K, Mg, Ca, and Cl one month after theearthquake (Fig. 3a–i). The observed increase in the Cl/
Li ratios in these samples could be related to post-earthquake mixing of Uenae groundwater with another water
source, such as seawater with a higher Cl concentration. The increases in the concentrations of Li, Rb, Cs, Sr,
Na, K, Mg, Ca, and Cl can be explained by mixing withanother water source at the Uenae site.
The presence of water or fluid after the M6.7 earthquake has also been proposed by Hua et al. (2019), who
employed P- and S-wave attenuation tomography; aclear low-Q (high attenuation) belt at depths of 10 and
40 km exist under central Hokkaido, including the areaof the earthquake epicenter. Groundwater mixing after
the earthquake through the ITTFZ fault system near the
Uenae site, where the pore pressure was high and permeability along the fault plane was enhanced, has been
discussed by Sano et al. (2020a). After the earthquake,the Uenae site groundwater aquifer mixed with anothernew fluid; however, meaningful changes in hydrochemical compositions and the Cl/Li ratio were not observedin Eniwa site samples before or after the earthquake(Fig. 3k–t). This difference can be attributed to the larger
distance of the Eniwa site from the epicenter and theactive faults in the ITTFZ, or to the small number ofmeasurements.

5 Conclusions
We documented a marked decrease in Na/K ratio ingroundwater in the Uenae area, approximately 20 km
west of the epicenter, a few months before the September
2018 M6.7 earthquake in Hokkaido, Japan.
The pre-earthquake decrease in the Na/K ratio at the
Uenae site did not result from an input of deep-derived
fluid because no variations in the 7Li/6Li and 87Sr/86Sr

Zandvakili et al. Progress in Earth and Planetary Science(2024) 11:32

Page 11 of 12

ratios of the groundwater were observed before theearthquake. The timing of the pre-earthquake decrease in
the Na/K ratio observed in this study coincided well withthe beginnings of the changes in δ13C, δ18O, and δD values reported by Sano et al. (2020a); these changes wererelated to the injection of C
­ O2 into the aquifer.
Therefore, the pre-earthquake Na/K decrease observedin the Uenae groundwater may be attributed to injectionof ­CO2 into the groundwater layer, without the involvement of deep-derived fluids, because the concentrationof each element in the solution was altered as a result ofthe ­CO2 influx into the aquifer.

Availability of data and material
All newly obtained data from this study are presented in the manuscript, anddata sharing is not applicable.

Appendix 1
Na/K ratios of the waters of several rivers in western
Hokkaido (Shigino 2011) that were used to calculate theaverage Na/K ratio of surface waters in the study area.
Name

Longitude Latitude Na/K
(deg. E)(deg. N) (w/w)

Changliuchan

141.11842.642

3.6

Tobi River

141.153

42.539

6.8

Shiraoi River

141.186

42.652

5.1

Tobi River

141.266

42.513

4.8

Tobi River

141.266

42.513

5.1

Average

5.1

Abbreviations
CCS	Carbon dioxide capture and storage
ITTFZ	Ishikari–Teichi–Toen fault zone
MC-ICPMS	Multi-collector inductively coupled plasma mass spectrometer
PTFE	Polytetrafluoroethylene
TIMS	Thermal ionization mass spectrometer
Acknowledgements
We thank Reiko Hirose, Kazuya Nagaishi, and Kei Okamura for their assistancewith the analytical procedures. Constructive comments from Kenji Shimizu,
Yohei Hamada, and Tsuyoshi Ishikawa were very helpful throughout thisresearch. Grateful acknowledgment is extended to Junji Yamamoto and an
anonymous referee for their thoughtful and constructive reviews, whichsignificantly contributed to the clarity of the presentation. Special thanks arealso extended to Madhusoodhan Satish-Kumar for the editorial handling of
this work.
Author contributions
ZZ conducted the analysis, data processing, and manuscript writing andrevision. YN conceived the study, obtained funding, and conducted theanalysis, data processing, and assisted with manuscript writing and revision. YS
conducted the sampling and assisted with manuscript writing and revision. All
authors have read and approved the final manuscript.
Funding
This study was supported by Takahashi Industrial Economic Research Foundation and by three Grants-in-Aid for Scientific Research (Nos. 20H01995,
18H03894, and 24H01031) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan.

Declarations
Competing interests
The authors declare that they have no competing interest.
Received: 8 September 2023 Accepted: 26 May 2024

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