Groundwater oxygen isotope anomaly before the M6.6 Tottori earthquake in Southwest Japan

Geochemical monitoring of groundwater in seismically-active regions has been carried out since 1970s. Precursors were well documented, but often criticized for anecdotal or fragmentary signals, and for lacking a clear physico-chemical explanation for these anomalies. Here we report – as potential seismic precursor – oxygen isotopic ratio anomalies of +0.24‰ relative to the local background measured in groundwater, a few months before the Tottori earthquake (M 6.6) in Southwest Japan. Samples were deep groundwater located 5 km west of the epicenter, packed in bottles and distributed as drinking water between September 2015 and July 2017, a time frame which covers the pre- and post-event. Small but substantial increase of 0.07‰ was observed soon after the earthquake. Laboratory crushing experiments of aquifer rock aimed to simulating rock deformation under strain and tensile stresses were carried out. Measured helium degassing from the rock and 18O-shift suggest that the co-seismic oxygen anomalies are directly related to volumetric strain changes. The findings provide a plausible physico-chemical basis to explain geochemical anomalies in water and may be useful in future earthquake prediction research.


Sampling site of groundwater. To document the geochemical background of groundwater in central
Tottori before and after the earthquake, we collected commercial bottled water covering a period of 22 months in between the Tottori earthquake event. This approach was suggested by a study of groundwater chemical variations prior and after the M7.2 Kobe earthquake in 1995 19 . Cretaceous-Paleogene granitoids are exposed in the Tottori region 20 and high-quality mineral water is available from the altered granite layers. All samples were collected at the Hakusan Meisui (HKM) site, located approximately 5 km west of the Tottori earthquake epicenter (Fig. 1). There is a drilled well up to 1500m-deep. Groundwater pumped from a 240m-deep aquifer with approximately 15 °C is filtered and sealed in polyethylene terephthalate bottles and distributed on the market. Twenty-seven samples were bottled water distributed from September 2015 to July 2017. Hydrogen (D/H) and oxygen ( 18 O/ 16 O) isotopes of water were measured by a cavity ring-down spectroscopy (L2120-i Analyzer, PICARRO Co. Ltd) without any preprocessing. Observed hydrogen and oxygen isotopic ratios were calibrated against our in-house water standard and converted into the conventional V-SMOW unit, expressed as per mil (‰) in STable 1. Instrumental errors of δ 18 O and δD values were less than 0.05‰ and 0.3‰ at 2σ. Figure 2a shows the secular variation of 18 17

Discussion
Most of the geochemical precursors in water reported in literature 1,8 have been rejected as fragmentary. Among a few convincing pieces of evidence, there is the groundwater radon anomaly 21 and the chlorine and sulfate variations in mineral water 19 prior to the 1995 Kobe earthquake in Japan. Chlorine contents started increasing about 4 months before the Kobe earthquake, while radon increase began 3 months before. Inspired by the anomalous period of 3-4 months in Kobe earthquake, we masked our δ 18 O data from August 2 nd to October 18 th 2016 and calculated a general trend for the temporal variation of δ 18 O values using a spline function method. We calculated the difference between the observed and the smoothed δ 18 O values. Figure 2b shows temporal variation of the δ 18 O differences. There are two anomalies of groundwater oxygen before the M6.6 event beyond the accuracy. One is an obvious positive peak with the maximum of +0.24 ± 0.05‰ (2σ) found on August 16 th . The other is a small but substantial negative peak with the minimum of −0.07 ± 0.05‰ (2σ) on October 18 th , three days before the M6.6 event. The latter indicates that the co-seismic increase of the δ 18 O value is 0.07‰.
To clarify the mechanism controlling the δ 18 O variations, the δD values were also measured and reported on a Craig's plot 10 Fig. 3a). Then we can assess the secular variation of the difference between observed and calculated δD values. Note that there is no anomalous change of the δD differences before the M6.6 event (Fig. 3b). On the other hand, there were small increases of chloride (Cl − ) and sulfate (SO 4 2− ) contents three months before the 2016 Tottori earthquake (SFig. 3 and STable 2), even though experimental errors are large. The anomalous trends are similar to those observed before the 1995 Kobe earthquake 21 , suggesting a common basis. Note that there is not significant temperature change of groundwater after the Tottori earthquake as mentioned by the mineral water company.
The most probable mechanism to cause δ 18 O enrichment without δD change of the groundwater is a 18 O-shift caused by water-rock interaction 11 . Aquifer rocks in Tottori area are altered Cretaceous-Paleogene granitoids, whose δ 18 O values are ranging from +6.9‰ to +10.6‰ with the average of +9.0‰ 23 . We have measured δ 18 O values of aquifer rocks, ranging from +8.20‰ to +8.56‰. Since observed groundwater shows δ 18 O values of about −8.2‰, much lower than those of aquifer rocks, a progressive isotopic equilibration of oxygen would make the groundwater δ 18 O values increase. On the other hand, the δD values of groundwater remain unchanged because the hydrogen content of granitic rocks is significantly low compared to that of water. Crustal deformation related to the 2016 Tottori earthquake may have enhanced water-rock interaction (i.e., by micro-fracturing and the resulting increase in rock surface area) in 240m-deep aquifer, induced 18 O enrichment of water that was in contact with the fractured rocks and appeared as the precursor and co-seismic oxygen isotope anomalies (Fig. 2) together with slight increases of Cl − and SO 4 2− contents (SFig. 3). Independent from stable isotope analysis of bottled mineral water as stated above, we have conducted a survey of noble gas in hot spring in the Tottori region. Groundwater helium isotopic variations were reported during the 2016 Kumamoto earthquake (M7.3) in Kyushu, southwest Japan 24 . The earthquake was of an inland crustal type produced along a strike-slip fault and with a shallow hypocenter (10 km), similar to the Tottori M6.6 event. Observed helium isotopes ( 3 He/ 4 He) variations, soon after the Kumamoto earthquake, were coupled with volumetric strain changes estimated using a fault model of crustal deformation. The relation between helium isotopes and volumetric strain can be explained by helium degassing that occurred during compressional loading of rocks as demonstrated in laboratory 25 . In this model, groundwater helium can be regarded as an effective strain gauge 24 . In order to verify this hypothesis for the Tottori event, we have collected hot spring samples at three sites, Sekigane (SKG), Misasa (MSS) and Togo (TOG) (Fig. 1) located close to the epicenter, soon after the event. Their 3 He/ 4 He and 4 He/ 20 Ne ratios were measured by a noble gas mass spectrometer 26 (STable 3). This is entirely independent observation from δ 18 O analysis of groundwater at HKM (Fig. 2).
Following the method that was developed to explain helium variations after the Kumamoto earthquake 24 , the amount of radiogenic helium degassed from Tottori area was estimated at two sites, Sekigane (SKG) and Misasa (MSS) (STable 3). Results were compared with data before the 2016 Tottori earthquake 27 . Figure 4 shows the obtained relation between co-seismic volumetric strain changes estimated by a fault model of rock deformation 28 and helium degassing by aquifer rocks using this new set of data obtained for Tottori and the previous one from the Kumamoto study 24 . A dotted line is obtained by a least square method on the 2/3 power law for volumetric strain changes 25 . Observed Misasa data (MSS in Fig. 1) agree with the best fit line within experimental error, even though the estimated strain change is one or two order of magnitude smaller than that estimated by using the Kumamoto data. The Sekigane data (SKG in Fig. 1) are also consistent with the line under the upper limit. Therefore the Tottori data can be explained by the hypothesis that groundwater helium reflects strain change during the earthquake 24 .
In order to combine two individual observations: δ 18 O anomaly of groundwater and helium isotopic change in hot springs in region close to the epicenter of the 2016 Tottori earthquake, we have conducted a preliminary experiment where rock was crushed in frozen water 29 (see Method). There is no reference that showing oxygen and helium isotope anomalies at the same time related to a single seismic activity. Therefore this is the first attempt to show a linkage between the 18 O-shift and helium release during the crush experiment. Estimated helium degassing of 4.5 × 10 −9 ccSTP/g is corresponding to δ 18 O-shift of 0.36‰ by water-rock interaction. Then it is possible to calculate a conversion factor from helium degassing to 18 O-shift by rock fracturing, which is 8.0 ± 6.9 × 10 7 ‰/ccSTP/g (2σ). Co-seismic volumetric strain change was estimated to be 6.88 × 10 −8 at the HKM site using the fault model 28 . This value can be converted into helium degassing of 1.8 × 10 −9 ccSTP/g based on the 2/3 power law between strain change and helium degassing (see HKM in Fig. 3). If the result of the rock crushing experiment is applicable to the present case, the amount of helium degassing, 1.8 × 10 −9 ccSTP/g would be converted into the δ 18 O-shift of 0.14 ± 0.12‰ (2σ). The calculated shift would be consistent with observed co-seismic increase of δ 18 O value, 0.07 ± 0.05‰ (2σ) within the experimental error. We can therefore conclude that the co-seismic groundwater oxygen anomaly would be related to the volumetric strain change.
On the other hand, the pre-seismic increase of the δ 18 O value, up to 0.24‰ observed from July to August 2016 (Fig. 2b) is approximately 3.4 times larger than the co-seismic change of 0.07‰. Then volumetric strain change of 2.3 × 10 −7 (3.4 times of 6.88 × 10 −8 ) should have been observed at the HKM site two months before the M6.6 earthquake. Unfortunately, there is no record of volumetric strain change to verify the estimated pre-seismic strain change in the area close to the HKM (Fig. 1). At the 1995 Kobe earthquake, pre-seismic strain change of 3-6 × 10 −6 was observed by a multicomponent borehole strain meter at the Rokko-Takao station, 5 km away from the groundwater observation well where geochemical anomaly was reported 19 . Crustal deformation at the estimated level of 2.3 × 10 −7 may have occurred in the region 5 km away from the epicenter before the Tottori earthquake. However further discussion is difficult, because there is no geophysical observation as well as helium anomaly data from July to November 2016.

Conclusion
We found an increase of +0.24‰ in oxygen isotopes of groundwater a few months before a M 6.6 earthquake in October 21 st 2016. The signal would be equivalent with volumetric strain change of 2.3 × 10 −7 estimated by a relation between helium and oxygen changes by rock crushing experiment in a laboratory and helium isotopic variation observed in hot springs after the earthquake. This suggests that monthly monitoring of groundwater δ 18 O value together with helium abundance in deep well, up to 1000 m may detect a possible strain change prior to inland earthquakes in subduction zones.

Methods
Oxygen and hydrogen isotope analysis. Oxygen and hydrogen isotopes of mineral water (HKM) and recovered water after a vacuum crushing experiment were measured by Cavity Ring-Down Spectroscopy (CRDS) using a L2120-i (Picarro Inc.) connected to an automatic injection system and high-precision vaporizer. Three working standards (SHOKO Science Co.,Ltd., SI Science Reference Material), calibrated against international reference material (VSMOW), were analyzed with samples.
Helium isotope analysis. Helium abundance and isotopic compositions of hot spring water (MSS, SKG and TOG) and head space gas after crushing in a stainless-steel ball mill were measured by a Helix SFT mass spectrometer after separation and purification usung a vacuum line. The 3 He/ 4 He ratio was calibrated against our inhouse standard and expressed as Ra notation where Ra is the atmopsheric ratio of 1.38 × 10 −6 . Anion analysis. Chlorine and sulfate ions of groundwater water at the HKM site were measured by ion chromatography (ICS 1500, DIONEX Thermo Fisher Scientific) after removal of suspended fine particles by 0.20 μm syringe-mounted filter. The samples were calibrated by five working standards (Wako Pure Chemical Industries Ltd, Standard Solition) analyzed in the sequence of sample measurement.
Frozen crush experiment. Two granitic rock samples recovered from a drilled core of the HKM site ( Fig. 1) at depths of −240 m and −1500 m were provided by the company that supplied the water samples. The rock sample derived from 240 m deep is primary important because it is potential aquifer rock. There is not enough amount of the 240 m rock to reproduce experiment. So we measured 1500 m deep rock because it is similar granitic rock in the same place. These rock samples were put in a stainless-steel vacuum crusher with 1 cm 3 of the mineral water and a stainless-steel ball. Water was frozen by cooling the ball mill into liquid nitrogen and head space gases were evacuated. Then, the ball mill was shaken 500 times and helium in rock was extracted by mechanical fracturing 29 . At first gas extracted by the crushing was introduced into vacuum purification line and helium abundance and isotopic composition were measured by a noble gas mass spectrometer after separation 26 . Then reacted water was recovered and the δ 18 O and δD values were measured by a cavity ring-down IR spectrometer. In this experiment, hot and cold blanks mean blank test with crushing and no crushing, respectively. These are not related to temperature, both are cooled by liquid nitrogen.
Measured helium abundances are higher than hot blank, while isotopic ratios show only an upper limit because of a tail effect of high HD background at mass number 3 (STable 4). In order to obtain the 3 He/ 4 He ratio, we conducted a vacuum crushing experiment without water. The dry experiment indicated helium release with radiogenic 3 He/ 4 He ratios with 0.15-0.71Ra. Average helium abundance is 4.5 × 10 −9 ccSTP/g for the 240 m and 1500 m depth samples by wet experiment with water (STable 3). On the other hand, reacted water and hot blank water had much higher δ 18 O and δD values than those of the initial water, probably due to an evaporation effect 10 . It is possible to calculate amounts of 18 18 O shifts of +0.30 ± 0.43‰ (2σ) in a similar way. When one takes error for the weighted mean of the two results, 18 O shift due to crushing experiment becomes +0.36 ± 0.31‰ (2σ), which is statistically not zero and larger than instrumental error of less than 0.05‰.