Geochemical monitoring of groundwater and soil gas emission pointed out precursor and/or coseismic anomalies of noble gases associated with earthquakes, but there was lack of plausible physico-chemical basis. A laboratory experiment of rock fracturing and noble gas emission was conducted, but there is no quantitative connection between the laboratory results and observation in field. We report here deep groundwater helium anomalies related to the 2016 Kumamoto earthquake, which is an inland crustal earthquake with a strike-slip fault and a shallow hypocenter (10 km depth) close to highly populated areas in Southwest Japan. The observed helium isotope changes, soon after the earthquake, are quantitatively coupled with volumetric strain changes estimated from a fault model, which can be explained by experimental studies of helium degassing during compressional loading of rock samples. Groundwater helium is considered as an effective strain gauge. This suggests the first quantitative linkage between geochemical and seismological observations and may open the possibility to develop a new monitoring system to detect a possible strain change prior to a hazardous earthquake in regions where conventional borehole strain meter is not available.

Groundwater‐level changes after earthquakes provide insight into changes in hydrogeological properties such as permeability and pore pressure. Quantifying such changes, both their location and magnitude, is usually hindered by limited data. Using extensive high‐resolution water‐level monitoring records, we provide direct evidence of significant groundwater drawdown (4.74‐m maximum) over a 160‐km2 area along crustal ruptures after the Mw 7.0, 2016, Kumamoto earthquake. Approximately 106 m3 of water disappeared within 35 min after the main shock. The loss of water was not caused by static‐strain driven pore‐pressure decrease nor by releasing of water through structural pathways, but most likely by water transfer downwards through open cracks. Such changes may impact the security of water resources, the safety of underground waste repositories, and contaminant transport in seismically active areas.

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.

Abstract. The height of water levels in two wells located near Usu volcano, Japan, changed in a systematic fashion for several months prior to the eruption of Usu volcano on 31 March 2000. In one well, water-level decrease relative to normal levels was first observed at the beginning of October 1999. The decreasing water-level is postulated to result from groundwater flow into cracks widened by intruding magma during dike formation. From the beginning of January 2000, the rate of decrease became higher. During this time, the water level of the second well increased by 0.05 m and then gradually decreased. The water-level changes are consistent with volumetric expansion of magma inside the magma chamber, followed by intrusion of magma into the fracture system associated with widening of cracks. We conclude that waterlevel observations can provide information that may potentially be used to predict further volcanic eruptions.

[1] Abrupt and large rises in well water levels were observed three days before the 2000 eruption of Usu volcano, simultaneously with the increase of seismic activity. After the groundwater level in the DT1 well rose 4.07 m, the groundwater discharged one day before the eruption. The groundwater level rose 0.95 m in the DT2 well before the eruption. These water-level rises are caused by crustal deformation from magma migration, and enable us to estimate 0.63À11.2 Â 10 À6 of compressional volumetric strain by applying the wells' responses to earth and ocean tides. These strain changes are explained by a 2.8 -16.5 Â 10 6 m 3 spherical inflation source model located at the Usu volcano. A 5 Â 10 6 m 3 inflation source model is estimated by a horizontal displacement observed by GPS near DT2, which is consistent with the volumetric strain estimated by the water-level rises in DT2.