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Solid earth sciences
202406202406
Geofluid behavior prior to the 2018 Hokkaido Eastern Iburi earthquake: insights from groundwater geochemistry
Zahra Zandvakili, Yoshiro Nishio, Yuji SanoZahra Zandvakili, Yoshiro Nishio, Yuji Sano
Na/K ratio, Hokkaido Eastern Iburi earthquake, Lithium isotope, Earthquake precursor, Carbon dioxide
Fig.1
Decreasing groundwater Na/K ratios observed at the Uenae site prior to the 2018 Hokkaido earthquake (blue dots), which is roughly contemporaneous with the carbon isotope ratios reported by Sano et al. (2020) (green dots) and the start of the decrease in Na/K ratios found here.
Fig.2
CO2 transport model in the subsurface of Uenae near the epicenter of the 2018 Hokkaido earthquake in Japan.Sano et al. (2020) suggested that the carbon isotope ratio characteristics may be derived from a large-scale carbon dioxide capture and storage (CCS) demonstration project in Tomakomai, about 15 km away from Uenae. On the other hand, the possibility of a natural origin deep underground remains.
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 cause of 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 epicenter of 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, CO2 influx into groundwater may be a common phenomenon preceding earthquakes.
