Abstract
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.
Highlights
Volatile element degassing from the solid Earth has been a major subject of geochemistry because it may provide a constraint on the evolution of atmosphere and ocean[1] together with mechanism of climate change and environmental consequence of volcanism[2,3]
The 3He/4He ratios of samples derived from the upper mantle show a high value of 8 Ra (Ra is the 3He/4He ratio of 1.382 × 10−6)[19], whereas those of crustal fluids are characterized by a radiogenic ratio of ~0.02 Ra20
The same trend was first recognized in the San Andreas fault system in California[21] and confirmed by later studies in the North Anatolian fault zone in Turkey[22] and the Karaforam fault in Tibet[23]
Summary
Volatile element degassing from the solid Earth has been a major subject of geochemistry because it may provide a constraint on the evolution of atmosphere and ocean[1] together with mechanism of climate change and environmental consequence of volcanism[2,3]. It is necessary to connect gas geochemistry, model of strain change and rock fracturing experiment in a comprehensive study. The 2016 Kumamoto earthquake (M 7.3 in the Japan Meteorological Agency scale)[17] occurred in Kumamoto City on Kyushu Island, Southwest Japan on April 2016, causing 50 fatalities, over 1800 injured and serious damage to local infrastructures. It was an inland crustal earthquake with a strike-slip fault and a shallow hypocenter (10 km depth) close to highly populated areas (see Methods). We discuss the connection between emanation of crustal helium, estimated volumetric strain change, and a quantitative comparison with a rock fracturing experiment
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