Anomalous changes of CH4/Ar ratio in subsurface gas bubbles as seismogeochemical precursors at Matsuyama,Japan |
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Authors: | Iwao Kawabe |
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Institution: | (1) Department of Earth Sciences, Faculty of Science, Ehime University, Bunkyo-cho, 2-5, Matsuyama, Japan |
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Abstract: | The results of seismogeochemical monitoring of subsurface gases associated with ground-waters from deep artesian wells in Matsuyama, Japan, are presented. The weekly collections of bubble-gas samples from the two borehole wells and gas chromatographic determinations of their He/Ar, H2/Ar, N2/Ar, and CH4/Ar ratios for the recent nineteen months have revealed that the bubble-gas CH4/Ar ratios at the two stations are correlated with the seismic activity in this area. Positive anomalies of the CH4/Ar ratio were accompanied by five earthquakes with relatively large magnitudes and/or relatively small epicentral distances among thirty earthquakes that occurred during the monitoring period. The anomalous increases in the CH4/Ar ratio had evidently begun prior to the respective earthquakes.The occurrence of the geochemical anomaly appears to depend strongly upon the earthquake magnitude and distance, because the earthquakes that accompanied the anomalies and those that did not can be reasonably distinguished in the plot of magnitude vs. distance. Similar anomalies can be also seen in the variations of He/Ar and N2/Ar ratios, but their magnitudes are much smaller than those of the CH4/Ar ratio. These features have been discussed in relation to the possible mechanism of the seismogeochemical phenomena. |
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Keywords: | CH4 Ar Gas bubble Earthquakes Geochemical Hydrological Precursors |
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