Measurements of 3He/4He and 20Ne/4He ratios in 12 CH4-rich natural gas samples were made with a magnetic deflection mass spectrometer equipped with a metallic high-vacuum purification line. In order to check the resolving power, sensitivity and stability of the mass spectrometer, atmospheric air in Tokyo was measured repeatedly. The mean 3He/4He ratio in Tokyo air was (1.43 ± 0.03) × 10-6. CH4-rich gases with significantly high 3He/4He ratios were first observed in this study. The 3He/4He ratios for 5 natural gas samples collected from hot springs and mineral springs in inland basins were as high as (1.7–7.3) × 10-6, probably due to the large contribution of the upper mantle-derived He. In comparison, the 3He/4He ratios for 2 samples from water wells distributed in coastal areas facing the Pacific Ocean and 5 samples from gas fields in the southern Kanto district were as low as ∼10-7. Most of the He in these gases is inferred to be radiogenic and of crustal origin.
A clear coseismic anomaly of groundwater radon was observed for a magnitude 5.6 earthquake that occurred on May 11, 1992. The coseismic radon anomaly was observed at a station which is located right on a major active fault in northeast Japan, and about 140 km away from the hypocenter. This was the first time that an earthquake with M<6 had ever been accompanied by a clear radon anomaly at the station; although we had observed 12 similar coseismic radon anomalies at the station during the observation period from 1984 to 1987, all of the earthquakes that were accompanied by radon anomalies in that period had been with magnitude 6.0 and over. Surprisingly, the radon concentration has become more sensitive to show coseismic anomalies even for M<5 earthquakes since October 1992. This enhancement of sensitivity of the coseismic radon response may be attributed to the progress of micro‐crack formation in the fracture zone of the active fault, which could be related to unusual stress accumulation in the region.
This paper provides a new approach to detect changes in the groundwater radon concentration related to an earthquake. We express changes in radon concentration in a radon-detection chamber by using stochastic linear differential equations. These equations are represented by the state space notation, and then its solution is replaced by an estimation of the state vector at discrete points in time with an assumption that the coefficients describing the stochastic differential equations are constant for a sufficiently small time interval. Since the solubility of radon in water depends strongly on temperature, the separation of radon from liquid water, which is necessary for radon detection, causes fluctuations in the observed radon concentrations due to water temperature changes in the chamber. We applied our procedure to some actual data sets on groundwater radon concentration with those on simultaneously observed water temperature, and found that the temperature effects on the fluctuations in the observed radon concentration can be satisfactorily described by our procedure. Furthermore, we were able to estimate the original radon concentration in groundwater before it was introduced into the radon-detection chamber, which was not affected by water temperature changes. The obtained original radon concentrations are very stable during normal periods, and anomalous changes associated with earthquakes were easily detected. Our new method will be very useful to examine time-variation patterns of changes in groundwater radon and will provide important information about the mechanism of radon changes related to earthquakes.
