George Igarashi

Earthquake-related water-level changes at 16 closely clustered wells in Tono, central Japan

Water-level/pressure data recorded at a dense network of 16 wells of depths ranging from 23 to 201 m within 400 m of the Tono Mine in Gifu prefecture, central Japan, have been studied in search of possible earthquake-related changes. The study area is traversed by the east-west trending Tsukiyoshi fault, which is ordinarily impermeable, sustaining a significantly higher groundwater pressure on the north side of the fault than the south side. On March 16, 1997, a local earthquake of magnitude 5.8 occurred about 50 km south of Tono. The related water-level/pressure changes recorded at different sites showed different features. At three deeper wells on the north side of the fault the water level and pressure showed large coseismic drops (up to 29 cm) followed by larger rises (of as much as 1.8 m). Two days before the earthquake the water level at another deeper well began to show a total drop of about 3 m during the next 2 months. Most of the shallow wells on the north side of the fault showed water-level drops after the earthquake. On the south side, however, only a small delayed surge and a subsequent gradual increase were recorded at a deeper well. Long-term data since 1989 at one of the deeper wells on the north side showed coseismic drops and recoveries for 25 moderate local and large distant earthquakes up to about 1000 km away. The different observed earthquake-related water-level/pressure changes cannot be explained by the static strain field calculated on the basis of poroelastic dislocation models; the changes require earthquake-related permeability changes at the monitoring sites. The high sensitivity of some of the water wells to seismic shaking may be due to the near-critical condition of the tapped aquifer/barrier system beginning several years prior to the March 16 earthquake, such that the local seismic shaking caused by each of the above mentioned earthquakes was sufficient to produce some quickly recoverable fissures in the system, resulting in the observed water-level changes. The sensitivity appears to be variable, showing an apparent decrease during a 1-year period after the earthquake.

Tidal responses and earthquake-related changes in the water level of deep wells

Monitoring of the water level has been continued in 100-m-deep and 500-m-deep wells in the southern Kanto District, Japan, to examine the relationship between changes in the well water level and earthquake occurrence. By using a Bayesian approach, the observed data were decomposed into four components: tidal response, barometric response, trend (drift), and irregular (noise) components. Loading effects of the ocean tide on aquifer dilatation were evaluated by using Schwiderskis ocean tide models and the harmonic constants compiled by the International Hydrographic Bureau. Well-aquifer properties were examined based on the tidal and barometric responses of the water level. Coseismic water level steps were detected in both wells for two earthquakes during the observation period from April 1985 to December 1989. By calibrating the strain sensitivity of the well water levels based on the tidal responses, strain changes equivalent to the coseismic steps in the water levels were estimated. The coseismic strain changes obtained for the deeper well can be explained reasonably by earthquake fault models in a viscoelastic half-space. However, those observed in the shallower well were too large and in the wrong direction to be explained by the earthquake fault models.

An anomalous radon decrease in groundwater prior to an M6.0 earthquake: A possible precursor?

A significant decrease in the radon concentration of artesian groundwater was observed at a well-established observation site in Northeast Japan about two days before an M 6.0 earthquake occurred about 200 km from the site. On the basis of a concurrent in situ ({minus}126 m) temperature drop of the groundwater, the authors infer that precursory changes in regional stress were locally amplified near this site and resulted in the temporary mixing of cooler, radon-poor groundwater into the normal artesian flow.

The primordial noble gases in the Earth: a key constraint on Earth evolution models

Abstract We made an extensive compilation of noble gas data obtained from MORB, oceanic island basalts (OIB) and diamonds. On the basis of this data base, we discuss the primordial noble gases in the Earth and its implications on mantle evolution. We conclude that in the contrary to a common assumption, the primordial Ne, Ar, Kr, and Xe in the Earth are distinctly different from the solar noble gases; the primordial noble gases were derived from the solar noble gas through Rayleigh distillation-like, mass-dependent fractionation. The primordial Ar, Kr, Xe are identical with the atmospheric components, but Ne observed in the mantle-derived materials differs significantly from the atmospheric Ne. We suggest that Ne was further fractionated in the atmosphere. While the abundances of Ar, Kr, and Xe in the mantle-derived materials are well correlated to each other, He (and Ne in MORB) is totally uncorrelated with the heavier noble gases. This indicates that He (and Ne in MORB) is decoupled from the heavier noble gases in the mantle source. He–Ne–Ar systematics indicate that He was open to the MORB and OIB mantle sources (and Ne in the MORB mantle source); while He is excess in the MORB mantle source, it is deficient in the OIB mantle source relative to the closed system values. However, it is not known whether this apparent complementary characteristic corresponds to some fundamental mantle dynamics or accidental coincidence.

Precursory and coseismic anomalies in well water levels observed for the February 2, 1992 Tokyo Bay Earthquake

Possible precursor water level changes of a moderate earthquake were detected by the long-term groundwater observation sites operated over 5 years in Japan. Three observation wells showed anomalous changes: rises and a fall in water levels of 3–10 cm which began simultaneously 1–1.5 days prior to the February 2, 1992 Tokyo Bay earthquake (M5.9). The distance from the hypocenter to the wells were 90–110 km. These possible precursors were followed by coseimic water level changes of 1–20 cm. Some portions of rises may be attributed to effects of concurrent precipitation, however, the water level fall of 10 cm observed at one well indicates that the change is anomalous and is highly likely to be an earthquake precursor.

TIME-VARIABLE RESPONSE CHARACTERISTICS OF GROUNDWATER RADON TO EARTHQUAKES

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.

Secular variations in helium isotope ratios in an active volcano: Eruption and plug hypothesis

Abstract Secular variations in 3 He/ 4 He and 4 He/ 20 Ne ratios are reported for October 1986 to April 1991 in gas samples from a 90°C steam well located about 3 km north of Mt. Mihara, an active volcano in Izu-Oshima Island, Japan. The 3 He/ 4 He and 4 He/ 20 Ne ratios increased anomalously from 4.18 R atm to 4.59 R atm and from 0.93 to 1.08, respectively, about 3 months prior to the explosive eruption of the volcano on 4 October 1990. The 3 He/ 4 He enhancement may be related to the concurrent decrease in height of volcanic smoke at the central cone. The gas pressure in the conduit may have significantly increased before the eruption owing to solidified cap lava or ash in the cone, which may have strengthened the transfer of magmatic helium from the source to the observation well through fissures or permeable channels in the volcanic edifice.

Sulfur chemistry in laser-simulated impact vapor clouds: implications for the K/T impact event

Abstract One of the most promising mechanisms for the mass extinction at the K/T boundary event is blockage of sunlight by sulfuric acid aerosol, which is induced by impact vaporization of sulfate in evaporite deposits around the K/T impact site. One of the advantages of this hypotheses is that it may cause an impact winter much longer than that by silicate dust and soot due to a global wildfire. However, the residence time of sulfuric acid aerosol in the stratosphere depends strongly on the ratio of SO 2 /SO 3 in the K/T impact vapor. If SO 3 was dominant, the blockage of sunlight by the sulfuric acid aerosol would not last longer than that by silicate dust and soot. The chemical reaction of sulfur oxides in an impact vapor cloud has not been studied extensively before. This study carries out chemical equilibrium calculations, kinetic model calculations, and laser irradiation experiments with a quadrupole mass spectrometer to estimate the SO 2 /SO 3 ratio in the K/T impact vapor cloud. The results strongly suggest that most of sulfur oxides in the K/T impact vapor cloud may have been SO 3 , not SO 2 . The sulfuric acid aerosol may not have been able to block the sunlight for a long time.

Noble gases in hydrothermal plumes of Loihi Seamount

Abstract Seawater samples from hydrothermal plumes of Loihi Seamount show very high 3 He/ 4 He ratios (up to 15.8 × 10 −6 ) and 4 He/ 20 Ne ratios higher than that of air-saturated seawater (ASSW). The 3 He/ 4 He ratio in the source from which the helium is released into the seawater is estimated as (23.1 ± 2.3) × 10 −6 from the correlation plot of 3 He/ 20 Ne vs. 4 He/ 20 Ne. The 3 He/heat ratio at Loihi is 5–49 times higher than that at the Galapagos Rift and at 21°N EPR. The difference may be attributed to the differences in the structure of the mantle beneath hot spots and the mid-ocean ridge, the former representing the deeper mantle region and the latter the shallower, depleted mantle. These observations are consistent with the view that helium in hydrothermal plumes of Loihi Seamount was derived from a deeper mantle region via a hot spot, where more primordial helium is still preserved.

Mantle helium flux from the bottom of Lake Mashu, Japan

Abstract Most of the 52 water samples from Lake Mashu, Hokkaido, Japan show large anomalies in their3He/4He ratios (up to 4.6 times the atmospheric ratio) and4He/20Ne ratios, indicating continuous injection of mantle He due to hot spring activity at the lake bottom. The3He/4He ratio of the injected mantle He is estimated at3He/4He= 9.43 ± 0.17 × 10−6 , which is close to the highest value reported for volcanic gases from the Japanese Islands. Accumulation of mantle He in the near-bottom water can be reasonably explained by a diffusion process with a constant He flux from the lake bottom off4He = 9.2 × 107 atoms/cm2/s andf3He = 8.7 × 102 atoms/cm2/s. The heat/3He ratio ( 1.7 × 10−7J/atom ) falls in the range of values reported for submarine hydrothermal systems (0.4–26 × 10−7 J/atom), whereas the C3He/ ratio ( 1.8 × 1011 ) is about two orders of magnitude higher than the MORB average ( 2 × 109 ). The mantle He injected from the lake bottom has a fairly short mean residence time in the lake water, and is estimated at about 2–4 years. Overturn of the lake water, which occurs twice a year, probably plays an important role in the rapid transportation of the mantle He from the bottom to the surface.