Hiroshi Wakita

Venting of carbon dioxide-rich fluid and hydrate formation in mid-okinawa trough backarc basin.

Carbon dioxide-rich fluid bubbles, containing approximately 86 percent CO2, 3 percent H2S, and 11 percent residual gas (CH4 + H2), were observed to emerge from the sea floor at 1335- to 1550-m depth in the JADE hydrothermal field, mid-Okinawa Trough. Upon contact with seawater at 3.8�C, gas hydrate immediately formed on the surface of the bubbles and these hydrates coalesced to form pipes standing on the sediments. Chemical composition and carbon, sulfur, and helium isotopic ratios indicate that the CO2-rich fluid was derived from the same magmatic source as dissolved gases in 320�C hydrothermal solution emitted from a nearby black smoker chimney. The CO2-rich fluid phase may be separated by subsurface boiling of hydrothermal solutions or by leaching of CO2-rich fluid inclusion during posteruption interaction between pore water and volcanogenic sediments.

Isotopic composition of helium, and CO2 and CH4 contents in gases produced along the New Zealand part of a convergent plate boundary

New Zealand straddles an active tectonic boundary between the Indo-Australian and Pacific plates. To the NE and SW oblique convergence of oceanic and continental crusts leads to the establishment of subduction zones; in the center continental crusts collide along a transform boundary. With regard to mantle degassing, and on the basis of chemical and He isotopic analyses of 140 samples from all over New Zealand, four major environments can be distinguished: 1. (1) R/RA values approaching MORB values are observed in areas of andesitic and rhyolitic volcanism and high-temperature geothermal activity over the center of the North Island. C3He ratios there vary from 40 × 109, suggesting considerable addition of CO2 from other than mantle sources, in the E. 2. (2) In areas of recent igneous activity, residual mantle He is extracted from rocks through hydrothermal alteration by percolating groundwater. The gases are low in CO2 due to conversion to carbonate; any CH4 present is generated within the crust. C3He ratios are generally well below those of any magmatic vapor. 3. (3) In the absence of recent igneous activity, but in areas of increased seismicity, mantle He may reach the surface along fractures provided by the movement of the subducting slab in forearc regions or by dilatancy pumping or fault-valving in highly compressional, but seismically active parts of an orogenic plate boundary. 4. (4) Mantle degassing is greatly impeded in areas of crustal thickening as indicated by negative gravity anomalies, low seismicity and rapid uplift. There 3He4He ratios approach typical crustal values of <0.1 RA. Except in areas of present-day volcanic and geothermal activity, production, transport, storage, and release processes of 3He, 4He, CO2, and CH4 appear to be effectively decoupled.

Precursory Chemical Changes in Ground Water: Kobe Earthquake, Japan

Chloride (Cl–) and sulfate (SO42–) ion concentrations of ground water issuing from two wells located near the epicenter of the Kobe earthquake in Japan fluctuated before the disastrous magnitude 7.2 event on 17 January 1995. The samples measured were pumped ground water packed in bottles and distributed in the domestic market as drinking water from 1993 to April 1995. Analytical results demonstrate that Cl–and SO42– concentrations increased steadily from August 1994 to just before the earthquake. Water sampled after the earthquake showed much higher Cl– and SO42– concentrations. The precursory changes in chemical composition may reflect the preparation stage of a large earthquake.

Water Wells as Possible Indicators of Tectonic Strain

Coseismic water level changes associated with the Izu-Hanto-oki earthquake of 9 May 1974 were recorded in 59 among 95 observation wells located in the districts of Tokai and Kanto, Japan. The spatial distribution of wells in which the groundwater level rose or fell is rather systematic. The areas in which these wells are located closely coincide with the areas of contraction and dilatation expected by the faulting. This strongly suggests a possible correlation between the observed changes in groundwater level and the tectonic strain. The results may indicate that the water level of wells is able to monitor at least acute coseismic strain changes.

Hydrogen Release: New Indicator of Fault Activity

The hydrogen concentration in soil gas has been measured in the area around the Yamasaki Fault, one of the active faults in southwestern Japan. Degassing of a significant amount of hydrogen (up to more than 3 percent by volume) has been observed for sites along the fault zone. The hydrogen concentration in soil gas at sites away from the fault zone was about 0.5 part per million, almost the same as that found in the atmosphere. The spatial distribution of sites with high hydrogen concentrations is quite systematic. A hypothesis on the production of hydrogen by fault movements is postulated.

Peculiar features of Suiyo Seamount hydrothermal fluids, Izu-Bonin Arc: Differences from subaerial volcanism

Abstract This is the first report of the dissolved gas composition of a deep-sea venting hydrothermal system originating from arc volcanism; the origin of dissolved gases in hydrothermal fluids from the Suiyo seamount in the southern Izu-Bonin (Ogasawara) Arc is discussed on the basis of chemical and isotopic compositions. Twelve high-temperature (up to 310°C) fluid samples were collected from three vents of the hydrothermal system using the submersible Shinkai 2000. The average 3 He 4 He ratio of the dissolved He is 8.1 ( R R atm ), which is within the range of values reported for N-MORB. The CO 2 3 He ratio is 12 × 109, which is within the range of arc volcanic gas values. The δ13C values of CO2 and CH4 are −1 and −8.5‰ respectively, which is quite high compared with other subaerial arc volcanic gases and those from mid-ocean ridges. These results, which do not accord with those from subaerial volcanism, are unlikely to result from fractionation or addition of carbon sources during the fluid circulation, but are regarded as characteristic of the magma source of Suiyo Seamount. A simple mixing of 97% MORB mantle with 3% subducting slab explains well the composition of the helium and carbon in the parent magma.

Helium and carbon geochemistry of hydrothermal fluids from the Mid-Okinawa Trough Back Arc Basin, southwest of Japan

Abstract Hydrothermal systems located in the Okinawa Trough Back Arc Basin are considered to be related to rifting activities in the continental margin. Hydrothermal fluids sampled from two sites (JADE and CLAM) are significantly enriched in gas species compared to those from mid-oceanic ridges. A high-temperature fluid (320°C) from the JADE site shows a CO 2 concentration of 200 mmol kg −1 , which is more than one order of magnitude higher than that of sediment-starved ridge crest hydrothermal systems. Both helium R R A = 6.1 −6.5 and carbon ( δ 13 C CO2 = −5.0 to −4.7‰ PDB) isotopic compositions are typical of island arc magmatic gases. Also the abundance ratio of CO 2 / 3 He (= 3·10 10 ) is within the range of values found in island arcs. Chemical and isotopic composition of the JADE fluid are considered to indicate that both CO 2 and helium are derived from a magmatic source beneath the hydrothermal system without significant contamination. Contrary to these species, CH 4 is attributed to a thermogenic origin based on its isotopic composition δ 13 C (CH 4 ) = −41 to −36% . PDB) and high concentration ( = 7.1 mmol kg −1 ), which values are similar to those observed in sediment-hosted hydrothermal systems. The CLAM fluids show gas geochemistry much more affected by fluid-sediment interaction. Lower 3 He/ 4 He ratios R R A =3.7–3.8 ) and higher CO 2 3 He ratios ( >10 11 ) may reflect involvement of gas species introduced during secondary fluid circulation within the sedimentary pile. Magmatic activity is the main contributor of gas species to the hydrothermal fluids in both the JADE and CLAM sites, although fluid-sediment interaction modifies their composition. The relatively high abundance of CO 2 in the Mid-Okinawa Trough hydrothermal systems reflects the geochemical nature of island arc magma.

Short-term and intermediate-term geochemical precursors

Detection of precursory phenomena in observation data is essential to earthquake prediction studies. Continuous monitoring of radon concentration in groundwater in Japan in one case showed a short-term anomaly related to a nearby earthquake. With the exception of the 1978 Izu-Oshimakinkai earthquake (M7.0), however, no abnormal change has been noted. This may be due partially to difficulty in detecting insignificant precursory signals from observation data, which ordinarily contains a noise-induced fluctuations, and partially to lack of understanding of the mechanism controlling the appearance of precursory phenomena. In order to increase our knowledge of the variation pattern of precursory changes in radon concentration of groundwater, hydrologic precursors with significant features are examined in this paper. Complexity of appearance of precursory phenomena and problems in assignment of the specific earthquake are discussed.

Diffuse emission of carbon dioxide, methane, and helium‐3 from Teide Volcano, Tenerife, Canary Islands

Diffuse emission of CO2, CH4 and ³He was investigated in the summit crater of Teide volcano, Tenerife, Canary Islands. The results indicate that Teide volcano releases abundant CO2 not only from its active crater, but also from its flanks as diffuse soil emanations. The spatial distribution of these emanations correlates quite closely with that of geothermal anomalies and manifestations. Our flux measurements indicate a total output of 380 ton·day−1 of CO2 and 1.6 on·day−1 of CH4 by diffuse degassing. From the ³He/CO2 ratios and the CO2 flux we estimate a diffuse ³He flux of 0.052 mol·year−1. Isotopic analysis of carbon in soil gas samples from sites of high soil CO2 flux indicates a magmatic origin for the CO2, also reported by high ³He/ 4He ratios of 5.50 and 5.87 Ra. The origin of methane is more uncertain with low δ13C values suggesting a biogenic contribution.

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.