Miyakawa, Kazuya; Tokiwa, Tetsuya; Murakami, Hiroaki
Geochemistry, Geophysics, Geosystems (Internet), 14(12), p.4980 - 4988, 2013/12
The origin of mud from the mud-volcano-like sediments in the Horonobe area of northern Hokkaido, Japan was characterized by analyzing the mineral and chemical compositions of the mud. Mineral and chemical compositions of the mud are similar to those of the Hakobuchi Formation located at approximately 1500-2500 m depth. The results suggest that the mud originates from the Hakobuchi Formation, and also that the mud ascended from a depth of 1500-2500 m to ground surface. This result could be an exemplification of the occurrence of mud volcanism in oil and gas field and tectonic compressional region. This is useful information that should be taken into consideration in the planning and construction of underground facilities for geological disposal of HLW.
Niwa, Masakazu; Takeuchi, Ryuji; Onoe, Hironori; Tsuyuguchi, Koji; Asamori, Koichi; Umeda, Koji; Sugihara, Kozo
Geochemistry, Geophysics, Geosystems (Internet), 13(5), p.Q05020_1 - Q05020_15, 2012/05
In the 2011 off the Pacific coast of Tohoku Earthquake, groundwater pressure changes were observed in and around the Mizunami Underground Research Laboratory (MIU). Coseismic changes of groundwater pressure are believed to correspond to crustal dilation/contraction induced by earthquakes. In this study we calculated volumetric strain changes due to the Tohoku Earthquake. The calculation indicates 210 of dilational strain around the MIU. The dilation corresponds to drawdowns of several tens of centimeters or more, and is almost the same as the drawdown observed in the boreholes at distances greater than 1 km from the MIU. In contrast, rapid elevation of groundwater pressures was observed in the boreholes within the 500 m vicinity of the MIU. The anomalous elevation is explained by a temporary recovery of the drawdown due to excavation of the shafts and a unique permeability increase induced by the coseismic dilation of impervious faults.
Umeda, Koji; Ninomiya, Atsushi
Geochemistry, Geophysics, Geosystems (Internet), 10(8), p.Q08010_1 - Q08010_10, 2009/08
A magnitude () 7.3 crustal earthquake occurred in western Tottori Prefecture, southwest Japan, on 6 October 2000. However, there was no indication of an earthquake surface fault corresponding to the 2000 earthquake. In order to elucidate the geographic distribution of the He/He ratios around the seismic source region, new helium isotope data for groundwater samples from drinking water wells were acquired. The maximum He/He ratio was observed from the well located nearest to the epicenter of the main shock. In addition, there appears to be a clear trend of decreasing He/He ratios with distance away from the main trace of the estimated fault segments. The observations provide significant evidence that the source fault of the 2000 earthquake is associated with leakage of mantle volatiles through the crust to the Earth's surface. We suggest that helium isotopes can be regarded as a tool for investigating concealed active faults with no surface expression.
Umeda, Koji; Ninomiya, Atsushi; McCrank, G. F.*
Geochemistry, Geophysics, Geosystems (Internet), 9(12), p.Q12003_1 - Q12003_7, 2008/12
Recently, two shallow MW 6.6 inland earthquakes, the 2004 mid-Niigata Prefecture Earthquake and the 2007 Niigataken Chuetsu-oki Earthquake occurred in the Niigata sedimentary basin, near the boundary between the Eurasian and the North American Plate. In order to elucidate the geographic distribution of He/He ratios in and around the seismic source regions, new helium isotope data from hot spring gases and water samples with a much denser sampling were determined. Higher He/He ratios were observed in a zone along 37.5N latitude including the source regions than in other regions in the Niigata sedimentary basin. Moreover, the He/He ratios are much higher than the 3.8 RA value, which is the expected He/He ratio at the present time for volcaniclastics of Middle Miocene age. Previous geophysical models of the mechanism for initiation of the 2004 mid-Niigata Prefecture Earthquake proposed that enhanced pore-fluid pressure around the fault system, due to infiltration of aqueous fluids derived from the subducting Pacific Plate slab into the lower crust, reduces frictional strength and triggered the main shock rupture. Geochemical evidence for emergence of hot spring and natural gases with anomalously high He/He ratios also indicates that upward migration of aqueous fluids with mantle volatiles from the subcrustal lithosphere to the Earth's surface could be attributed to the hydraulic gradient along the high permeability of the rupture zone.
Umeda, Koji; Asamori, Koichi; Negi, Tateyuki*; Ogawa, Yasuo*
Geochemistry, Geophysics, Geosystems (Internet), 7(8), p.Q08005_1 - Q08005_8, 2006/08
Mesozoic crystalline mountains (Iide Mountains) in a non-volcanic region of the Northeast Japan arc were imaged by wide-band magnetotelluric soundings. A two-dimensional model shows that an anomalous conductive body is clearly visible beneath the Iide Mountains. The conductor widens with increasing depth, and extends from the near-surface down to the base of the crust and perhaps into the upper mantle. Considering several independent indications of the possibility of crustal magma storage, such as a high-temperature regime, hot spring gases with high He/He ratios, thinning of the brittle seismogenic layer and anomalous low seismic velocity, it is reasonable to suppose that the conductor reflects partial melt and/or high-temperature aqueous fluids in the crust, related to renewed magmatism in the present-day subduction system.
Umeda, Koji; Kanazawa, Sunao; Kakuta, Chifumi*; Asamori, Koichi; Oikawa, Teruki
Geochemistry, Geophysics, Geosystems (Internet), 7(4), p.Q04009_1 - Q04009_11, 2006/04
New helium isotopic data from hot springs on Shikoku Island with much denser sampling than previous studies were obtained, in order to elucidate the geographical distribution of He/He ratios in the fore-arc regions of Southwest Japan. Systematic, regional variations in He/He ratios were observed on both Sikoku Island and the Kii Peninsula. Most of the hot springs having He/He ratios in the range 1.0 RA to 2.0 RA were determined throughout Shikoku Island. On the other hand, the majority of hot springs in the Kii Peninsula have He/He ratios higher than 2.0 RA. It has been suggested by some authors that the anomalously high He/He ratios in the fore-arc region of Southwest Japan are related to the migration of mantle-derived helium with aqueous fluids generated by dehydration of the subducting PHS and mixing with terrestrial waters [e.g., Matsumoto et al., 2003]. According to recent seismological observations of crustal microearthquakes and their focal mechanisms, the geotectonic environment beneath Shikoku Island is subject to a stress field without normal faulting and the extremely low crustal seismicity, which does not favor migration of aqueous fluids from the subcrustal lithosphere, resulting in a relatively small influence of MORB-type helium at surface. As a consequence, the regional variation in He/He ratios of hot springs on Shikoku Island are interpreted as a three-component mixture of mantle-derived helium associated with magmatism of Middle Miocene of age ( 3.4 RA), crustal, radiogenic helium and atmospheric helium.