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Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo; Hama, Yuki*
Journal of Geophysical Research; Solid Earth, 120(4), p.2279 - 2291, 2015/04
Times Cited Count：3 Percentile：12.07(Geochemistry & Geophysics)Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow normal-faulting swarm sequence occurred near the Pacific coast in the southeast Tohoku district. The observed 
He/
He ratios are significantly lower than the atmospheric value, indicating mantle helium contributed less than 10%. The plausible source of fluids can be attributed to waters released owing to sediment porosity collapse, and smectite-illite and opal-quartz reactions in the subducting sediments, rather than dehydration reactions of the subducting altered basalts and/or hydrated mantle. The aqueous fluids driven off the subducting slab migrate into the fore-arc crust, because of the pressure gradient between lithostatic pore pressure along the plate interface and hydrostatic pore pressure in the overriding crust. The swarm earthquake sequence would have been triggered by stress change associated with the Tohoku-Oki earthquake, enhanced by fluid flow along inherited weakened zones in the crust.
Yasue, Kenichi; Ishimaru, Tsuneari; Kobori, Kazuo; Umeda, Koji; Nakatsuka, Noboru
Chishitsugaku Zasshi, 120(12), p.XIII - XIV, 2014/12
From the viewpoint of present and future use of underground space such as the geological disposal of radioactive waste and carbon dioxide, this is a need to understand subsurface geological structures in the Japanese islands. In general, conventional methods of subsurface mapping are widely used to convert subsurface geological information into two-dimensional space. We present subsurface geological structures on horizontal-section maps at depths of 0, 500, and 1,000 meters below sea level. We also map surface geological units consisting of pre-Neogene basement rocks. Data from existing geological maps, geological cross sections and boreholes were used as input to the maps. The data has been gathered from publications prior to 2001.
Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo
Tectonophysics, 634, p.116 - 126, 2014/11
Times Cited Count：4 Percentile：11.18(Geochemistry & Geophysics)The geophysical and geochemical observations are significant indications that the invasion of mantle fluids into the crust, driven by upwelling asthenosphere from the Okinawa trough, triggers the notable left-lateral shearing in the zone in the present-day subduction system. In addition, the existence of aqueous fluids in and below the seismogenic layer could change the strength of the zones, and alter the local stress regime, resulting in the occurrence of the 1997 earthquake doublet.
Yasue, Kenichi; Asamori, Koichi; Niwa, Masakazu; Kokubu, Yoko; Kobori, Kazuo; Makuuchi, Ayumu; Matsubara, Akihiro; Shibata, Kenji; Tamura, Hajimu; Tanabe, Hiroaki; et al.
JAEA-Review 2014-033, 43 Pages, 2014/09
JAEA-Review-2014-033.pdf:16.91MB
The concept of geological disposal of HLW in Japan is based on a multi-barrier system which combines a stable geological environment with a robust barrier system. Potential geological host formations and their surroundings are chosen, in particular, for their long-term stability, taking into account the fact that Japan is located in a tectonically active zone. This report is a plan of research and development (R&D) on geosphere stability for long-term isolation of HLW in JAEA, in fiscal year 2014. The objectives and contents in fiscal year 2014 are described in detail based on the outline of 5 years plan (fiscal years 2010-2014). In addition, the planned framework is structured into the following categories: (1) Development and systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques.
Yasue, Kenichi; Asamori, Koichi; Niwa, Masakazu; Hanamuro, Takahiro; Kokubu, Yoko; Sueoka, Shigeru; Makuuchi, Ayumu; Ikuta, Masafumi; Matsubara, Akihiro; Tamura, Hajimu; et al.
JAEA-Research 2013-047, 109 Pages, 2014/03
JAEA-Research-2013-047.pdf:6.81MB
This annual report documents the progress of R&D in the 3rd fiscal year during the JAEA 2nd Midterm Plan (FY 2010 - 2014) to provide the scientific base for assessing geosphere stability for long-term isolation of the high-level radioactive waste. The planned framework is structured into the following categories: (1) development and systematization of investigation techniques for selecting suitable sites in geosphere stability, (2) development, application and verification of prediction models for evaluating the changes of geological environment in thermal, hydraulic, mechanical and geochemical conditions for a long period of time, and (3) development of new dating techniques for providing information about geologic history and the timing of geologic events. In this paper, the current status of R&D activities with previous scientific and technological progress is summarized.
Umeda, Koji; Asamori, Koichi; Kobori, Kazuo
no journal, ,
Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow normal-faulting swarm sequence occurred near the Pacific coast in the southeast Tohoku district. In order to provide geochemical constraints on the source of the fluids triggering the swarm activity, new helium isotope data were acquired from gas and water samples around the seismic source region. The observed He/He ratios in these samples are significantly lower than the atmospheric value of 1.4
10
, indicating that mantle helium contributed less than 10% of the total helium. Plausible sources of the fluids can be attributed to waters released from sediment porosity collapse and from smectite-illite and opal-quartz reactions in the subducting sediments, rather than dehydration reactions in subducting, altered basalts and/or hydrated mantle. The swarm earthquake sequence would have been triggered by stress changes associated with the Tohoku-Oki earthquake, enhanced by fluid flow along inherited weakened zones in the crust.
Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo
no journal, ,
An E-W trending discontinuity along 32 N latitude in GPS velocities across southern Kyushu Island is clearly defined, indicating a highly active left-lateral shear zone. However, there was no obvious prefaulting indication at surface in relation to the shear zone. The geophysical and geochemical observations provide significant evidence that the invasion of mantle fluids into the crust, driven by upwelling asthenosphere from the Okinawa trough, triggers off the notable left-lateral shear zone in the present-day subduction system.
Ishimaru, Tsuneari; Kobori, Kazuo; Umeda, Koji; Nakatsuka, Noboru
no journal, ,
no abstracts in English
Umeda, Koji; Asamori, Koichi; Kobori, Kazuo; Hama, Yuki
no journal, ,
Earthquake swarms, often interpreted to result from fluids invading the brittle seismogenic zone, have seismicity patterns that are significantly different from an aftershock sequence. Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow normal-faulting swarm sequence occurred near the Pacific coast in the southeastern Tohoku district. An integrated approach combining geophysical and geochemical methods was utilized to establish the presence of aqueous fluids around the seismic source region and their derivation.
