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Journal Articles

Triggering of earthquake swarms following the 2011 Tohoku megathrust earthquake

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:2 Percentile:12.15(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 $$^{3}$$He/$$^{4}$$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.

Journal Articles

Earthquake doublet in an active shear zone, southwest Japan; Constraints from geophysical and geochemical findings

Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo

Tectonophysics, 634, p.116 - 126, 2014/11

 Times Cited Count:3 Percentile:11.27(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.

JAEA Reports

Research plan on geosphere stability for long-term isolation of radioactive waste (Scientific programme for fiscal year 2014)

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.

JAEA Reports

Annual report for research on geosphere stability for long-term isolation of radioactive waste in fiscal years 2012

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.

JAEA Reports

Research plan on geosphere stability for long-term isolation of radioactive waste (Scientific programme for fiscal year 2013)

Yasue, Kenichi; Asamori, Koichi; Niwa, Masakazu; Hanamuro, Takahiro; Kokubu, Yoko; Makuuchi, Ayumu; Ikuta, Masafumi; Matsubara, Akihiro; Ishimaru, Tsuneari; Umeda, Koji

JAEA-Review 2013-023, 42 Pages, 2013/10

JAEA-Review-2013-023.pdf:2.82MB

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 2013. The objectives and contents in fiscal year 2013 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.

JAEA Reports

Annual report for research on geosphere stability for long-term isolation of radioactive waste in fiscal years 2011

Asamori, Koichi; Niwa, Masakazu; Hanamuro, Takahiro; Yamada, Kunimi; Kusano, Tomohiro; Makuuchi, Ayumu; Takatori, Ryoichi; Kokubu, Yoko; Matsubara, Akihiro; Ishimaru, Tsuneari; et al.

JAEA-Research 2012-024, 132 Pages, 2012/09

JAEA-Research-2012-024.pdf:15.68MB

This annual report documents the progress of R&D in the 2nd 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 planed 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.

JAEA Reports

Research plan on geosphere stability for long-term isolation of radioactive waste; Scientific programme for fiscal years 2012

Asamori, Koichi; Niwa, Masakazu; Hanamuro, Takahiro; Yamada, Kunimi; Kusano, Tomohiro; Makuuchi, Ayumu; Takatori, Ryoichi; Kokubu, Yoko; Ishimaru, Tsuneari; Umeda, Koji

JAEA-Review 2012-024, 44 Pages, 2012/07

JAEA-Review-2012-024.pdf:1.29MB

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 2012. The objectives and contents in fiscal year 2012 are described in detail based on the outline of 5 years plan (fiscal years 2010-2014). In addition, the planed 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.

JAEA Reports

High density electric survey around the Horonobe Underground Research Laboratory, 3

Nakamura, Takahiro; Sanada, Hiroyuki; Sugita, Yutaka; Teshima, Minoru*; Kasagi, Toshio*; Kishimoto, Munemaru*; Deguchi, Tomonori*; Makuuchi, Ayumu*

JAEA-Research 2010-006, 68 Pages, 2010/06

JAEA-Research-2010-006.pdf:33.19MB

The present document is to report the result of geophysical survey by electric survey around a site proposed for the Horonobe Underground Research Laboratory Project at Horonobe-cho, Hokkaido by the Japan Atomic Energy Agency. This Electric survey using pole-pole array in Horonobe Underground Research Laboratory Area was the third survey. Resistivity distribution near surface was about 20 $$Omega$$$$cdot$$m and it become to about 2 $$Omega$$$$cdot$$m as the depth increasing. Resistivity distribution was good agreement with existing survey results and existing electrical resistivity log data. By comparing resistivity model with hydrology model, it was confirmed that resistivity reflected the density of salinity. The range and distribution trends of apparent resistivity in this year were similar to the results of last year. It is thought that the data in which reproducibility is high is acquired in this survey. The influence on the underground water flow by construction is not admitted now.

JAEA Reports

High density electric survey around the Horonobe Underground Research Laboratory

Matsuo, Koichi*; Kishimoto, Munemaru*; Makuuchi, Ayumu*; Teshima, Minoru*; Sanada, Hiroyuki; Niunoya, Sumio*; Matsui, Hiroya

JAEA-Research 2008-071, 44 Pages, 2008/09

JAEA-Research-2008-071.pdf:29.04MB

Correlation between resistivity in rock mass and groundwater quality is shown from the results of investigation such as borings and geophysical exploration. Electrical resistivity survey had been done to understand impact on behavior of groundwater by construction of underground research laboratory. This paper shows distribution of resistivity around underground research laboratory obtained from electrical resistivity survey.

Oral presentation

Three-dimensional magnetotelluric imaging of the 1997 Kagoshima earthquake doublet, Southwest Japan

Asamori, Koichi; Makuuchi, Ayumu; Umeda, Koji

no journal, , 

Two shallow moderate earthquakes occurred in the northwestern part of Kagoshima province, on March 26 (Mw 6.1) and on May 13 (Mw 6.0) in 1997. In this study, we conduct magnetotelluric (MT) soundings in and around the source region of the 1997 Kagoshima earthquake sequence and perform a 3-D inversion of wideband MT data above a depth of 30 km. The obtained 3-D resistivity model acquired through the inversion shows as follows. (1) Two anomalous conductive bodies are clearly visible in the upper and lower crust beneath the seismic source region, and seem to be combined and extend down to the uppermost mantle. (2) These conductive bodies are located just beside the two mainshocks in the upper crust. (3) These results suggest that the generation of two large earthquakes is not a pure mechanical process, but is closely related to heterogeneities in the material property and stress field due to the relatively effective transfer of mantle fluids from the upper mantle.

Oral presentation

A Data processing technique of removing DC railway noise from time domain magnetotelluric data

Makuuchi, Ayumu; Asamori, Koichi; Negi, Tateyuki*

no journal, , 

Even though applying the far remote reference magnetotelluric (MT) method, we need long recording period to obtain usable data from the contaminated data by strong and coherent noise in DC railway area. In the recorded MT time series sampled continuously at 15Hz, the noises from DC railway are recorded as square waves in electric time series and as pulses in magnetic time series measured by induction coil. In this study, we attempt to identify and delete the short time windows which are contaminated by the DC railway noises in magnetic time series automatically, and then we attempt to interpolate these windows by plausible data which are calculated as a response of reference magnetic time series. MT sounding curves obtained by above processing are improved over curves obtained from raw data.

Oral presentation

An Active shear zone, southwest Japan; Electromagnetic geophysics and noble gas geochemistry

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.

Oral presentation

A Data processing technique of removing DC railway noise from time domain magnetotelluric data, 2

Makuuchi, Ayumu; Asamori, Koichi; Negi, Tateyuki*

no journal, , 

Even though applying the far remote reference magnetotelluric (MT) method, we need long recording period to obtain usable data from the contaminated data by strong and coherent noise in DC railway area. In this study, we consider the electric time series model including a trend component, natural magnetic signal response, correlated noise components, and white noise, then attempt to separate to each component with a Kalman filter algorithm. The method was applied to the magnetotelluric data observed near the DC railway and seems to work well in the time domain.

Oral presentation

The Influence of the noise included in far remote reference site on the magnetotelluric survey, and the noise reduction method, 2; A Validation study using survey data

Hama, Yuki*; Makuuchi, Ayumu; Negi, Tateyuki*; Asamori, Koichi

no journal, , 

The remote reference technique is widely used in the magnetotelluric method to decrease local noise by acquiring data simultaneously at a remote site. This technique can reduce the local noise component and extract the magnetotelluric signal by stacking the cross power spectrum between the remote data and field data. Remote reference is especially efficient when applying ideal remote data with no noise component. Yet it is usually difficult in practical surveys. The best way to compensate in such a difficulty is to select multiple remote sites. With multiple remote sites we can expect the same effect obtainable from an ideal remote site without noise. In this paper we applied multiple remote reference method using robust weighted stacking to noisy survey data, in order to redact the noise component included in the remote site which causes the escalation of data scattering.

Oral presentation

Geophysical and geochemical signatures associated with mantle fluids beneath an active shear zone, southwest Japan

Shimizu, Mayuko; Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Sueoka, Shigeru; Tamura, Hajimu

no journal, , 

In 1997, the Kagoshima earthquake doublet, consisting of two closely associated Mw 6 strike-slip events, five km and 48 days apart, has occurred in southwest Japan. The location is where an E-W trending discontinuity along 32N latitude on southern Kyushu Island is clearly defined in GPS velocities, indicating the presence of a highly active left-lateral shear zone. However, there have not been any obvious indications of active faulting at the surface prior to the earthquake doublet, which could be associated with this shear zone. Three-dimensional inversion of magnetotelluric sounding data obtained in the source region of the earthquake doublet reveals a near-vertical conductive zone with a width of 20 km, extending down to the base of the crust and perhaps into the upper mantle toward the Okinawa trough. The prominent conductor corresponds to the western part of the active shear zone. Elevated $$^{3}$$He/$$^{4}$$He ratios in groundwaters sampled from hot spring and drinking water wells suggest the emission of mantle-derived helium from the seismic source region. 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.

Oral presentation

Three-dimensional magnetotelluric inversion regarding distortion tensors as unknown parameters

Negi, Tateyuki*; Makuuchi, Ayumu*; Hama, Yuki; Asamori, Koichi

no journal, , 

In magnetotelluric (MT) soundings, Galvanic distortions, which might disturb the reliability of the resistivity model estimated by inversion analyses, are often recognized. In order to solve this problem, we developed a new three-dimensional MT inversion method that can estimate both resistivity model and distortion tensors without prior information. Using synthetic examples of data under distorted and noisy conditions, we demonstrated that the method can estimate both resistivity model and distortion tensors, and that the method could be expected to enhance the reliability of the resistivity model estimated by the MT inversion.

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