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Ogawa, Hiroki; Hiratsuka, Shinya; Asamori, Koichi; Shimada, Koji; Niwa, Masakazu
Butsuri Tansa, 77, p.15 - 23, 2024/00
Understanding pathways of volcanic fluids or non-volcanic slab-related fluids by means of investigations from the surface is effective for preventing the loss of the function as a natural barrier in the geological disposal system of high-level radioactive waste. There are few long active faults and is no record of remarkable groundwater discharge in the fore-arc region of Kyushu, Japan. However, previous studies on seismic wave velocity and resistivity structures suggest the existence of the zones of slab-related fluids derived from the Philippine Sea Plate in the crust. To associate pathways of fluids in the fore-arc region of Kyushu with distribution and properties of cracks, we applied a shear wave splitting analysis to waveform data of shallow earthquakes in this region. In the inland part of Kyushu, the orientation of the faster polarized shear wave is subparallel to the axes of the maximum horizontal compressional stress, which can attribute the shear wave polarization anisotropy mainly to the distributions of cracks aligned along the crustal stress. The faster polarized shear wave at the seismic stations in the coastal area of Hyuganada Sea shows the orientation of NNE-SSW
NE-SW or NNW-SSENW-SE different from that of the crustal stress. The average intensity of anisotropy over the ray-path length from hypocenters to each seismic station is also calculated. As a result, a few ray paths acquired at the seismic station TAKAZA to the east of Kirishima Volcano show larger anisotropic intensity, 5.6-7.0%. There is no denying the possibility that these ray paths reflect the pathways of the hydrothermal fluids. In the coastal area of Hyuganada Sea, however, it is confirmed that we do not obtain the ray paths that indicate continuous fluid pathways extending from hypocenters to each seismic station because the anisotropic intensity over the ray-path length is less than 5%.
Hiratsuka, Shinya; Asamori, Koichi; Saiga, Atsushi
JAEA-Research 2022-002, 38 Pages, 2022/06
JAEA-Research-2022-002.pdf:4.49MB
Deep groundwater originates from dehydration of Pacific and Philippine Sea slab subducting beneath Japanese islands, which has characteristics of high temperature and is rich in carbonate species. In this respect, it is very important for geological disposal of high-level radioactive waste to estimate reservoir and migration pathway of deep groundwater. The region where cracks are densely distributed can be regarded as the migration pathway of slab-derived fluid. It is highly probable that the region has strong anisotropy. Shear wave propagating through anisotropic media splits into two mutually orthogonally polarized waves due to shear wave polarization anisotropy. In this report, we applied shear wave splitting analysis to Hongu area of Tanabe City, Wakayama Prefecture and estimated the spatial distribution of leading shear wave polarization direction (LSPD) and arrival time difference between leading and lagging shear waves (dt). Based on comparison with helium isotope ratio of ground water and bubbling gas samples and two-dimensional resistivity structure estimated by previous study, we attempt to estimate migration pathway of slab-derived fluid in Hongu area of Tanabe City, Wakayama Prefecture. The main results are summarized as follows. When helium isotope ratio of groundwater and bubbling gas samples is high, dt value tends to be large. Shear wave propagating through high and low resistivity anomaly zone show small and large dt values, respectively. Previous study suggested that slab-derived fluid migrates from deeper part of western side of Hongu area and wells out in Yunomine and Kawayu hot springs. This is consistent with spatial distribution of dt values estimated by this study.
Niwa, Masakazu; Shimada, Koji; Watanabe, Tsuyoshi; Goto, Akira; Hiratsuka, Shinya
no journal, ,
As a case study to visualize crustal deformation induced by concealed faults and related structures, we applied integrated approaches of geodesy, geomorphology, and geology to a high-strain shear zone in southern Kyushu, Japan. Our GNSS observation for recent 4.7 years by a network composed of 10 stations revealed that locking depth and slip velocity of the deeper extension of the fault were estimated to be 8 km and left-lateral slip of 9 mm/year. In the geomorphological approach, concentration of E-W trend lineaments is identified in northwestern Kagoshima, near the center of the sinistral high-strain shear zone detected by the GNSS observation. The field survey showed that minor faults are common in the study area. Thus, we analyzed stress field estimated from slip data of the minor faults, and compared it with the stress field corresponding to the sinistral high-strain shear zone. The result of the analysis suggests that the subsurface deformation due to the sinistral high-strain shear zone is localized rather than dispersed.
Watanabe, Tsuyoshi; Hiratsuka, Shinya*; Shimada, Koji
no journal, ,
Fluid has a significant influence on the seismogenic process because it reduces the fracture strength of rocks. Fluid migration also affects the long-term safe confinement function of a geological disposal systems. To clarify the existence and the migration channels of such fluid, shear wave splitting analysis is a useful method, which can estimate the anisotropy related to cracks and fluid in the crust. We conducted the shear wave splitting analysis in Iwaki area, northeast Japan, where seismic activity was increased and anomalous discharge of hot springs was observed about one month after the 2011 Tohoku Earthquake (Mw 9.0). As a result, we showed that the crack orientations turn to generally arc-parallel direction in Iwaki area, and additionally, that the spatial distribution of anisotropic intensity is relatively high at a seismic site near the hot springs with anomalous discharge. This suggests that fluid from deep underground gather in the crust in steady-state.
Hiratsuka, Shinya; Saiga, Atsushi; Asamori, Koichi
no journal, ,
Hydrothermal activity is generated in the Kii Peninsula although it is a non-volcanic region. High helium isotope ratio, low seismic velocity and low resistivity suggest that hydrothermal water is originated from deep fluid. This study attempt to estimate the migration process of deep fluid using seismological method so-called S-wave splitting analysis. The result shows that high anisotropic region may correspond to migration process of deep fluid, but it is necessary for more accumulation of seismological data.
Watanabe, Tsuyoshi; Hiratsuka, Shinya*; Shimada, Koji
no journal, ,
The crustal fluid is one of the important factors in considering the seismogenic process because it reduces the fracture strength of rocks. To clarify the existence and the migration channels of such fluid will greatly contribute to disaster prevention, mitigation, and safety geological disposal. In this study, we conducted shear wave splitting analysis in the region from Fukushima to Ibaraki Prefectures to investigate the crustal fluid. As a result, we showed the spatio-temporal distributions of the crack orientations and anisotropic intensity. Then, we succeeded in capturing how crustal fluid propagated from deep to shallow areas along active faults or structures.
Ogawa, Hiroki; Hiratsuka, Shinya*; Asamori, Koichi; Shimada, Koji
no journal, ,
One of the issues related to the assessment of geosphere stability for long-term isolation of HLW is the crucial effect of geofluids such as slab-related fluid on the disposal site. As the fore-arc region of Kyushu Island, Japan, is related to subduction of the Philippine Sea Plate, it is believed that the Miyazaki Plain and its neighboring areas have been influenced by slab-related fluid. With a view to capturing pathways of slab-related deep-seated fluid by means of subsurface seismic anisotropy, we performed a shear wave splitting analysis by utilizing seismic waveform data recorded at stations located throughout Miyazaki prefecture. It reveals that many of the stations show the orientation of the seismic anisotropy parallel to the axes of the maximum horizontal compressional stress, while several stations in the Miyazaki Plain show the orthogonal orientation to that of these stations. Regarding the strength of the seismic anisotropy, relatively large values are partially distributed in the southern part of the Miyazaki Plain and the area above the low S-wave velocity anomaly estimated in the previous study.
Hiratsuka, Shinya; Asamori, Koichi; Saiga, Atsushi
no journal, ,
no abstracts in English
