BUTSURI-TANSA(Geophysical Exploration)
Online ISSN : 1881-4824
Print ISSN : 0912-7984
ISSN-L : 0912-7984
Rapid Letter
Estimation of shear wave polarization anisotropy in the fore-arc region of Kyushu, Japan
Hiroki OgawaShinya HiratsukaKoichi AsamoriKoji ShimadaMasakazu Niwa
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2024 Volume 77 Pages 15-23

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Abstract

 Understanding pathways of volcanic hydrothermal 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 (HLW). There are few long active faults and is no record of remarkable groundwater discharge in the Miyazaki Plain and its adjacent areas located 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. Moreover, it is reported that the partial distribution of groundwater indicates the influence of upwelling slab-related fluids. To associate pathways of such crustal 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 earthquakes at depths shallower than 20 km in this region. In the inland part of Kyushu, the orientation of the faster polarized shear wave (φ) is subparallel to the axes of the maximum horizontal compressional stress derived from focal mechanism solutions, 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-SSE~NW-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%.

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© 2024 The Society of Exploration Geophysicists of Japan
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