高圧下における地殻・上部マントル岩石の弾性波速度異方性

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Seismic velocity anisotropy is often observed in crustal and mantle rocks. The velocity anisotropy that is caused by preferred orientation of rock-forming minerals has signiflcant implications on the structure and the deformation process in the crust and the upper mantle. In this study, we have developed the method for determining velocity anisotropy in rocks at pressures to 1.3GPa by using a piston-cylinder high-pressure apparatus. Our new method is simple and suitable to measure rock anisotropy, compared with the method using gas or liquid pressure medium. The method allows us to use large samples (〜30 mm long) at high pressure, and to employ both pulse transmission and reflection techniques to determine accurate travel times in the sample. By using a digital oscilloscope of fast sampling rate (500 MHz), accuracy in the velocity measurement carried out at 0.4 to 1.3 GPa is ±0.7%. The method is applied to determine P-wave velocities along two orthogonal directions of crustal and mantle rocks; the directions parallel to the lineation and normal to the foliation. Velocity differences in the two directions amount to 0.5-0.7 km/s for strongly foliated peridotites, but they are small (0〜O,1km/s) for weakly foliated gabbros. Obtained P-wave velocity anisotropy in peridotite (up to 9%) is consistent with previously reported values, and is mostly caused by the preferred orientation of olivine. Small velocity anisotropy in gabbro (less than 2%) is consistent with the values reported for rocks mainly composed of plagioclase and pyroxene.
特定非営利活動法人日本火山学会の論文
1997-04-25

高圧下における地殻・上部マントル岩石の弾性波速度異方性

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