[S-CG42] ハードロック掘削科学：陸上・深海底掘削、そしてオフィオライト

[SCG42-01] Seismic anisotropy in the Horoman peridotites calculated from crystal-fabrics of olivine, orthopyroxene, and clinopyroxene

*松山 和樹¹、道林 克禎¹ (1.名古屋大学大学院 環境学研究科 （岩鉱）)

キーワード：カンラン岩、結晶方位定向配列、幌満カンラン岩体

In this study, we calculated the P-wave and S-wave velocities and anisotropies using bulk elastic tensors from the measured crystallographic preferred orientations (CPOs) of olivine, orthopyroxene, and clinopyroxene in Horoman peridotites, Hokkaido (Matsuyama and Michibayashi 2023). Three peridotites were used to present the deformation structures and CPOs of Horoman peridotites. The calculations were conducted for a range of possible modal percentages of minerals: 50–100 % olivine, 0–50 % orthopyroxene, and 0–50 % clinopyroxene. We used a Vp-Flinn diagram (Michibayashi et al. 2016) to explore the relative influence of the three minerals on the seismic properties (e.g. anisotropy, propagation pattern, and Vp/Vs ratio) of the peridotites. As a result, the bulk P-wave velocities and S-wave velocities were 7.85–9.15 km/s and 4.79–5.2 km/s, respectively. The azimuthal anisotropies of the P-wave velocity ranged from 2.89–15.2%, whereas the maximum polarization anisotropies of the S-wave ranged from 2.48–9.99%. VpVs ratios were 1.58–1.75. In Horoman peridotites, increasing the amount of orthopyroxene and/or clinopyroxene caused a decrease in the bulk seismic velocities and anisotropies, and modification of seismic propagation patterns. Because CPOs in the Horoman peridotite complex are different in the basal fault zone (Matsuyama and Michibayashi 2023), their seismic properties could be different from the others. In this study, we discuss the relationships among the crystal-fabric, the calculated seismic properties, and the lithospheric structure inferred from geophysical observatories.

References
Mainprice, 2007, G. Schubert (Ed.), Treatise in Geophysics vol. 2, Elsevier, Oxford, UK.
Matsuyama and Michibayashi, 2023, Journal of Geodynamics 158.
Michibayashi et al., 2016, Earth and Planetary Science Letters 443.