Changes in elastic wave velocity during brittle deformation of gabbro and peridotite: Implications for oceanic Moho reflectivity
Changes in elastic wave velocity during brittle deformation of gabbro and peridotite: Implications for oceanic Moho reflectivity
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DOI：
10.1016/j.epsl.2021.117036
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发表时间：
2021-08
影响因子：
5.3
通讯作者：
Y. Akamatsu;I. Katayama;T. Tonegawa
中科院分区：
文献类型：
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作者：
Y. Akamatsu;I. Katayama;T. Tonegawa
脆性变形过程中产生的裂缝很容易改变岩石的物理性质;因此，了解大洋岩石变形过程中的地震特性对于解释大洋岩石圈的地震观测至关重要。在这里，我们测量了辉长岩和橄榄岩在室温、围压为20 MPa、应变速率为10 - 6s-1的三轴变形实验中的弹性波速度。辉长岩在接近最大应力时弹性波速度明显降低，与典型的结晶岩相当。另一方面，橄榄岩表现出相对突然和小的弹性波速度接近最大应力的减少。基于有效介质理论，将观测到的速度反演为裂纹密度张量。辉长岩和橄榄岩之间的弹性波速度的不同变化被解释为不同的特征的裂纹发展，有限数量的裂纹在橄榄岩比辉长岩打开。计算得到的辉长岩-橄榄岩界面的反射系数随着应力的增大从<0.1增大到<0.25。由海洋岩石圈裂纹相关速度结构计算的合成地震图表明，莫霍面反射率对壳幔边界裂纹损伤的对比度非常敏感。大洋岩石圈中存在着可能与板内地震有关的不均匀破裂损伤带，这可能导致莫霍面反射的空间变化。
Cracks that develop during brittle deformation easily modify rocks' physical properties; therefore, understanding of seismic properties of oceanic rocks during deformation is essential for interpretation of the seismic observations of the oceanic lithosphere. Here we measured elastic wave velocities of gabbro and peridotite during triaxial deformation experiments at room temperature, a confining pressure of 20 MPa, and a strain rate of ∼10−6s−1. Gabbroic rocks showed significant decrease in elastic wave velocities approaching the maximum stress, which is comparable to that of typical crystalline rocks. On the other hand, peridotite showed relatively sudden and small decrease in elastic wave velocities close to the maximum stress. We inverted the observed velocities to crack density tensor based on an effective medium theory. The different changes in elastic wave velocities between gabbroic rocks and peridotites were interpreted by different characteristics of crack development, where limited number of cracks were opened in peridotite than in gabbroic rocks. The calculated reflection coefficient of the interface between the gabbro and peridotite increases from <0.1 to ∼0.25 as approaching the maximum stress. Synthetic seismograms computed from the crack-dependent velocity structures of the oceanic lithosphere indicate that the Moho reflectivity is highly sensitive to the contrast of crack-damage across the crust-mantle boundary. The existence of a heterogeneous crack-damaged zone in the oceanic lithosphere, which is likely related to intra-plate earthquakes, may result in the spatial variations of Moho reflection.