Shear-wave attenuation anisotropy: a new constraint on mantle melt near the Main Ethiopian Rift

J. Asplet, J. Wookey, Micheal Kendall, Mark Chapman, Ritima Das
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Abstract

The behaviour of fluids in preferentially aligned fractures plays an important role in a range of dynamic processes within the Earth. In the near-surface, understanding systems of fluid-filled fractures is crucial for applications such as geothermal energy production, monitoring CO2 storage sites, and exploration for metalliferous sub-volcanic brines. Mantle melting is a key geodynamic process, exerting control over its composition and dynamic processes. Upper mantle melting weakens the lithosphere, facilitating rifting and other surface expressions of tectonic processes.Aligned fluid-filled fractures are an efficient mechanism for seismic velocity anisotropy, requiring very low volume fractions, but such rock physics models also predict significant shear-wave attenuation anisotropy. In comparison, the attenuation anisotropy expected for crystal preferred orietation mechanisms is negligible or would only operate outside of the seismic frequency band.Here we demonstrate a new method for measuring shear-wave attenuation anisotropy, apply it to synthetic examples, and make the first measurements of SKS attenuation anisotropy using data recorded at the station FURI, in Ethiopia. At FURI we measure attenuation anisotropy where the fast shear-wave has been more attenuated than the slow shear-wave. This can be explained by the presence of aligned fluids, most probably melts, in the upper mantle using a poroelastic squirt flow model. Modelling of this result suggests that a 1% melt fraction, hosted in aligned fractures dipping ca. 40° that strike perpendicular to the Main Ethiopian Rift, is required to explain the observed attenuation anisotropy. This agrees with previous SKS shear-wave splitting analysis which suggested a 1% melt fraction beneath FURI. The interpreted fracture strike and dip, however, disagrees with previous work in the region which interprets sub-vertical melt inclusions aligned parallel to the Main Ethiopian Rift which only produce attenuation anisotropy where the slow shear-wave is more attenuated. These results show that attenuation anisotropy could be a useful tool for detecting mantle melt, and may offer strong constraints on the extent and orientation of melt inclusions which cannot be achieved from seismic velocity anisotropy alone.
剪切波衰减各向异性:埃塞俄比亚主裂谷附近地幔熔体的新约束条件
流体在优先排列的裂缝中的行为在地球内部的一系列动态过程中发挥着重要作用。在近地表,了解充满流体的裂缝系统对于地热能源生产、监测二氧化碳封存地点以及勘探火山下卤水等应用至关重要。地幔熔化是一个关键的地球动力学过程,对地幔的组成和动态过程具有控制作用。上地幔熔化削弱了岩石圈,促进了断裂和其他构造过程的地表表现形式。排列整齐的充满流体的裂缝是地震速度各向异性的有效机制,所需的体积分数非常低,但此类岩石物理模型也预测了显著的剪切波衰减各向异性。在这里,我们展示了一种测量剪切波衰减各向异性的新方法,并将其应用于合成实例,利用在埃塞俄比亚 FURI 站记录的数据首次测量了 SKS 衰减各向异性。在 FURI 站,我们测量到快速剪切波比慢速剪切波衰减更多的衰减各向异性。这可以用上地幔中存在排列整齐的流体(很可能是熔体)来解释,使用的模型是孔弹性喷流模型。对这一结果的建模表明,要解释所观测到的衰减各向异性,需要有1%的熔体成分,寄存在垂直于埃塞俄比亚主裂谷的倾角约为40°的排列断裂中。这与之前的 SKS 剪切波分裂分析一致,该分析表明 FURI 地下有 1%的熔融部分。然而,对断裂走向和倾角的解释与该地区以前的研究不一致,以前的研究解释了与埃塞俄比亚主裂谷平行的次垂直熔融包裹体,而这种包裹体只在慢剪切波衰减较大的地方产生衰减各向异性。这些结果表明,衰减各向异性可作为探测地幔熔体的有用工具,并可对熔体包裹体的范围和方位提供有力的约束,而这一点仅靠地震速度各向异性是无法实现的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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