Collaborative Research: Seismic Attenuation and Anelasticity in the Upper Mantle: the Effect of Continuous Far-Field Dislocation Creep

地幔中的热对流驱动板块构造，是地震和火山爆发等众多灾害的起源。上地幔位于地壳之下，深度达410公里，基本上无法到达。 因此，地震学是研究地幔特征的主要工具。 地震波能量可以被它们所经过的物质吸收，这个过程称为地震衰减。这允许识别深度处的结构，如熔体的存在。每个衰减过程必须首先在实验室中进行表征。 位错是一种线性缺陷，它在变形过程中对矿物产生剪切作用，位错的往复运动可使波幅衰减。在这里，研究小组通过实验测量了岩石微观结构（如位错和晶粒取向）对地震波衰减的影响。 研究人员使用水冰作为地幔岩石的模拟物，因为冰的物理性质是众所周知的。冰也可以在适度的压力和应力条件下变形到高应变。在变形过程中，冰试样在这里暴露于低振幅振荡应力，如地震波引起的，而衰减量化。这项研究的结果为理解冰川和冰盖的行为以及岩石微观结构对地震衰减的作用提供了重要的见解。该项目对地震学有直接影响，对材料科学和行星科学（冰冷的行星体）有更广泛的影响。它还为一位职业生涯早期的女科学家提供支持，为岩石物理学领域的博士后助理提供培训，并向高中生提供宣传。该团队通过探索地震波衰减作为多晶冰应变的函数，扩展了他们以前对冰行为的研究。这项研究旨在探索上地幔条件的微观结构：位错密度，亚晶粒尺寸和晶体择优取向。目标是测量在位错蠕变状态下经历高背景应力的材料的衰减特征。初步的实验室研究已经确定了在高度应变的样品中衰减的增加。然而，控制参数和性质的位错阻尼尺度如何地幔是未知的。在这里，样品在宾夕法尼亚大学的高压低温变形装置中以剪切扭转或压缩的方式预变形至高应变。其微观结构的特点是在低温扫描电子显微镜的光学显微镜和背散射衍射。然后在拉蒙特-多尔蒂地球观测站（哥伦比亚大学）的环境压力低温设备中测试样品在宽频率范围内的衰减。结果被用来估计地幔应力大小和组构强度对地震衰减的影响。他们也将应用于地球上的冰川和冰盖以及经历潮汐力的冰冷行星体。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Thermal convection in the Earth's mantle drives plate tectonics, at the origin of numerous hazards such as earthquakes and volcanic eruptions. The upper mantle, which lies beneath the crust to depths of 410 km, is largely unreachable. Seismology is, thus, a major tool when investigating mantle features. Seismic-wave energy can be absorbed by the materials through which they pass, a process called seismic attenuation. This allows to identify structures at depth like the presence of melt. Each attenuation process must first be characterized in the laboratory. Wave amplitude can be attenuated by back-and-forth motions of dislocations, which are linear defects shearing minerals during deformation. Here, the team measures experimentally the effects of rocks' microstructure, such as dislocations and grain orientations, on the attenuation of seismic waves. The researchers use water ice as analog for mantle rocks because ice physical properties are well known. Ice can also be deformed to high strain at modest pressure and stress conditions. During deformation, ice specimens are here exposed to low-amplitude oscillating stress like those induced by seismic waves, while the attenuation is quantified. Results from this research provide critical insights to understand glaciers' and ice-sheet behavior and the role of rocks microstructures on seismic attenuation. The project has direct implications in Seismology and broader impacts in Material Sciences and Planetary Science (icy planetary bodies). It also provides support for an early-career female scientist, training for a post-doctoral associate in the field of Rocks Physics and outreach to high-school students.The team expands on their previous studies of ice behavior by exploring seismic-wave attenuation as a function of strain in polycrystalline ice. The study is designed to explore upper mantle conditions in terms of microstructure: dislocation density, sub-grain size and crystal preferred orientation. The goal is to measure the attenuation signature for materials experiencing a high background stress in the dislocation creep regime. Preliminary laboratory studies have identified an increase in attenuation in highly strained samples. Yet, the controlling parameters and the nature of how dislocation damping scales to the mantle is not known. Here, samples are pre-deformed to high strain in either shear torsion or compression in the high-pressure cryogenic deformation apparatus at University of Pennsylvania. Their microstructure is characterized by light microscopy and back scatter diffraction in a cryo-scanning electron microscope. Specimens are then tested for attenuation over a broad frequency range in the ambient-pressure cryogenic apparatus at Lamont-Doherty Earth Observatory (Columbia University). The results are used to estimate the effect of both mantle stress magnitude and fabric strength on seismic attenuation. They will also have applications to glaciers and ice sheets on Earth and icy planetary bodies experiencing tidal forcing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.