Collaborative Research: Seismic attenuation and anelasticity in the upper mantle: the effect of continuous far-field dislocation creep

合作研究：上地幔的地震衰减和滞弹性：连续远场位错蠕变的影响

• 批准号: 1855423
• 负责人: Christine McCarthy
• 金额: $ 45.25万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-05-01 至 2023-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1855423&HistoricalAwards=false
• 关键词: Collaborative; Research; Seismic; attenuation; anelasticity; Collaborative; Research; Seismic; attenuation; anelasticity

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.

地球地幔中的热对流驱动板块构造，其起源是地震和火山喷发等众多危害的起源。上地幔位于地壳下至410公里的深处，在很大程度上是无法达到的。 因此，在研究地幔特征时，地震学是一种主要工具。 地震波能通过它们通过的材料吸收，这是一种称为地震衰减的过程。这可以像熔体的存在一样识别深度的结构。每个衰减过程必须首先在实验室中进行表征。 波浪振幅可以通过脱位的来回运动来减弱，它们是变形过程中剪切矿物质的线性缺陷。在这里，团队通过实验测量岩石微观结构（例如位错和晶粒方向）对地震波的衰减的影响。 研究人员将水冰用作地幔岩石的类似物，因为冰的物理特性是众所周知的。在适度的压力和应力条件下，冰也可以变形为高应变。在变形过程中，冰样样品在这里暴露于低振幅的振荡应激，例如地震波引起的压力，而衰减是量化的。这项研究的结果为了解冰川的行为以及岩石微观结构在地震衰减中的作用提供了关键见解。该项目对地震学具有直接影响，并在物质科学和行星科学（冰冷的行星体）中产生更广泛的影响。它还为早期职业女性科学家提供了支持，为岩石物理学领域的博士后助理培训，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层的菌株来扩展其先前对冰行为的研究。该研究旨在根据微观结构来探索上地幔条件：位错密度，亚谷物大小和晶体优选方向。目的是测量在错位蠕变状态下遇到高背景应力的材料的衰减签名。初步实验室研究已经确定了高度紧张的样品的衰减增加。然而，尚不清楚控制参数和脱位阻尼量表的性质。在这里，在宾夕法尼亚大学的高压低压低压变形设备中，样品在剪切扭转或压缩的高应变中被预先定为高应变。它们的显微结构的特征是光学显微镜和冷冻扫描电子显微镜中的背部散射衍射。然后，在拉蒙特·霍尔蒂（Lamont-Doherty）地球天文台（哥伦比亚大学）的环境压力低温设备（哥伦比亚大学）的环境压力型中的频率范围内测试了标本。结果用于估计地幔应力幅度和织物强度对地震衰减的影响。他们还将在地球上的冰川和冰盖和经历潮汐强迫的冰冷的行星机构上提出申请。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的知识分子和更广泛影响的评估评估标准来通过评估来支持的。