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着板块构造，这是地震和火山喷发等众多灾害的根源。上地幔位于地壳下410公里深的地方，基本上是无法到达的。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通过的材料吸收，这一过程被称为地震衰减。这使得能够识别深度的结构，如熔体的存在。每个衰减过程都必须首先在实验室中进行表征。波幅可以通过位错的来回运动来衰减，位错是变形过程中剪切矿物的线性缺陷。在这里，研究小组通过实验测量了岩石的微观结构，如位错和颗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类似物，因为冰的物理性质众所周知。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，冰样暴露在像地震波那样的低幅度振荡应力下，同时衰减是量化的。这项研究的结果为理解冰川和冰盖的行为以及岩石微结构在地震衰减中的作用提供了关键的见解。该项目对地震学有直接影响，对材料科学和行星科学(结冰的行星体)有更广泛的影响。它还为职业生涯早期的女科学家提供支持，为岩石物理领域的博士后助理提供培训，并向高中生提供服务。该团队通过探索多晶冰中作为应变函数的地震波衰减，扩展了他们之前对冰行为的研究。这项研究旨在从位错密度、亚晶尺寸和晶体择优取向等微观结构方面探索上地幔条件。目标是测量在位错蠕变区域经历高背景应力的材料的衰减特征。初步实验室研究发现，在高度应变的样品中，衰减会增加。然而，位错衰减如何扩展到地幔的控制参数和性质尚不清楚。这里，在宾夕法尼亚大学的高压低温变形装置中，样品被预变形为高应变，无论是剪切扭转还是压缩。用光学显微镜和低温扫描电子显微镜的背散射衍射仪对其微观结构进行了表征。然后，在拉蒙特-多尔蒂地球天文台(哥伦比亚大学)的环境压力低温设备中，对样品进行宽频率范围的衰减测试。该结果被用来估计地幔应力值和组构强度对地震衰减的影响。他们还将应用于地球上的冰川和冰盖，以及经历潮汐强迫的冰行星。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.