Collaborative Research: Depth Distribution of Anisotropic Fabric in the Oceanic Mantle

合作研究：大洋地幔各向异性织物的深度分布

• 批准号: 0830798
• 负责人: Dayanthie Weeraratne
• 金额: $ 12.79万
• 依托单位: The University Corporation, Northridge
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-03-13 至 2015-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0830798&HistoricalAwards=false
• 关键词: Collaborative; Research; Depth; Distribution; Anisotropic

The Earth's surface is divided into a small number of tectonic plates that move as units. The cold, upper part of the earth, called the lithosphere, is stiff, enabling the plates to move without significant internal deformation above a deformable, softer layer called the asthenosphere. Thus, it is the physical properties of the lithosphere that control the surface expression of convection within the Earth's interior, enabling plate tectonics. Despite its fundamental role in governing tectonics, the thickness of the lithosphere is difficult to measure. We propose to measure the azimuthal anisotropy of Rayleigh wave propagation within two ocean-bottom seismometer (OBS) arrays in the western Pacific as a means of unambiguously determining the thickness of the old oceanic lithosphere.Thermal models of seafloor subsidence indicate that the oceanic plates should be ~ 90 - 125 km thick, with temperatures approaching steady state in very old seafloor. In contrast, seismic surface wave studies indicate that velocities continue to increase as a function of age, with the velocity changes occurring at depths greater than the thickness of the best-fitting cooling slab models. The most direct and unambiguous way to determine the thickness of the lithosphere and to resolve this controversy is to map the transition from static structure frozen in the plate to actively deforming fabric in the convecting, deforming asthenosphere. This change should induce a change in anisotropic fabric associated with the alignment of the mineral olivine in a deforming Earth, which we propose to detect by measuring the variation of azimuthal anisotropy of Rayleigh waves as a function of period.In a relatively small area of the western Pacific in seafloor approximately 155 million years old, there are major changes in the direction of spreading in seafloor of the same age and similar spreading rate. Thus, the fossil component of anisotropy in the lithosphere should change direction dramatically, but the asthenospheric component due to flow beneath the plate should be nearly constant. With a deployment of arrays of OBSs where the spreading directions change, it should be possible to clearly distinguish the fossil component of anisotropy from the dynamically maintained component in the asthenosphere. We will collect continuous seismic records of earthquakes occurring around the world. In addition to measuring the azimuthal anisotropy of Rayleigh waves as a function of period, we will look for lateral heterogeneities in velocity within and in the vicinity of the arrays, measure shear wave splitting, P and S delays, and study the regional propagation of surface waves in the oldest parts of the Pacific.Broader Impacts. An important component of the proposed activity is education of students and communication with local public schools. Graduate students will be supported at Brown and at CalState Northridge and undergrads will work as assistants. At least four students will participate in each of the two seagoing legs; a good way to introduce oceanography as a field to students. Student participants will be expected to visit local elementary and middle schools before and after the cruise to communicate the excitement of going to sea and to prepare a daily weblog on board to communicate with the classes they have visited. We expect that of the Brown University participants, at least 50% will be women, and we will attempt to recruit underrepresented minorities from the CalState Northridge student body.In addition to presentations at scientific conferences and publication in professional journals, we will work with our local press officers to prepare press releases to communicate findings to the general public. Data gathered will be archived at the IRIS Data Management Center and made available to seismologists and the general public.

地球表面被分成几个构造板块，这些板块作为一个单位运动。地球上被称为岩石圈的寒冷部分是坚硬的，这使得板块能够在一个可变形的、更柔软的被称为软流圈的层上移动，而不会发生明显的内部变形。因此，正是岩石圈的物理性质控制着地球内部对流的表面表现，使板块构造成为可能。尽管岩石圈在控制构造方面起着基础性作用，但岩石圈的厚度很难测量。我们建议测量西太平洋两个海底地震仪(OBS)阵列中瑞雷波传播的方位各向异性，作为明确确定旧海洋岩石圈厚度的手段。海底下沉的热模型表明，海洋板块厚度应为~90-125公里，非常旧海底的温度应接近稳定状态。相比之下，地震面波研究表明，速度随着年龄的增加而继续增加，速度变化发生在比最佳拟合冷却板模型的厚度更大的深度。确定岩石圈厚度和解决这一争议的最直接、最明确的方法是绘制从冻结在板块中的静态结构到对流变形的软流圈中的活跃变形组构的过渡图。这种变化应该引起与变形地球中矿物橄榄石对齐有关的各向异性组构的变化，我们建议通过测量瑞利波的方位各向异性随周期的变化来检测这种变化。在西太平洋大约1.55亿年的海底相对较小的区域，相同年龄和相似扩张速度的海底在扩散方向上发生了重大变化。因此，岩石圈各向异性的化石成分应该发生巨大的方向变化，但由于板块下方流动而产生的软流圈成分应该几乎是恒定的。随着OBSS阵列的部署，在那里传播方向发生变化，应该可以清楚地区分各向异性的化石成分和软流圈中动态保持的成分。我们将收集世界各地发生地震的连续地震记录。除了测量瑞雷波的方位各向异性作为周期的函数外，我们还将在阵列内和附近寻找速度的横向不均匀，测量横波分裂、P和S延迟，并研究面波在太平洋最古老部分的区域传播。拟议活动的一个重要组成部分是教育学生和与当地公立学校交流。布朗大学和加州州立大学的研究生将得到支持，本科生将担任助理。至少有四名学生将参加两个海洋航段；这是向学生介绍海洋学作为一个领域的一个好方法。预计学生参与者将在邮轮前后参观当地的中小学，交流出海的兴奋之情，并在船上准备每天的博客，与他们参观过的班级进行交流。我们预计，在布朗大学的参与者中，至少有50%将是女性，我们将尝试从加州州立大学北岭分校的学生团体中招募人数较少的少数族裔。除了在科学会议上发表演讲和在专业期刊上发表文章外，我们还将与当地新闻官员合作准备新闻稿，向公众传达调查结果。收集的数据将在IRIS数据管理中心存档，供地震学家和公众使用。