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: 0648387
• 负责人: Donald Forsyth
• 金额: $ 44.44万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-04-01 至 2014-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0648387&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以单位运动的构造板块。寒冷的地球上部称为岩石圈，它是坚硬的，这使得板块能够在没有明显内部变形的情况下，在一个可变形的、更柔软的称为软流层的层上移动。因此，正是岩石圈的物理特性控制了地球内部对流的表面表达，从而实现了板块构造。尽管岩石圈的厚度在控制构造方面起着重要作用，但它很难测量。我们建议在西太平洋的两个海底地震仪（OBS）阵列中测量瑞利波传播的方位角各向异性，作为明确确定旧海洋岩石圈厚度的手段。海底沉降热模型表明，大洋板块厚度应为~ 90 ~ 125 km，非常古老的海底温度接近稳定状态。相反，地震表面波研究表明，速度随年龄的增长而继续增加，速度变化发生在深度大于最合适的冷却板模型的厚度的地方。确定岩石圈厚度并解决这一争议的最直接和最明确的方法是绘制从板块中冻结的静态结构到对流变形软流圈中主动变形结构的过渡图。这种变化应该引起与变形地球中矿物橄榄石排列相关的各向异性结构的变化，我们建议通过测量瑞利波的方位角各向异性随周期的变化来检测。在西太平洋一个相对较小的约1.55亿年历史的海底区域，在相同年龄和相似扩张速度的海底，出现了主要的扩张方向变化。因此，岩石圈各向异性的化石成分方向应该发生巨大的变化，但由于板块下流动的软流圈成分应该几乎是恒定的。在传播方向发生变化的OBSs阵列的部署中，应该可以清楚地区分软流层中各向异性的化石成分和动态维持的成分。我们将收集世界各地发生地震的连续地震记录。除了测量Rayleigh波的方位角各向异性作为周期函数外，我们还将在阵列内部和附近寻找速度的横向非均质性，测量横波分裂，P和S延迟，并研究太平洋最古老部分表面波的区域传播。更广泛的影响。拟议活动的一个重要组成部分是对学生的教育以及与当地公立学校的交流。布朗大学和加州州立大学北岭分校的研究生将得到支持，本科生将担任助理。两个航段各至少有四名学生参加；这是一个向学生介绍海洋学的好方法。学生们将在航行前后参观当地的小学和中学，以交流出海的兴奋之情，并在船上准备每日博客，与他们参观过的班级交流。我们预计，在布朗大学的参与者中，至少有50%是女性，我们将尝试从加州州立大学北岭分校的学生群体中招募代表性不足的少数族裔。除了在科学会议和专业期刊上发表报告外，我们还将与当地新闻官员合作，准备新闻稿，向公众传播研究结果。收集到的数据将在IRIS数据管理中心存档，并提供给地震学家和公众。