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.

地球表面分为少量的构造板块，这些板块作为单位移动。 地球的寒冷上部，称为岩石圈，是坚硬的，使板块能够在称为软流圈的可变形、较软的层上方移动，而不会发生明显的内部变形。 因此，岩石圈的物理特性控制着地球内部对流的表面表现，从而实现了板块构造。 尽管岩石圈在控制构造方面发挥着重要作用，但它的厚度却很难测量。我们建议测量西太平洋两个海底地震仪（OBS）阵列内瑞利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一种手段。海底沉降的热模型表明，海洋板块应该厚约 90 - 125 公里，在非常古老的海底温度接近稳定状态。 相比之下，地震表面波研究表明，速度随着年龄的变化而持续增加，速度变化发生在大于最适合的冷却板模型厚度的深度。确定岩石圈厚度并解决这一争议的最直接、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织物的转变。 这种变化应该会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各向异性随周期的变化来检测。西太平洋海底大约有1.55亿年的历史，相同年龄和相似扩张速度的海底，扩张方向发生了重大变化。 因此，岩石圈中各向异性的化石成分应该显着改变方向，但由于板块下方的流动而产生的软流圈成分应该几乎恒定。 通过在传播方向发生变化的 OBS 阵列的部署，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维持的成分。 我们将收集世界各地发生的地震的连续地震记录。 除了测量瑞利波随周期变化的方位各向异性外，我们还将寻找阵列内部和附近速度的横向不均匀性，测量剪切波分裂、P 和 S 延迟，并研究表面的区域传播太平洋最古老地区的波浪。影响更广泛。拟议活动的一个重要组成部分是学生教育以及与当地公立学校的沟通。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。 两条航程各至少有四名学生参加；这是向学生介绍海洋学这一领域的好方法。 学生参与者将在航行前后参观当地的小学和中学，传达出海的兴奋感，并在船上准备每日博客，与他们参观过的班级进行交流。 我们预计布朗大学的参与者中至少有 50% 是女性，并且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。除了在科学会议上发表演讲和在专业期刊上发表文章外，我们还将与我们的当地新闻官员准备新闻稿，向公众传达调查结果。 收集到的数据将存档在 IRIS 数据管理中心，并提供给地震学家和公众。