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

地球的表面被划分为少量的构造板块，这些板块作为一个单元移动。 寒冷的，地球的上部，称为岩石圈，是坚硬的，使板块能够在没有明显内部变形的情况下移动，上面是一个可变形的，较软的层，称为软流圈。 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达，使板块构造成为可能。 尽管岩石圈在控制大地构造方面起着基本作用，但它的厚度很难测量。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各向异性，作为明确确定老海洋岩石圈厚度的一种手段。海底沉降的热模型表明，海洋板块应该是~ 90 - 125 km厚，在非常古老的海底温度接近稳定状态。 与此相反，地震面波研究表明，速度继续增加作为年龄的函数，与速度的变化发生在深度大于最适合的冷却板模型的厚度。确定岩石圈厚度和解决这一争议的最直接和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主动变形组构的过渡图。 这种变化应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方位各向异性随周期的变化来检测。在西太平洋约1.55亿年的海底相对较小的区域中，在相同年龄和相似扩张速率的海底，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急剧改变方向，但由于板块下方的流动而产生的软流圈成分应该几乎是恒定的。 通过部署扩展方向发生变化的OBS阵列，应该可以清楚地区分软流圈中各向异性的化石成分和动态维持成分。 我们将收集世界各地发生的地震的连续地震记录。 除了测量作为周期函数的瑞利波的方位各向异性外，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，并研究太平洋最古老地区表面波的区域传播。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。研究生将在布朗大学和加州州立大学北岭分校得到支持，本科生将担任助理。 至少有四名学生将参加每两个出海腿;一个很好的方式介绍海洋学作为一个领域的学生。 预计学生参与者将在游轮前后访问当地的小学和中学，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进行交流。 我们希望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中招募代表性不足的少数民族。除了在科学会议上发表演讲和在专业期刊上发表文章外，我们还将与当地的新闻官员合作，准备新闻稿，向公众传达研究结果。 收集的数据将在IRIS数据管理中心存档，并向地震学家和公众提供。

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.