Establishing a Long-Term Geodetic Network at the East Pacific Rise Ridge 2000 Integrated Studies Site

摘要世界上大多数板块边界都淹没在海洋之下，人们对这些边界如何适应月度到年度时间尺度上的板块运动知之甚少。在大洋中脊，当两个大洋板块分开时，新的大洋地壳就形成了，岩浆从山脊下涌出的地幔中被挖掘出来。使用GPS和InSAR等各种大地测量技术跟踪了陆地活火山下的岩浆运动，然而，到目前为止，对大洋中脊下的岩浆提取几乎没有限制。在地壳内和地壳地幔边界都发现了熔融透镜(岩浆)，并在地壳岩浆透镜下方发现了低地震速度的混杂(部分熔融)带。据推测，山脊顶端的喷发与这些熔融透镜有关，但与山脊下岩浆运动有关的基本尺度仍然模糊不清。这项研究要解决的问题包括：1)火山喷发时，地壳熔融透镜的体积有多大？熔融的透镜从上升的地幔中补充的速度有多快？喷发依赖于浅岩脉中岩浆的水平运移吗？脊线的流变结构是什么，包括低粘度糊状区的宽度？地壳在喷发过程中和喷发后的力学行为是什么？喷口是否会随着下面岩浆透镜的演化而改变温度、化学流量和位置？我们计划通过监测2006年初发生的一次喷发附近EPR上北纬9度50‘的海底垂直位移来解决这些问题。我们将在三个战役风格的调查中使用移动压力记录器(MPR)来测量不同海底位置之间的相对水压，以确定海底基准的相对高程的变化。由于岩浆的浮力，地表形变直接反映了岩浆在地表以下的运动。我们希望追踪岩浆在喷发和堤防事件之间的逐渐运动。我们计划将这一快速蔓延的山脊段下的过程与其他地方的火山进行比较，后者通常具有集中的岩浆源。岩浆室喷发后补给的时间和空间尺度将取决于下伏混杂带的物理特征，并提供对周围地壳流变性的洞察。当与将于2007年2月部署的共置阵列海底压力记录器(BPR)配对时，将可以获得阶段性和长期形变信息，以小时到年度的时间尺度跟踪岩浆运动。这将使我们能够与其他研究人员合作，检查系统关系，如岩浆运动、热液喷口系统变化和微震之间的相互作用。广泛影响这项工作将建立长期大地测量监测的基础设施，并开始积累大地测量时间序列。海脊系统的所有方面都受到地壳内岩浆运动的热量的影响或驱动，这使得这项工作对于理解所有海脊系统和过程非常重要。借用Zumberge博士的工具将促进我们两个机构之间的合作。这项工作的公开传播将通过同行评议的出版物以及在科学会议、跨学科会议和其他大学的研讨会上发表。我们将与Ridge 2000教育和外展办公室合作，教育公众了解这个项目在了解RONG系统方面的重要性，以及这项工作如何适应更大的图景。

Abstract Most of the world's plate boundaries are submerged beneath the oceans, and little is known about how these boundaries accommodate plate motion on monthly to yearly time scales. At mid-ocean ridges, new oceanic crust is formed as two oceanic plates move apart and magma is tapped from mantle upwelling beneath the ridge. Magma movement beneath active volcanoes on land has been tracked using a variety of geodetic techniques such as GPS and InSAR, however, to date there are few constraints on magma extraction beneath mid-ocean ridges. Lenses of melt (magma) have been identified both within the crust and at the crust mantle boundary, as well as a low seismic velocity mush (partial melt) zone beneath the crustal magma lens. Eruptions at the ridge crests presumably tap into these melt lenses but the fundamental scales associated with magma movement beneath ridge crests remain obscure. The questions to be addressed in this study include 1) What volume of the crustal melt lens is tapped by an eruption? How quickly does the melt lens get replenished from upwelling mantle? Does an eruption depend on horizontal transport of magma in a shallow dike? 2.) What is the rheologic structure of the ridge, including the width of the low viscosity mush zone? What is the mechanical behavior of the crust during and after an eruption? 3.) Do vents change temperature, chemistry flow rates, and location as the underlying magma lens evolves?We plan to address these questions by monitoring the vertical displacements of the seafloor at 9degrees50'N on the EPR in the vicinity of an eruption that occurred in early 2006. We will use a Mobile Pressure Recorder (MPR) in three campaign style surveys to measure the relative water pressure between different seafloor locations to determine changes in the relative elevation of benchmarks placed on the seafloor. Due to the buoyancy of the magma, surface deformation directly reflects the movement of magma below the surface. We expect to track the gradual movement of magma between eruption and diking events. We plan to compare processes under this fast-spreading ridge segment, which is underlain by a continuous, linear body of melt, to that of volcanoes elsewhere, which typically have a focused magma source. The temporal and spatial scales of recharging of the magma chamber post eruption will depend on the physical characteristics of the underlying mush zone and provide insight into the rheology of the surrounding crust. When paired with the co-located array Bottom Pressure Recorders (BPR) to be deployed in February 2007, both episodic and long-term deformation information will be available, tracking magma movements with hourly to yearly time scales. This will allow us, in collaboration with other researchers, to examine system relationships, such as the interplay between movements of magma, changes in hydrothermal vent systems, and microseismicity.Broader impacts This work will establish the infrastructure for long-term geodetic monitoring and begin accumulating a geodetic time series. All aspects of the ridge system are influenced or driven by the heat from magma that moves within the Earth's crust, which makes this work important in understanding all ridge systems and processes. Borrowing an instrument from Dr. Zumberge will promote collaboration between our two institutions. Public dissemination of the work will take place through peer reviewed publications and presentations at scientific conferences, interdisciplinary meetings, and seminars at other universities. We will work with the Ridge 2000 Education and Outreach office to educate the public about the significance of this project in understanding ridge systems, and how this work fits into the larger picture.