Global Imaging of the Lithosphere-Asthenosphere Boundary using Scattered Waves

我们星球的表面由许多构造板块组成，就像一个已经破裂但没有打开的蛋壳。这些板块被称为岩石圈。岩石圈在一个被称为软流圈的弱层上方移动。这种运动被称为板块构造。岩石圈不断遭到破坏，一个板块被拖下，或俯冲到另一个板块之下，进入软流圈。它也不断地在大洋中脊产生，在那里两个板块被拉开，导致熔体上升到空隙中，冷却形成新的地壳。地球是由许多层组成的，这些层的位置以及分层的原因(例如，岩石类型或状态的变化)是相对众所周知的。然而，岩石圈-软流圈的边界并不是全球性的，定义它的机制也不是众所周知的。岩石圈和软流圈之间的界面是一个非常重要的边界，因为边界的性质对板块构造的驱动力和我们所生活的大陆的起源和演化具有影响。板块构造是导致地震、火山喷发和海啸等自然灾害的原因。大陆形成令人费解，因为它已经不再发生，而大多数大陆内部都是数十亿年前的事情。我们想知道它们是如何形成的，是什么使它们能够形成，并随着时间的推移保持稳定，因为它们构成了地球上适合人类居住的区域。为了研究这一边界，我使用来自地震的能量，即地震波，在遥远的台站记录下来，以成像地球的边界，因为地球速度的变化会影响波的路径。在过去的20年里，地震学家在主要位于大陆的永久地震台站收集了大量的地震数据。我们还从高密度部署的临时地震仪阵列收集数据。这些数据为我们提供了高分辨率成像能力，这使我们能够非常详细地约束地震速度梯度。这些约束告诉我们定义岩石圈-软流圈边界的机制。在岩石上进行的实验有助于我们确定各种参数，如温度、成分和熔融对地震波的影响。他们告诉我们的是，逐渐的速度梯度可以用从较冷的岩石圈到较热的软流层的转变来解释。然而，地震尖锐的边界需要其他机制来解释。成分变化，即矿物含量和/或水化作用，或软流圈中的少量熔融，可能是造成强烈速度对比的原因。尖锐的边界意味着岩石圈和软流圈是非常不耦合的，板块运动是由致密板块的引力驱动的，在那里它们俯冲到软流圈。逐渐的边界表明耦合增强，岩石圈下地幔中的运动可能发挥更大作用的概念。我们计划寻找与岩石圈-软流圈边界相关的尖锐边界，并调查不同构造环境下边界深度和特征的变化。在海洋下，清晰的边界经常被拍摄到，偶尔也会被拍摄到大陆下。人们常常认为，不同的机制界定了大陆和海洋之下的边界。然而，为什么在不同的地点以不同的方式定义这样的边界仍然是一个谜。我们计划通过使用高频能量对边界进行全局建模来解决这个问题，该能量为我们提供了关于界面特征的信息。在某些情况下，我们也可以想象岩石圈内部的边界。但这些也很有趣，因为它们可以告诉我们组成大陆的积木，以及它们的形成和演化的含义。

The surface of our planet is composed of a number of tectonic plates, resembling an eggshell that has been cracked, but not opened. These plates are called the lithosphere. The lithosphere moves over a weak layer that is called the asthenosphere. This movement is referred to as plate tectonics. The lithosphere is constantly being destroyed, where one plate is dragged down, or subducts, beneath another, and enters the asthenosphere. It is also constantly being created, at mid-ocean ridges, where two plates are pulled apart, causing melt to rise into the void, and cool to form new crust. The earth is made of many layers, and the locations of the layers as well as the cause for the layering (e.g. changes in rock type or state) are relatively well known. However, the lithosphere-asthenosphere boundary is not globally located, nor is the mechanism that defines it well known. The interface between the lithosphere and the asthenosphere is a very important boundary in that the nature of the boundary has implications for the driving forces of plate tectonics and the origin and evolution of the continents on which we live. Plate tectonics is what drives natural disasters like earthquakes, volcanic eruptions, and tsunamis. Continent formation is puzzling since it is no longer occurring, and most continental interiors are billions of years ago. We would like to know how they formed and what enabled their formation, and stability through time, since they make up the area of the earth that is hospitable to humans. To investigate this boundary I use the energy from earthquakes, seismic waves, recorded at distant stations to image boundaries in the earth, since changes in the velocity of the earth affect the path of the waves. Seismologists have collected much seismic data over the past ~20 years at permanent seismic stations located primarily on continents. We also collect data from high density deployments of temporary arrays of seismometers. The data gives us high resolution imaging capabilities, and this allows us to constrain seismic velocity gradients in great detail. Such constraints tell us about the mechanism that defines the lithosphere-asthenosphere boundary. Experiments done on rocks help us determine the effects of various parameters like temperature, composition, and melting have on seismic waves. What they tell us is that gradual velocity gradient can be explained by the transition from a cool lithosphere, to a hotter asthenosphere. However, seismically sharp boundaries require other mechanisms to explain them. Compositional changes, i.e. mineral content and/or hydration, or a small amount of melting in the asthenosphere could be responsible for sharp velocity contrasts. Sharp boundaries mean that the lithosphere and the asthenosphere are very decoupled, and plate motions are driven by the gravitational pull of dense plates where they subduct into the asthenosphere. Gradual boundaries indicate increased coupling, and the notion that motions in the mantle beneath the lithosphere may play a larger role. We plan to look for sharp boundaries associated with the lithosphere-asthenosphere boundary, and investigate variations in the depth and character of the boundary in a variety of tectonic environments. Beneath oceans sharp boundaries are frequently imaged, and they are occasionally imaged beneath continents. It is often assumed that different mechanisms define the boundary beneath continents and oceans. However, it remains a puzzle why such a boundary would be defined in different ways in different locations. We plan to resolve this issue with global modeling of the boundary using high frequency energy that gives us information about the character of the interface. In some cases, we may also image boundaries that are interior to the lithosphere. But these are also interesting since they can tell us about the building blocks that compose the continents, with implications for their formation and evolution.