Global Imaging of the Lithosphere-Asthenosphere Boundary using Scattered Waves

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.