Investigating the Pervasiveness of Complex Seismic Anisotropy and Its Origin Beneath Continents

Missouri University Of Science And Technology, Rolla MO

Investigators

Abstract

Numerous geoscientific investigations over the past 50 years have convincingly demonstrated that the Earth's surface is divided into seven large and many small blocks known as tectonic plates. The relative movement of the plates against each other produces earthquakes, volcanoes, majestic mountains, and deep ocean basins. The physical and chemical processes that drive plate motion, however, are still poorly understood. Improving our understanding of such processes is essential not only for understanding how the Earth works, but also for achieving the ultimate goal of reliably predicting and mitigating natural hazard such as earthquakes and volcanic eruptions. One of the effective tools to investigate plate motion is shear wave splitting analysis, which is based on the observation that when a shear wave originated from an earthquake travels through an anisotropic area formed by plate motion, it will split into two waves. Previous shear wave splitting studies were mostly conducted under the assumption that there is only one layer of anisotropy. The recent dramatic increase in the number of seismic stations and recorded earthquakes suggests that the actual situation is more complicated than this single layer assumption. This project will develop and test a set of sophisticated tools to systematically investigate complex anisotropy on a global scale, for the purpose of providing constraints on a number of hypotheses related to plate dynamics and plate motion. This project will also support training of a graduate student, two undergraduate students, and a summer intern. Shear wave splitting (SWS) has been increasingly used to quantify seismic azimuthal anisotropy and to understand the geodynamic processes responsible for its formation. Mostly due to the limited amount of data available at most of the stations on Earth and the resulting poor azimuthal coverage of the incoming XKS (including SKS, SKKS, and PKS) rays, the vast majority of existing SWS studies, including most of the studies conducted by the PIs, assumed the simplest form of anisotropy, i.e., a single layer with a horizontal axis of symmetry. On the other hand, some SWS studies have identified systematic variations of the observed splitting parameters as a function of the azimuth of the incoming XKS rays. This dependence is the most important diagnostic of complex anisotropy (e.g., anisotropy with a dipping axis of symmetry and/or with two or more layers with a horizontal axis). By taking advantage of the recent dramatic increase in the quantity and quality of broadband seismic data, this project will address the important question of the pervasiveness of complex anisotropy using long-running seismic stations on the continents. The project also intends to characterize the complex anisotropy by determining the splitting parameters associated with the layers. The resulting spatial distribution of complex anisotropy will be a valuable constraint and input parameter for various geoscientific studies in the areas of geodynamic modeling, seismic tomography, and mineral physics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.