Investigation of sources of global seismic scattering using advanced array processing and Bayesian inversion

University Of Utah, Salt Lake City UT

Investigators

Abstract

When an earthquake happens, it not only vibrates the Earth’s surface but also the Earth’s interior. The strength and timing of the vibration recorded on the Earth’s surface provide crucial information about what’s inside the Earth as vibrations travel in different directions and probe different depths and areas. This study analyzes compressional (P) waves, similar to sound waves, that pass through the core-mantle boundary about 2900 km beneath the surface. The P-waves passing through certain regions of the Earth arrive at the surface before (pre-cursors) and after (post-cursors) P waves due to small structures within the earth on the order of several tens of kilometers wide. These pre- and post-cursor signals are generally weaker than the main P-waves. In addition to the strength and timing of these signals, the angle at which they reach the surface also carries crucial information about where these structures are. This project analyzes the strength, timing, and angles of these signals, considering their consistency across nearby stations so that the weaker signal is amplified. While past studies have analyzed these signals, they have struggled to pinpoint the unique locations and properties of the structures that give rise to these pre- and post-cursors. To address this challenge, this proposal will develop an advanced statistical approach to uniquely map the locations of the features giving rise to these scattered seismic waves. This technique will be applied to a new global dataset that covers approximately 99% of the core-mantle boundary’s surface area. Identifying where these structures are located and what their physical properties are will provide insights into the ongoing dynamic processes within the Earth and their connection to surface tectonic activity. The results of this work will enhance our understanding of scatterer locations and their properties, benefiting a broad community of researchers, including seismologists, mineral physicists, geochemists, and geodynamicists. The method developed for this study will be generalized for use in an introductory undergraduate course, which will be tested in the "Introduction to Geophysics" class at the University of Utah and publicly shared through recorded videos. Seismic waves are scattered when they encounter velocity and density anomalies along their paths. Quantification of the depth, velocity, density, and size of these seismic anomalies and their uncertainties in the Earth’s mantle is crucial for understanding the Earth’s dynamic processes, chemical composition and variability, and its long-term thermal and chemical evolution. The scatterer’s location and physical properties have been previously studied using various seismic phases, including the P-wave precursor arrival that pass through the outer core (termed PKP precursors), but unique identification of the scatterer lateral location and depth is challenging. This proposal will further advance recently developed array processing techniques, which can uniquely identify anomaly location, and its uncertainty associated with PKP precursors. This proposal will further expand the new techniques to include scatterer depth and develop a Bayesian back-projection scheme to locate scatterer lateral position (i.e., latitude and longitude) and depth along with their associated uncertainties. In addition to PKP precursor arrivals, this proposal will also analyze prominent scatterers in the post-cursory wavefield that may be associated with scatterer locations higher up in the mantle. This study will apply these techniques to a new global data set that covers approximately 99% of the core-mantle boundary by surface area. This study proposes to systematically map and identify where the scatterers exist, and the seismic velocity structure associated with it through 2.5- and 3-D waveform modeling. With this approach, this study aims to identify, catalog, and map the lateral variability of the structure and its dynamic implications. 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.