We developed a means of quantifying the global P- and S-velocity structures of the lowermost mantle using seismic waves that diffract around Earth’s core, with the aim of determining how much heat passes from the core into the mantle. Measuring the rate of heat flow out of the core requires knowing the vertical velocity structure of the lowermost mantle, right above the core-mantle boundary. However, most seismic data provide very poor resolution of this vertical structure or resolution in only a few locations. By using the core-diffracted P and S waves from many earthquakes recorded at many seismometers around the globe, we have begun to map out an averaged vertical velocity structure across the entire core-mantle boundary. Key to is that these waves travel around the core at different speeds at different frequencies, a property called dispersion. By measuring and modeling the dispersion of the P and S waves as they diffract around the core, as well as measuring the rates at which the amplitudes of the waves decrease, we are determining a global velocity structure at the core-mantle boundary that can be combined with mineral physics data to measure how much heat flows into the mantle from the core. We found that the speeds of both Pdiff and SHdiff vary systematically with geographic position and are consistent with the locations of the large, low shear-velocity provinces resolved by whole-mantle tomography models.  The magnitude of these variations are ± 2% for Pdiff and ± 4% for SHdiff, which corroborates the lowermost mantle variations seen by tomography of other seismic phases.  The geographic variations of the decay constants for both Pdiff and SHdiff (rates at which the amplitudes decrease with distance) do not appear correlated with geographic variations in the wave speeds, which likely reflects regional variations in the vertical velocity gradients of the lowermost mantle as opposed to absolute velocities.  The Pdiff wave speed increases with decreasing period (increasing frequency), implying that the lowermost mantle has a positive gradient in Vp in the lowermost mantle.  In contrast, the SHdiff ray parameter is found not to be significantly dispersed, indicating that there is not a substantial vertical gradient in shear velocities in the lowermost mantle.  Geographic variations in the frequency bands of 0.1Hz - 0.025Hz are broadly consistent with full broadband waveform variations, although we show some evidence of significant variations in dispersion at regional scales, suggesting the presence of strong lateral velocity gradients in some regions.

In terms of Broader Impacts, the geophysical understandings gained through this research has allowed us to continue to make significant educational contributions in several areas that include (1) helping to develop free-access non-profit middle school and high school geoscience curricula as part of Washington University’s MySci program, which is used by more than 100 schools in the St. Louis region, (2) creating and coauthoring a national K-8 (elementary and middle school) science textbook program with Pearson Education called “Elevate Science,” which is built around the principles of the NRC report “A Framework for K-12 Science Education, and (3) creating and coauthoring a national high school chemistry textbook program (“Experience Chemistry”) and physics textbook program (“Experience Chemistry”) with Savvas Learning (formerly the K-12 textbook division of Pearson) that are built around the revolutionary Next Generation Science Standards using phenomenon-based learning and 5E principles of student learning.

Last Modified: 01/16/2021
Modified by: Michael E Wysession