Seismic tomography provides images of the fast and slow regions within the Earth's lower mantle using the travel times of a variety of body waves produced by earthquakes and recorded at seismometers around the globe. The investigators are examining the seismic and mineralogical behavior of the tomographically fastest regions of the lower mantle. The goal is to discover whether or not fast anomalies are uniformly associated with subducted material, and to resolve the likely effects of temperature and composition in generating these anomalies. Recent seismic tomographic models have excellent resolution in the mid and lower mantle and reveal a change in character with depth of fast anomalies. In the mid mantle, fast anomalies resemble linear features that can be associated with paleo-subduction zones. In the lower mantle, the fast anomalies take on a circum-Pacific pattern that essentially wraps around the large slow velocity provinces under the central Pacific and Africa that rise from the core-mantle boundary. In the canonical view of whole-mantle convection, seismically fast subducting oceanic lithosphere descends to the core-mantle boundary, where it drives the flow of the passive, seismically slow material. While much attention has been paid to detailed investigations of these slow features, the seismic characteristics of the fast features are poorly documented. We plan to use the most current catalog of long-period travel times of lower mantle S, P, Sdiff, Pdiff, ScS, SKS, and PKP phases to better-define the shear, compressional, and bulk sound speed of these anomalies in the lower mantle. The investigators are developing a formulation for the relative variations of these seismic waves that is interpretable from a mineral physics framework, and which is suitable for investigating 3D structure. Presently, the seismic behavior of basalt is not well understood in the lower mantle, but the investigators' preliminary calculations indicate that it is a likely candidate for producing fast seismic velocities. The investigators are using an iterative approach to discern whether the fast anomalies can be explained simply as thermal anomalies, or it they require a contribution from a fast component such as oceanic basalt, and if so, in what quantity. The investigators are also probing the anomalies with short-period seismic data from regional arrays that sample the fast features. If reflectors or velocity gradients are present, they will provide additional constraints on the thermo-chemical state of the material. Near the core-mantle boundary, the investigators anticipate that some of the fast anomalies may not be generated by temperature and/or chemical anomalies, and may require incorporation of the seismic characteristics of the post-perovskite phase transition into their thermo-chemical models. This project is funding the postdoctoral work of Dr. C. Reif who has made major contributions to Earth science outreach in elementary schools and the general community, and who is continuing her work to meet the needs of Earth science K-12 educators in the Santa Cruz area.
