Collaborative Research: Detecting Seismic Anisotropy in the Upper Mantle and Upper Mantle Transition Zone

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The heat that escapes from Earth's core is brought towards the surface through convection, a process that causes solid rocks in the mantle to flow and deform over geological time scales. Hot materials rise to the surface, while cold materials sink to the bottom. The overturn of the mantle through convection is thought to be the driving mechanism behind the motion of the rigid plates that divide the Earth's crust, which in turn generates earthquakes and volcanoes. Fundamental questions remain regarding the nature of the boundary that separates the rigid plates at the surface from the underlying, more deformable convecting mantle. In particular, the nature of the mantle transition zone between 410 and 670 km depth plays an important role in determining the nature of convection in the Earth. Flow or deformation of the rocks in the mantle will align minerals with the flow direction, which can be detected with seismic waves through the observation of seismic anisotropy. Here, the velocity with which waves travel becomes a function of the orientation of the travel path. In this project, the PIs will model three-dimensional variations in seismic anisotropy in the upper 800 km of the mantle. By combining multiple types of seismic data, the investigators will greatly enhance the accuracy of their model, particularly in the mantle transition zone. Their numerical forward modeling technique allows the team to quantitatively assess model uncertainties. This key element is necessary to interpret their models in terms of mineral physics, geodynamics, or mantle geochemistry, and to guide future research. The results will benefit the geoscience community as a whole through improved models of mantle deformation and plate tectonics, public outreach presentation, and training of graduate students in deep earth research science.The proposed work will address three major questions: (1) What is the seismological character of the lithosphere-asthenosphere boundary (LAB)? (2) Is there detectable seismic anisotropy in the deep upper mantle and mantle transition zone (MTZ)? (3) What is the nature of the MTZ and it's role in convection? To answer these questions, the investigators will model global, three-dimensional (3-D) variations in radial and azimuthal seismic anisotropy in the upper 800km of the mantle using a joint forward modeling approach for fundamental and higher mode surface wave dispersion measurements, surface wave arrival angle measurements, SS precursor travel times, and SKS splitting data. The proposed research will produce (1) a new surface wave arrival angle dataset that will greatly enhance the imaging of small-scale anisotropy in the uppermost mantle. This will allow us to obtain new, improved insight on the nature of the oceanic and continental LAB; (2) a new 3-D model of azimuthal and radial anisotropy in the upper 800km of the mantle. It will enable us to test for the presence and sign of radial anisotropy in the deep upper mantle, which can impose constraints on the dominant shear direction and mantle flow at these depths. It will also test the ability to resolve lateral variations in radial and azimuthal anisotropy below 250km and how such structures are related to mantle dynamics; (3) the integration of a new global dataset of SS precursor travel times providing topography at the MTZ boundaries. This will reduce trade-offs between MTZ boundaries topography and 3-D structural variations in the MTZ, as well as provide new constraints on the thermal versus compositional nature of this depth shell of the Earth. An important facet of this research is the use of numerical forward modeling to statistically identify well-constrained features of the new models. With forward modeling the team will be able to assess model resolution by quantifying parameter trade-offs and uncertainties, which is key to determining which model parameters are robust. It will also guide future research in determining what other type of data is needed to further improve resolution.