ULVZs Beneath the Southern Hemisphere

Ultra-low velocity zones (ULVZs) are laterally-varying structural heterogeneities that have been identified along some portions of the core-mantle boundary.  These features have thicknesses on the order of tens of kilometers and are generally associated with reduced seismic wave velocities and increased density; however, ULVZ properties vary amongst different studies (see review by Yu and Garnero, 2018), making determination of the source of these anomalous structures challenging.  Only a small percentage of the core-mantle boundary has been surveyed for the presence of ULVZs; thus, investigations with new sampling geometries are desired to further constrain ULVZ origins. 

Collaborating with colleagues at Arizona State University and The University of Leeds, our research group has been investigating ULVZ structure beneath the southern hemisphere using different types of core-reflected waveforms (ScP, PcP), and our work is highlighted in the following papers.

Agboola, K.J., S.E. Hansen, E.J. Garnero, S. Rost, M. Li, and S.-H. Shim, Ultra-low Velocity Zones beneath the Southern Hemisphere Imaged with Double-Array Stacking of PcP Waveforms, J. Geophys. Res. Solid Earth, 129, doi: 10.1029/2023JB028170, 2024. Agboola_etal_JGR_2024.pdf

Hansen, S.E., E.J. Garnero, M. Li, S.-H. Shim, and S. Rost, Globally Distributed Subducted Materials along the Earth’s Core-Mantle Boundary: Implications for Ultra-low Velocity Zones, Science Adv., 9, doi: 10.1126/sciadv.add4838, 2023.  Hansen_etal_SciAdv_2023.pdf

Hansen, S.E., E.J. Garnero, and S. Rost, Historical Interstation Pattern Referencing (HIPR): an application to PcP waves recorded in the Antarctic for ULVZ imaging, J. Geophys. Res. Solid Earth, 126, doi: 10.1029/2021JB022741, 2021.  Hansen_etal_JGR_2021.pdf

Hansen, S.E., S.E. Carson, E.J. Garnero, S. Rost, and S. Yu, Investigating Ultra-low Velocity Zones in the Southern Hemisphere using an Antarctic Dataset, Earth Planet. Sci. Lett., 536, 116142, doi: 10.1016/j.epsl.2020.116142, 2020. Hansen_etal_EPSL_2020.pdf

Maps from Hansen et al. (2023), showing (left) the locations of identified ULVZs and (right) the distribution of subducted materials in the lowermost 50 km of the mantle. Our investigations suggest that subducted materials, advected along the core-mantle boundary, may provide an explanation for the distribution and range of reported ULVZ properties.

Prior work has been supported by NSF grants ANT-1148982, PLR-1643551, EAR-1855624, EAR-2019565, and EAR-2216564 as well as NSF-NERC award NE/R012199/1. We were also recently awarded new funding (NSF grants CSEDI-2348823 and CSEDI-2348824) to further investigate ULVZs beneath the southern hemisphere using a multi-disciplinary approach that includes seismic analyses, mineral physics experiments, and geodynamic modeling. So, more to come!