To understand the dynamics of the Earth on a global scale (e.g., how tectonic plates move, resulting in destructive earthquakes and volcanic eruptions), we must understand the nature of the mantle, 3000 km of solid rock that lies beneath the Earth's surface. In particular, two enigmatic dome-shaped structures called the "Large Shear Wave Velocity Provinces" (or LLSVPs) rise from the base of the mantle to around 1000 km above, one beneath Africa and the other beneath the Pacific Ocean. Seismic waves that emanate from earthquakes sometimes travel through these LLSVPs and are slowed down when they enter these structures. It is still unclear why this might be the case: LLSVPs could be hotter (and thus more buoyant) than their surroundings, or they could be chemically distinct (and likely denser) than their surroundings. Both suggest fundamental differences in how the mantle flows over millions of years and therefore how it influences tectonic plate motions and the related natural disasters. The PIs plan to, for the first time, combine information from traditional seismic data sets but also measurements from special types of seismic waves and earth tides to shed new light on the buoyancy of these LLSVPs. The special type of seismic waves are called Stoneley modes and are vibrations trapped along the core-mantle boundary (CMB) and the earth tides are the twice-daily deformation of the solid Earth under gravitational forces from the Sun and Moon. This award will support the training of two graduate students at UC Berkeley and two female PIs, one of whom is an early career scientist and new faculty member at UC Berkeley. The models developed in this project will be made available as part of a webpage specifically developed for this project, as well as through the Incorporated Research Institution for Seismology (IRIS) facility. They will be used in undergraduate workshops at UC Berkeley and at the interdisciplinary CIDER workshop geared towards senior graduate students and early career scientists.
Within the last two years, independent studies have presented seemingly contradictory results on the density structure of the large low shear velocity provinces (LLSVPs) imaged by seismic tomography in the earth's deep mantle. In particular, one study used a variety of seismological data (normal mode splitting, seismic travel time and waveform data), and another, semi-diurnal earth tide data, concluding that a significant part of the LLSVPs represent regions of excess density pointing to the source of the LLSVP anomalies being dominated by chemical heterogeneity. In contrast, a study featuring measurements of Earth's Stoneley modes concluded the opposite, implying that the source of LLSVP anomalies are in large part thermal. Both conclusions paint a very different picture of mantle convection: the former implies that the LLSVPs are potentially sluggish upwellings or stabilized piles, while the latter implies a more energetic mode of mantle circulation. In this project, the investigators will combine these different geodetic and seismic datasets - which have different depth sensitivities to elastic and density structure - to establish whether previous results can be reconciled under the hypothesis that the excess density in LLSVPs may be confined to a thin layer (~200 km or less) above the CMB. In doing so, they will investigate further trade-offs with shear and compressional wave speed structure, anisotropy, attenuation, as well as CMB topography and possible structure in the outermost core. First, they will consider a combination of existing global seismic waveform, tide and normal mode splitting data, including Stoneley mode data, and perform inversions for structure in the deepest mantle, using the classical first order mode perturbation formalism for modeling splitting data. They will implement both an optimization and a trans-dimensional Monte Carlo inversion method for this purpose. This, in large part, will act as a guide to prepare for the more advanced inversions to be undertaken in the next stage, while yielding interesting intermediate results. In the second stage, after completing their own dataset of normal mode spectra, they will consider a more rigorous theoretical approach and directly invert mode spectra, together with long period waveform and tide data (complemented by newly acquired data) for density and velocity structure in the vicinity of the CMB.
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.
