Three-Dimensional Seismic Velocity Structure of the Earth's Outermost Core and Mantle

Citation

Kohler, Monica Diane (1995) Three-Dimensional Seismic Velocity Structure of the Earth's Outermost Core and Mantle. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/q3fd-fd26. https://resolver.caltech.edu/CaltechETD:etd-10312007-090136

Abstract

Obtaining an accurate, detailed picture of deep-Earth structure is of fundamental importance in a wide range of geophysical applications such as fluid dynamic, magnetohydrodynamic, and mineral physics models of the Earth which incorporate properties determined from seismology. Because it is such a drastic chemical and thermal boundary layer, the nature of the core-mantle boundary has important implications for deep-Earth processes, particularly those which have their origin in the lower mantle or outer core. Seismic data provide the most direct method of sampling the Earth's interior and are, therefore, useful for determining deep-Earth material properties.

The goal of this work has been to present models of three-dimensional, shear and compressional velocity structure which are self-consistent with the data and which can be used in other geophysical applications. The numerical inversions consisted of determining the three-dimensional structure of the outermost core and mantle of the Earth from long-period seismic waveforms. This approach is distinct from other global models of deep-Earth heterogeneity because it accounts for possible lateral heterogeneity in an outermost core layer whose properties are constrained by seismic phases which travel through the core-mantle boundary region.

This method is different from previous core studies in several important ways: synthetic seismograms are constructed using short-period normal modes for the entire set of body-wave phases which travel through the interior of the Earth (e.g., P, PP, S, SS, SKS). Over 5000 seismograms from global digital seismic networks were collected and processed. First-order perturbations in P-wave velocities in one outermost core layer and S-wave velocities within 11 mantle layers of varying thicknesses comprised the least-squares solutions to the inverse problem. Spheroidal modes with periods between 33 and 100 sec were selected to model the body-wave portion of seismograms recorded from earthquakes which occurred globally.

The preferred model is a 12-layered model incorporating data weighted by inverse data variance. This model produces velocity anomalies in the mantle and outermost core which are acceptable for first-order perturbation methods. The results of one-layer inversions also point to the possible existence of lateral variations in the outermost core, most likely between ±0.5% but not as large as ±5%. This model suggests that outermost core P-wave velocity perturbations accompany S-wave velocity perturbations in the lowermost mantle to produce observed variations in SKS-S and SKKS-SKS travel times. In addition, the patterns of structure vary smoothly and exhibit both large and small scale features. The spectral amplitudes fall off more rapidly for the lower mantle layers than for the upper mantle. The depth resolution displayed by the c⁰₀ spherical harmonic term is 200-300 km for upper mantle layer midpoints and increases to 500-600 km for lower mantle layer midpoints.

The data variance reduction of entire body-wave portions as well as SnKS portions of seismograms are slightly better for the 12-layered model than for the 11-layered model; however, the total variance reductions were never very large. The results of the F ratio suggest that lateral velocity variations in the outermost core layer are not zero and that the deepest layer is statistically significant. This test does not require that the extra layer lie in the outermost core (as opposed to the lowermost mantle).

The results of pattern retrieval resolution tests support the conclusion that structure of the outermost core has been obtained independently from the mantle. Multiplicative factors have been calculated from the resolution tests using synthetic Earth models to place constraints on the amount of power leakage suspected from one region to another due to incomplete data coverage. An upper bound of 84% and a lower bound of 68% of the power of outermost core structure is, in fact, due to heterogeneity in the outermost core. By the same analysis, less than 100% of the power of structure initially placed in the lowermost mantle was retrieved in that layer after the resolution inversion. An upper bound of 60% and a lower bound of 53% of the power of lowermost mantle structure is, in fact, due to D" heterogeneity. Almost no leakage occurred from structure initially placed in the uppermost mantle layer.

Several possible sources of lateral velocity anomalies for the lowest layers are explored. Invoking thermal coupling between the mantle and core, one explanation is that the fluid surfaces are deformed due to cold downwellings of lower mantle, and as a result, outermost core fluid. This will give the appearance of lateral velocity anomalies. If lateral velocity anomalies indeed exist, they are likely to be due to a combination of lateral temperature variations and chemical inhomogeneity, suggested by mineral physics relationships.

Thesis Files