Advances in ambient noise tomography: Measurement, inversion methodology, and inference of azimuthal anisotropy.

摘要

Ambient noise tomography has recently emerged as one of the most important tools to study the crustal and uppermost mantle structure of the earth. In this thesis, I advance the methodology of ambient noise tomography and at the same time study the crustal and upper mantle structures of New Zealand and the western US. On the technique side, I show that (1) the strength of noise correlation signals is anticorrelated with local noise levels, (2) robust Rayleigh and Love wave phase travel time measurements can be obtained from ambient noise correlations without a measurable systematic bias, and (3) with a dense array of stations, a novel inversion technique that I call eikonal tomography possesses many advantages over the traditional straight ray inversion method and can be used to constrain both isotropic and anisotropic structures and their uncertainties. In particular, eikonal tomography allows direct inspection of the robustness of azimuthal anisotropy measurements at each location. On the structural side, the velocity anomalies observed in both New Zealand and the western US are strongly correlated with the major geological features such as the Southern Alps, the Taupo Volcanic Zone, the Taranaki Basin, the Canterbury Basins, and the Hikurangi accretionary prism in New Zealand and the Sierra Nevada, the Snake River Plain, the Colorado Plateau, the Great Basin and the High Lava Plains in the western US. I combine ambient noise tomography, surface wave tomography based on earthquakes, and SKS splitting measurements to study the stratification of azimuthal anisotropy in the western US. The inferred anisotropy model in the western US is consistent with a decoupled crust and an approximately 80 km thick uppermost mantle in which anisotropy is dominated by relatively shallow, regional-scale tectonic processes. Beneath these layers is an approximately 200 km thick more homogeneous asthenospheric layer in which anisotropy appears to be controlled by plate motions and the subduction of the Farallon slab.