Imaging upper mantle discontinuities and Earth's small-scale heterogeneities.

摘要

Deep earthquakes (depth > 400km) occurring in subducted slabs are used to image mantle discontinuities. Any reflector at depth 'x' kilometers, above the deep earthquake, can generate underside P- and S-wave reflections, e.g. pXP, sXP, and sXSH. Motivated by these observations, we developed a three-dimensional imaging method in the local dip angle domain to image reflectors in mantle wedges using readily observable underside reflections from multiple earthquakes. An illumination normalization scheme is developed to achieve balanced image amplitudes. Application of this imaging technique to the Tonga subduction zone yields fruitful results as many localized mantle discontinuities are imaged at depths between 90-450 kilometers, which cannot be explained by standard Earth models. Pervasive mantle metasomatism, is invoked to explain these spatially extensive reflectors, which are likely caused by phase transitions due to silica enrichment.Through performing joint waveform modeling of the surface reflection phase and its precursor, the underside reflection off the Moho, we put constraints on the crustal thickness, the Moho compressional and shear impedance contrasts and the Vp/Vs ratio of the crust and the uppermost mantle in the Sea of Okhotsk region. Crustal thicknesses vary from &sim15 km north of the Kurile basin to 19-25 km in the central Sea of Okhotsk and west to the Kamchatka Peninsula. Low Vp/V s ratios (1.6--1.7) were inferred for the uppermost mantle. The presence of fluids and extensive enrichment of SiO2 with possibly low-temperature veining are viable explanations for these anomalous ratios. This may represent an important process for continentalization taking place landward from the volcanic arc in subduction zones.Small-scale heterogeneities within Earth's mantle bear important information on the dynamics of convection and mixing. The amplitude and phase fluctuations of waves traveling through random media are used to form coherence functions, which depend on the spatial lag between stations and the lag of incident angle between different plane waves. These coherence functions are used to invert for the depth-dependent heterogeneity spectra. First, a correct phase measurement technique with phase unwrapping is proposed. Second, previous theory based on homogeneous background is extended to depth variable background velocity using the WKBJ Green's function. Theoretical predictions for coherence functions agree well with results from numerical modeling.In this thesis, other work is also reported this includes prestack Gaussian beam migration, fast three-dimensional offset plane wave modeling and migration, imaging resolution issues in global seismology.