Study of deep Earth structure using body waves.

摘要

I present a work in which both source and structure aspects of the observational seismology are studied. In the first chapter of the thesis, we address anomalous behavior of the Long Valley Caldera earthquakes, with moment tensors being dominated by CLVD and volumetric components. We conclude that the the anomalous events may have been triggered by net fault-normal stress reduction due to high-pressure fluid injection or pressurization of fluid saturated faults due to magmatic heating. We currently study non-double-couple earthquakes in Iceland, in order to get better understanding of the physics behind their source processes.; The rest of the thesis is dedicated to studying of the deep mantle structure (and the inner core indirectly) using broadband differential and absolute travel time measurements of mostly P waves that travel through the Earth's core, and those that bounce of the core-mantle boundary. We present three such global datasets of hand-picked high quality measurements: PKP(AB-DF), PKP(BC-DF) and PcP-P, which we hope, will also be helpful in the future studies of the Earth's interior. We confirm by forward modeling and inversion that the lowermost mantle is a very heterogeneous region and that in order to make conclusions about the physical and chemical properties of the Earth's core, these heterogeneities have to be taken into account. Our models of D" are characterized by prominent fast features under mid America and east Asia, a fast belt across Pacific, a slow region under the southwestern Pacific and southern Africa, as well as sharp transitions from fast to slow, for instance under Alaska and south Atlantic. Heterogeneity itself cannot completely explain the trends observed in differential time residuals, when plotted as a function of the angle between P wave paths in the inner core and the Earth's spin axis, but it can account for most of the trend. Inner core anisotropy, with the fast P axis oriented parallel to the Earth's spin axis, which was hypothesized to exist as the main cause of the trends in travel time residuals and inner core sensitive normal modes splitting, thus can not be ruled out. However, anisotropy has to be very complex, which is difficult to reconcile with its plausible physical causes.; We use the core-mantle boundary reflected PcP and ScS seismic waves to investigate the nature of heterogeneities in the lowermost mantle. The PcP-P and ScS-S differential travel time residuals, calculated with respect to 1D Earth reference models, exhibit coherent spatial variations, when the same are projected at the PcP and ScS reflection points. Our observations include good spatial correlation between PcP-P and ScS-S residuals, revealing the existence of short scale heterogeneities previously overlooked by global tomographic models, as well as large discrepancy between the observed PcP-P and ScS-S residuals for some paths bottoming under slow anomalies. All these observations could be explained by the presence of strong chemical heterogeneities at the base of the mantle.