| This thesis focuses on two main types of applications related to ambient noise: monitoring the temporal changes of subsurface velocity and imaging the structures of crust and upper mantle. On noise-based seismic monitoring, a new method of measuring velocity change was developed using three-component ambient noise, and applied to the epicentral area of the Ms8.0Wenchuan earthquake and Lushan Ms7.0earthquake to investigate the spatial and temporal changes of crustal velocity. The new understandings are listed as following:1. In our new method based on three-component, the velocity changes are measured from nine component combinations, rather than one component combination in the case of that only vertical component data are used. The average of all nine components increases the stability and accuracy of the velocity change measurements. The measurement of velocity changes in different sub-bands gives constrains on the depth of velocity change.2. We use seismic data recorded by six short-period stations in the Zipingpu Reservoir Seismic Network and one broadband station YZP from2004to2011to measure the velocity change in the period of1-8s (wide range of depth sensitivity from hundreds of meters to about10km). The results show that temporal velocity change has remained steady up to four years before the Wenchuan earthquake.3. The seismic velocity dramatically decreased at the time of the main shock in all period bands. The largest coseismic deduction,～0.2%, is observed in the period band of2-4s (approximate depth of1-4km).4. The seismic velocity overall recovers in approximate logarithmic form in first tens of days after the main shock. The velocity reduction is almost constant following the initial postseismic recovery. The velocity reductions for more than3years after the main shock in the period band of2-4s and4-8s are0.13%and0.08%, respectively.5. In different periods, the lateral distributions of coseismic velocity change are different, but the most significant coseismic changes occurred around the two surface ruptures of the mainshock. 6. Based on our measurements and seismic reflection and local tomography data, we infer that the observed temporal changes in the top4km are likely associated with a combined effect of damages in shallow sedimentary rocks and around active faults.7. In this study, we first found that the variation in the seismic velocities related to the reservoir impoundment. After the start of reservoir impoundment of the Zipingpu Reservoir, a clear negative correlation between velocity change in the period of1-2s and the water level was observed. The variation in the seismic velocity after the Wenchuan earthquake is significantly increased as compared with before the main shock. We hypothesize that opening of fractures in shallow crust by strong shaking of main shock may significantly increase permeability or water mobility, which could in turn enhance the impact of water level on velocity change.8. We use three-component waveforms recorded by13broadband stations about200km around the epicenter of the Lushan earthquake to study the temporal and spatial velocity changes caused by this event. The results show that the seismic velocity in the period of1-2s near the Lushan epicenter is reduced about0.03%, far less than the velocity reduction in the Wenchuan epicenter, which reflect that coseismic velocity changes may largely depend on the magnitude of earthquake.9. We find that even far from the Lushan epicenter (about200km) there are slightly coseismic velocity changes, including the increases of velocity, which are significant compared to the preseismic velocity perturbations. Whether these changes reflect the true coseismic subsurface changes needs further confirmation.On noise-based imaging, we use the continuous seismic waveforms from a dense array including272broadband stations to invert the Rayleigh wave phase velocity and azimuth anisotropy at period ranging from7to50s in northeast China. We obtain following new understandings:1. The Rayleigh phase velocity structure in the period of7s is strongly affected by the surface geological structures. The boundaries of low-velocity anomalies not only clearly map the large basins such as the Songliao Basin, but also display small basins, such as Hailar, Erlian, Xialiaohe and Sanjiang Basin. There are strong azimuthal anisotropy in the Songliao and Xialiaohe Basin, which may reflect the background of extensional tectonics in the formation and evolution of the basins. 2. The maps of phase velocity in the period of12-25s reflect the structure from middle-lower crust to uppermost mantle. The velocity anomalies are generally NE trending in accordance with the surface topography and fault strike. The direction of the fast wave near the Tanlu fault is similar with the strike, which may show the important effect of large fault on regional geology. The Rayleigh wave phase velocity and anisotropy both show significant differences on the two sides of the Gravity Lineament, which may reflect different Moho depths.3. In longer period (35s and50s), the inversion results of lithospheric mantle reveal there are low-velocity anomalies beneath the Changbaishan and Datong volcanoes and Songliao Basin. We also find there is a significant arc-like low-velocity body beneath from Erlian Basin to Bohai Bay, which may be associated with the formation and evolution of the Gravity Lineament. The difference of the azimuthal anisotropy between two sides of the north boundary of North China Craton is reduced, which may manifest the similar evolution history of both sides since the Mesozoic.The results obtained in this paper have some important implications:1. The results of temporal-spatial variations of seismic velocity caused by earthquakes could deepen the understanding of the preparation, occurrence, interaction and hazards of earthquakes.2. The study of seismic velocity changes around reservoir area can provide observational evidence of reservoir-induced earthquakes and also can guide human engineering practice effectively.3. The inversion of Rayleigh wave phase velocity and azimuthal anisotropy in northeastern China can lay a foundation for studying the effect of subducting Pacific slab on the structure of lithosphere. |
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