| On 12 May 2008, an Ms 8.0 earthquake, causing heavy casualties and economic losses,occurred in Wenchuan County, Sichuan Province, China. It was the most destructive earthquake since the founding of the PRC and the deadliest one to hit China since the 1976 Tangshan earthquake. This great earthquake occurred along the Longmenshan thrust belt, which is at the eastern margin of the Tibetan Plateau and adjacent to the Sichuan basin. It ruptured over a length of 300km along the NE direction and generated 240-km and 72-km surface rupture zone on the Longmenshan central fault and front-range fault, respectively. There was significantly large range of co-seismic deformation around the Longmenshan fault zone. In the meantime, the physical properties of subsurface material around the seisgenic zone also would be changed caused by the great earthquake. Studying these changes has important significance for understanding the mechanism of the Wenchuan earthquake and the evolution of fault zone and analyzing the seismic hazard. In this thesis, we applied ambient seismic noise correlation, combining with the coda wave interferometry technique, to detect the co- and post-seismic variations of the crustal velocity around the Wenchuan earthquake fault zone.The research of the Earth's interior structure and its temporal variation based on the ambient seismic noise is one of the hottest topics in seismology recently. The interaction of the ocean and atmosphere with the solid Earth creates the ambient noise field throughout the Earth. Computing the cross-correlations of ambient noise at two stations can obtain the interstation empirical Green function (EGF), which can be used to invert the velocity structure. The waveforms of EGF in different period times maintain high degree of similarity, which makes it possible to detect the temporal variations of seismic velocity of the medium using coda wave interferometry. Furthermore, the continuous monitoring of velocity changes can be achieved due to the continuity of the available ambient noise.In this study, we used the vertical component seismograms recorded by seismic stations in the Sichuan digital seismic network during the year 2001 - 2009. The data processing procedure was divided into three phases: (1) single station data preprocess to obtain the ambient noise;(2) cross-correlation and temporal stack to extract the inter-station empirical Green function;(3) estimation of relative velocity change by measuring travel time shifts between the daily and the reference empirical Green function. The key procedure in single station data preprocess was time domain normalization. After comparing some existing normalization method, we made some improvement of running absolute mean normalization, from which we achieved better results. We did a lot of trials and comparative analyses of the techniques (moving-window cross-correlation technique, moving-window cross-spectrum technique, stretching technique and moving-window stretching technique) and parameters for estimating temporal change of seismic velocity. Finally, we chose the moving-window cross-spectrum technique to measure the velocity changes in this study because it can give a stable estimate of velocity changes and own small computing error.Applying the above data processing procedure, we obtained more than 100 station pair results of relative seismic velocity changes around the Wenchuan earthquake fault zone in frequency 0.1-0.5Hz. We selected the results based on the distance of station pairs and the SNR and clock error of EGF waveforms. Then 41 results were selected for further analysis. The results revealed a sudden post-seismic velocity drop for the station pairs across the Longmenshan fault, the largest of which was more than 0.4%. The seismic velocity change with time exhibited a spatial difference: in the southwest segment of the Longmenshan fault, the maximum value of velocity reduction occurred at the time immediately after the Wenchuan earthquake, whereas in the northeast segment of the fault velocity reduction appeared 1-4 months after the main earthquake, which was corresponding to the spatial-temporal distribution of the aftershocks. Furthermore, the spatial extent and magnitude of the post-seismic velocity change in the Sichuan Basin exceeded that in the mountainous regions west of the fault zone. In order to explore the background of seismic velocity change in this area before the Wenchuan earthquake, we processed the data from January 2001 and found that the velocity change in every station pair was always stable and no obvious linear or periodic trends before the Wenchuan earthquake.We discussed the physical mechanism of seismic velocity change observed above. The velocity change calculated by ambient seismic noise correlation reflected the changes of the states and properties of 3D crustal medium around station pair. We analyzed the results combined with other geological and geophysical observation and found that the static stress change and near-surface physical damage caused by strong ground motion could not entirely explain the measurements in this study. We inferred the temporal changes of seismic velocity maybe related to the damages from faults rupture and stress changes around the fault zones.
|
PDF Full Text Request