| On 12 May 2008, the Mw 7.9 earthquake occurred on the Longmenshan (LMS) fault zone in Sichuan province of China. Geological surveys, inversion of the seismic rupture process and aftershock relocation indicate that this temblor resulted from a thrust on a high-angle fault with right-slip component. In tectonic setting, the LMS is located at the border of the east margin of the Tibet Plateau and South China block with a special seismogenic structure. The study of the fine velocity structure of this fault and earthquake-generating environment is of great importance to understanding the tectonic setting and dynamic process of the Wenchuan earthquake as well as the dynamics of the east edge of the Tibet Plateau.After the Wenchuan earthquake, scientists at home and aboard have made a lot of studies on velocity structure in crust and upper mantle, regional seismogenic conditions, characters of the regional tectonic stress field and so on for the seismic region and its adjacent areas, yielding abundant research results. They involve the study of receiver function inversion, seismic ambient noise tomography and seismic travel time tomography using data from regional seismic network and transportable array across the Sichuan basin, LMS fault, Songpan-Ganzi block and Chuandian block. The results show that there is wide distribution of low S wave velocity in the middle-lower crust of the Songpan-Ganzi block. The lower crust may be in the state of melt or partially melt. Meanwhile, the middle and upper crust beneath the LMS fault has high velocity anomalies. It is inferred that the low-velocity material in the middle-lower crust of Songpan-Ganzi block moves eastward, then upward when blocked by the rigid Sichuan basin crust, causing the rupture of the LMS fault zone. This may be the dynamic process of the Wenchuan quake at depth. The structural differences in the upper and middle crust of the Songpan-Ganzi block and the interaction with Sichuan basin and South China block may be the main control factors of the seismogenic process of the Wenchuan earthquake. The fine middle and upper crust structure beneath the LMS fault zone is the key evidence to confirm the mechanism of the Wenchuan event. However, the special resolution of those work on this sbject is usually larger than 25km, so that it is hard to give a fine structure with respect to the fault width around 30km. Some researchers used the double difference tomographic imaging method to deal with the data of aftershocks of the Wenchuan shock, and obtained the velocity structure of upper crust for the fault zone with lateral resolution around 5-10km. Although the high-velocity anomalies in the hanging wall and deep extension features of the fault are shown in the results, the bottom boundary of these anomalies cannot be seen since the aftershocks are all located above the depth 22km.Seismic ambient noise tomography has been proved to be a new and efficient way to study the crust and upper mantle structure in recent years. This method extracts surface wave Green’s Function between station pairs from continuous ambient noise data, then uses the surface wave tomography and S wave inversion method to obtain the velocity structure. The method has been widely used in the study of crust and upper mental structure in western Sichuan and co-seismic velocity changes for the Wenchuan earthquake. Since the spatial resolution for ambient noise tomography mainly depends on the distribution of the stations, short period surface waves from ambient noise have been increasingly used to study the shallow structure of faults with dense seismometer arrays.A transportable array with 40 broadband seismic stations was deployed across the LMS fault after the Wenchuan earthquake by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. Together with the Weatern Sichuan Seismic Array, it covered the LMS fault from Yaan in the south to Qingchuan in the north. The station spacing was from less than 20km to about 300km. The array operated for 13 months, yielding continuous records.This work is based on the vertical component data of 57 stations of the array aforementioned for 1 year (2008.11-2009.11). It utilizes the short-period seismic ambient noise imaging to invert the S-wave velocity structure of the top 25 km of the central and northern LMS zone. The procedures include:ambient noise data preprocessing, computing cross-correlation functions, measuring group velocity dispersion curves, surface wave tomography, and S-wave velocity inversion. During the preprocessing, a method with short time windows is used instead of time domain normalization (e.g. one-bit) to improve the SNR of cross-correlation functions. This study implements a frequency time analysis to measure Rayleigh wave group velocity for periods of 1-25s, and then uses a generalized inversion program to generate group velocity maps at 2-20s. According to the checkboard tests, the lateral resolution is about 10km. At last, this thesis inverts the group velocity at each grid node for S-wave velocity using a linearized inversion method.It is noted that the LMS fault zone is located on the steep boundary between Songpan-Ganzi block and Sichuan basin. The biggest elevation difference between two sides of the fault is almost 5km. The linearized inversion method is based on assumption of level ground, and does not consider terrain correction. Whereas the influence of the topography in this situation is obviously not negligible, especially when the shallow structure is concerned.In order to reduce the influence of the topography during the inversion, this work introduces a three-dimensional simulation iterative correction method. The whole process of correction is to fine tune the input group velocity curve for each node according to values evaluated from three-dimensional simulation. After nth iterative correction, there is velocity structure denoted by Vsmiu(n) from shear velocity inversion. The Vsmiu(n) is used as the input model for the next simulation with the velocity for terrain elevation that equals to the value at level surface in Vsmiu(n) for each point. After (n+1)th three-dimensional simulation and correction, there is velocity structure denoted by Vsmiu(n+1) from inversion. When the number n is big enough,Vsmiu(n+1) will be very close to Vsmiu(n). Then stable velocity structure is determined with consideration of the influence of topography. Different from the structure obtained before correction, here it uses the velocity value above water level. Several tests on different theoretical models proves that this method is effective.Appling the correction method to the study of the structure of the LMS fault zone, this work finally obtains the fine S-wave velocity structure above depth 25 km of the central and northern parts of the fault zone. The results of this thesis are summarized below.(1)According to the dense seismometer array with spacing 10-300km, using the short-period ambient noise method, this work obtains the middle and upper crust shear wave velocity structure with lateral resolution around 10km and vertical resolution 2-5km. The resolution is as high as results from seismic body wave tomography using arrival times of thousands of aftershocks with similar station density. The results of this work demonstrate the great potential of short period ambient noise tomography with data from dense passive seismic array in the study of fine velocity structure and fault zone imaging.(2) Velocity structure above 10 km keeps good consistency with the surface fault system around Longmenshan, and controls the deep extension features of most major faults. Below the depth of 15 km, the velocity structure exhibits a crossing tectonic pattern along both Longmenshan and Minshan. The complex structure may have affected the rupturing process of the Wenchuan earthquake.(3)The depth-velocity structure profiles give a good constraint on the deep geometry of major faults. The characteristics of the high angle, listric, and reverse structure of the LMS fault zone is further confirmed by the results of this thesis.(4) In southern part of the study area, low-velocity structure is found at about 20km depth beneath the Pengguan massif, which is related to the low velocity layer in the middle crust of the Songpan-Ganzi block. This may be one piece of evidence for the existence of brittle-ductile transition zone in southern rupture zone of the Wenchuan earthquake at the depth around 22km. |
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