Attenuation Tomography Of The Earth Media

Q value which is also called the quality factor of media depicts the inelasticity and inhomogeneity of the earth. Q value is related directly to the attenuation of seismic waves, and it is a key parameter to detect the fissure number, the density of pores and the fluid content in the undergroud media of the earth. The measurement of the lateral variation of attenuation not only supplies extra constraint on the heat structure, viscosity and rheological behavior, but also has considerable significance on interpreting the three dimensional (3-D) velocity structure, so it is a vital parameter to understand the lateral distribution of the seismic wave velocity and density of the earth media. In this thesis, we carried out the attenuation tomography of different-scale media by using different types of seismic waveform data. First of all, we conducted the studies of 3-D fine attenuation structure tomography in three typical reservoir areas by using the local earthquake data and then assessed the infiltration and diffusion status of underground fluid by attenuation structures. We inferred that the possible role of the water infiltration and diffusion to the occurrence and development of the Wenchuan Ms8.0 earthquake from the 3-D attenuation structure before and after the Ms8.0 earthquake in the Zipingpu reservoir area. And then, we performed attenuation tomography studies with the regional scale in the Xinjiang region using Lg waves to obtain high-resolution attenuation images of Lg waves in Xinjiang and adjacent regions. Finally, we carried out the attenuation tomography with the large scale in Chinese mainland based on seismic waves and ambient noise with short and middle periods and we enhanced the calculation efficiency greatly by improving the technology of amplitude extracting. What is worth mentioning that the study of attenuation tomography using the ambient noise data is an original research of performing attenuation tomography by correlation of ambient noise based on a complete theoretical system for the first time, and the complete data processing and calculation programs are formed systematically. After that, we further applied the method in the Chinese mainland to obtain the ambient noise attenuation images with high resolution.Local Earthquake Tomography (LET) is a main method on studying the media structure of typical tectonic zones in small areas, which is widely applied in the fault zones, subduction zones and volcanic areas and reservoir areas. Especially in reservoir areas, LET is a significant method of studying the distribution and status of underground fluid, and it is also an important basis to deduce the relationship among the seismic activities, the fine crust structures and the mechanism of earthquake occurrence.The permeation and diffusion of pore fluid in crustal media in reservoir areas are crurial reasons of reservoir inducing earthquakes, and they are also significant parameters on studying the formation mechanism and further judging the risk of reservoir inducing earthquakes. Combination of the seismic velocity of P wave (Vp), the seismic wave velocity ratio of P wave and S wave (Vp/Vs) and attenuation images are the most important approach in distinguishing the variation of crustal structures in the underground caused by structural discontinuities or fluid infiltration. We conducts the studies of 3-D Vp, Vp/Vs, Qp and Qs tomography in the Longtan, Three Gorges, and Zipingpu reservoir areas using LET technology in this thesis. High-resolution 3-D Vp, Vp/Vs, Qp and Qs images of the three reservoir areas reveal that the complexity of the media around the reservoirs and the influence of fluid permeability on media structures, in which the fluid infiltration and diffusion in fracture zones in the reservoirs may be one of the important origins of earthquake occurrence in reservoir areas. The media structures in the Longtan and Three Gorges reservoir areas show that obviously low Vp, high Vp/Vs, low Qp and Qs distribution characteristics under the shallow zones in the reservoir areas demonstrate the clear fluid infiltration phenomenon of the media in the shallow layers. The low Vp, high Vp/Vs, low Qp and Qs anomalies under the Longtan reservoir head region and the main rivers achieve 4 to 7 km depth, which indicates that the depth of water penetration in the Longtan reservoir may arrive at 4 to 7 km. Fluid penetration around the media of the Xiannvshan fault in the Three Gorges reservoir area may achieve the depth of about 6 km, but the fluid penetration under other main rivers could only arrive at the depth of 2 km or so. The 3-D Vp, Vp/Vs, the Qp and Qs images of the Zipingpu reservoir area show that the water penetration depth in the Zipingpu reservoir area possibly reach the depth of more than 10 km, which may be associated with the existence of deep and large faults around the reservoir area. We infer that the water infiltration in the Zipingpu reservoir area is probably a triggering factor of the Wenchuan Ms8.0 earthquake, but the further evidence is needed by combining the 3-D media structure before the impoundment of the Zipingpu reservoir and the results in this paper. After the Ms8.0 earthquake, the high attenuation region under the Zipingpu reservoir area further expands, suggesting that the Ms8.0 earthquake made the media surrounding the source region rupture obviously, and the fluid further penetrated and diffused along the faults and fissures, therefore led to the high attenuation zone is larger and the depth is greater than those before the Ms8.0 earthquake.Lg waves are often the most prominent phases with maximum amplitudes in the regional seismogram, so attenuation tomography of Lg waves are an important tool in understanding features of regional tectonics and searching anomaly areas of media. In this thesis, we performed attenuation tomography of Lg waves in Xinjiang and adjacent regions with the regional scale. The images of Lg wave attenuation show that the distribution of QLg is closely related to tectonics in the study area. For example, the northeastern Pamirs, the northwest of the Tibetan Plateau, the western south Tianshan Mountains, the north Tianshan Mountains, and the inner regions of the Junggar Basin located at the north of the northern Tianshan Mountains belong to low-Qo areas. On the contrary, the western Tarim Basin, the eastern Tarim Basin, the eastern Tianshan Mountains including the Turpan Hami Basin, the east part of the south Tianshan Mountains, and the north Tianshan Mountains belong to high-qo areas. According to the obvious correlation between the distribution images of QLg and terrain, we consider that Lg waves have obvious features of channel waves. Besides, we inferred that hidden faults may exist in the interior of the two rigid basins by the partition distribution of QLg in the Tarim Basin and the Junggar Basin.The advantage period of surface waves are greater than that of body waves, so surface waves have a good sampling on large-scale tectonic units than body waves. The energy of surface waves is often very large and attenuate slowly than that of other seismic waves when the epicentral distance more than a certain range, as a result, higher resolution of tomography images is also likely to get in the regions of poor station coverage. Therefore, tomography of surface waves is important to detect large-scale tectonic units, and they are also applicable to regions with few earthquakes or regions with low station density. In our research, we extracted the amplitude ratios of Rayleigh waves after phase matching filtering by using Rayleigh waves at the periods of 10 s and 20 s recorded by 188 broadband stations of the China national network and regional earthquake network. And then, we conducted inversions by spectral ratios of two stations and obtained the attenuation images of the surface waves at the periods of 10 s and 20 s in the China Mainland. We developed a method of automatic determination of seismic amplitude spectrum, and we also compared the results with those extracted from manual measurements, showing a good consistency. This improvement implement us to invert a 2-D attenuation model with high resolution of surface waves in the China Mainland based on a large number of seismic data using two-station method. The attenuation models show that the resolution of the models in the east of the China Mainland arrives at about 3°, and the resolution of the models is about 5° in the western China and the margin areas of the China Mainland. The tomography results in this thesis have a good similarity with the existing attenuation distribution of the China Mainland from other researches, and they also have a good consistency to the geological structures.In recent years, the technology of surface-wave tomography using ambient noise has a rapid development. It gets rid of the impact taken by earthquake locations and complex focal mechanisms, and it also not restricted by the irregular earthquakes occurred. So far, tomography of ambient noise is widely applied in velocity tomographic inversions. However, due to the complexity of intensity and distribution of ambient noise which change with the time, locations and directions, the studies on the attenuation tomography by extracting amplitudes from ambient noise correlations are far behind those of velocity tomography.We carried out serials of numerical simulation tests to invert attenuation coefficients of media by extracting surface wave amplitudes from correlations of ambient noise. The results demonstrate that it is feasible to extract Rayleigh wave attenuation from temporal flattening data exactly. After that, we illustrated the whole procedures how to retrieve the amplitudes of Rayleigh waves from ambient noise, and then introduced an improved method of temporal flattening. By comparing the attenuation coefficients to the real coefficients solved from real seismic waves, we consider that the approach is practicable to calculate 1-D attenuation structure from ambient noise. On this basis, we conducted studies of 2-D attenuation tomography. We generated long-time ambient noise records of 100 receiving stations by numerical simulation based on asymmetrical sources of ambient noise and a heterogeneous attenuation model. The study area was gridded by node intervals with two kinds of scales,180 km and 60 km, and tomography inversions were performed based on these two grid nodes separately. The attenuation models after inversions are mostly consistent with the models set in advance. Moreover, the checker-board tests also confirm that it can be successful to invert 2-D attenuation models by using the method and parameters of extracting surface wave amplitudes of ambient noise presented in this thesis.At last of this thesis, we used the real ambient noise data recorded by 146 broadband seismic stations of the China national and regional network, and conducted the study of attenuation tomography from correlations of ambient noise data in the China Mainland. We used the methods of narrow-band filtering and asynchronous temporal flattening to process ambient noise data. After correlating the ambient noise, we obtained the empirical Green functions of Rayleigh waves of more than 10 thousand of station pairs and inverted the Rayleigh wave attenuation coefficients at the periods of 10 s and 20 s by using the methods of phase matching filtering and two-station tomography. The images show that the resolution of images in the southern Xinjiang, the western Tibet Plateau, the eastern China Mainland and the margin of the study area is about 2.5°～5°, and the resolution in other regions of the China Mainland is not lower than 2.5°. The attenuation images agree fairly well with the tectonics, and they are also in accordance with previous research results from attenuation tomography images in the China Mainland. These studies demonstrate that it is feasible to carry out 2-D attenuation tomography by using Rayleigh waves after retrieving Rayleigh-wave amplitudes based on the real seismic ambient noise. Our studies provide another way for attenuation tomography of surface waves and get rid of dependencies on the occurrence of earthquakes in previous attenuation tomography studies. Furthermore, the methods proposed in these studies can improve tomographic resolution greatly, so they have significant application values.