| The city is developing rapidly in China, with some large facilities such as ultra-high-rise buildings, bridge, road, railway, nuclear-power plants, and so on. It will be devastating for the human life and property if the earthquake or geological disaster occur. Moreover, the urban active fault is a potential threat to induce earthquake and geological disaster. Therefore, the research on urban active fault is significant for urban construction and social sustainable development.The urban active fault has been detected by relevant departments using geological, geophysical and geodesic data in recent years. But the most deep information of fault, which is important for long-term disaster prevention, water resources planning and architecture preservation, is still unknown. Gravity data, reflecting the interior structure of the whole Earth, can be used to obtain the shallow and deep information of fault. In this paper, the high-precision gravity observations were used to detect urban active fault in domestic, aiming at providing scientific evidence for the safety of ultra-high-rise buildings and long-span structure. It could provide direct scientific basis for long-term geological disaster prevention, exploring the reasons of geological disaster and the underground exploration of public-works programme as well.The multi-scale gravity inverse method was studied in the paper:identifying fault by wavelet multi-scale decomposing method from gravity anomaly, detecting fault from multilayer horizontal tectonic stress inverse method and inverting fault parameters based on multi-scale edge of gravity anomaly. Then, they were used to detect Shenzhen active fault in order to obtain comprehensive information of fault. The main work and result were as follows:(1) On basis of reviewing a large number of domestic and foreign literature, the unique advantages and significance of multi-scale gravity for detecting urban active fault were expounded. As the basic theory of this paper, forward and inverse problem of gravity inversion and wavelet multi-scale analysis were discussed.(2) The wavelet multi-scale decomposing method from gravity anomaly for identifying fault was proposed. Based on multi-scale decomposition, estimating corresponding depths and recognition criteria of fault, the spatial distribution of urban active fault could be detect accurately by this method. It could solve effectively the problem that the fault depth was not determined by other data. Shenzhen active fault was detected by the method, and the main results were as follows:1) The depth of fault range from2.5km to20.0km in Shenzhen.2) As the depth increasing, the northwest fault (L1) was westward rotation and the distance between L4and L5in the east-west direction was increasing.3) The largest scale of fault was at6km depth.4) The results were accordance with the results from geological, geophysical and geodesic material. Therefore, the example analysis of active fault in Shenzhen verified the correctness and effectiveness of the wavelet multi-scale decomposing method from gravity anomaly to detect qualitatively the distribution of urban active fault. It also showed the effectiveness to detect fault from high-precision gravity observations.(3) The multilayer horizontal tectonic stress inverse method for identifying fault was proposed. According to the wavelet multi-scale decomposing results from gravity anomly, the multilayer density and horizontal tectonic stress could be obtained by the method. Then, the spatial distribution and stability of urban active fault were further obtained. The method was a new way for determining the fault distribution and stability. The main results from the case study on Shenzhen active fault using the method were as follows:1) The distribution of stress was accordance with the strike of fault, and the strike of fault could be obtained by the relationship.2) Each layer of the crust was nearly equipressure since the horizontal tectonic stress was O.1mp amplitude. It indicates that the main faults in Shenzhen are relatively stable and have no serious impact on planning and construction of the city. The successful study of active fault in Shenzhen verified the correctness and effectiveness of this method. It also showed that there were unique advantages for gravity method to research the interior structure of Earth.(4) The inverse method based on multi-scale edge of gravity anomaly for fault parameters was proposed. The formula of wavelet multi-scale edge of gravity anomaly from fault was derivated. Then, the relationship between fault parameters and wavelet multi-scale edge was established. Finally, the fault parameters were invert by genetic algorithm. The correctness and effectiveness of this method was verified by simulation results. The inverse results of active fault F5in Shenzhen using the method were:1) The fault was normal fault. The burial depths of upper and lower of hanging wall were6.1±2.3km km and19.3±5.6km km, respectively, and the depths of footwall were2.5±2.1km and14.7±5.7km. The dip was50.4°±5.7°.2) The results were consistence with the research of Yu Chenghua (2010).The effectiveness of the method was further verified by the inverse results of active fault parameter in Shenzhen.(5) A multi-scale gravity inverse software was developed independently. Its functions included the preprocessing of high-precision gravity observations, wavelet multi-scale decomposition, estimating the burial depth of source, inverting multilayer density and horizontal tectonic stress, calculating the wavelet multi-scale edge, inverting fault parameters, and so on. It apply to detect urban active fault and study related underground geological structure.In conclusion, the multi-scale gravity inverse method could detect effectively the depth of urban active fault, show dynamic process of fault distribution as the depth changed, evaluate the stability of fault, and determine the fault parameters. It is a new perspective for detecting urban active fault and inverting other related geophysical problem. |
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