| Gravity data is easy to get and it has a great coverage. We can do research on a large regional geology overall and in system using gravity data, and we can study regional geological structures combining with geological, seismic tomography data.Satellite gravity anomaly data and high-precision bathymetric data is collected to analyse, collect and sort geology and tomography data to carry on the structural analysis of subduction zone. Wavelet multiscale analysis method is used to process the abnormal data, and Parker-Oldenburg Iterative inversion method is applied to invert deep Moho, and the results of tomography data is recollected and re-edited, aiming to do the research on subduction zone mode.The exception details and approximation of 1-8 order are obtained by applying 1～8 order wavelet decomposition and reconstruction to Free space gravity anomaly and Bouguer gravity anomaly. Low-order (1-3 order) details reflect the shallow density anomaly’s distribution. Higher order reflects greater range of density and deeper depth of the anomaly body. The fourth-order approximation mainly reflects the updulation of the Moho. Combining with previous research results, the Moho depth distribution can be attained by inverting the fourth-order approximation of the Bouguer anomaly. The optimal inversion parameters are determined by adjusting the reference interface’s depth and density difference between interfaces. In different parts of subduction zones, the depth has great difference. In Philippine Sea area, Moho depth is shallow(about-11～-8 km), local region is even more shallow, with typical oceanic crust properties. Moho depth of Nankai Trough, Ryukyu Trench, Manila Trench and the Philippine Trench is between-25 and -10 km, with the property of transitional crust. Moho depth of Nankai Trough has little difference from north to south. The Ryukyu Trench is divided to two parts as the south one and the north one by Dadong Ridge with vertical strike. Moho depth’s gradient of Manila Trench is large at north and south ends, and is small in the middle. Philippine Trench’s Moho depth changes little from north to south. Okinawa Trough’s Moho depth changes between -25 and -10 km, with continental crust in the north, transitional crust in the south, and oceanic crust in some local region. In the shelf of the South China Sea, Moho depth changes between -28 and -15 km, with transitional crust. In the middle of the basin, Moho depth varies between -15 and -10 km and locally oceanic crust presents.The plate’s subduction angle is affected by many factors. However, among these factors, oceanic plate’s convergence rate have a significant impact on the angle. Generally, as plate’s age is getting older and subduction rate is getting smaller, its subduction angle becomes larger. The tomographic analysis of the study area shows that:from north to south,the rate of the Pacific plate subducttings beneath the Philippine Sea plate decreases and the subduction angle increases. The subduction of Philippine Sea Plate and Eurasian plate are significantly different in different areas. The subduction angle of Philippine Sea plate along Ryukyu Trench is 20°～40°, and the subduction angle and range both increase from north to south. The subduction angle of Eurasian plate along Manila Trench is much larger than Ryukyu Trench. From north to south, its subduction angle changes from approximately 30° to 90° and then to 45°. Meanwhile, the subduction angle of Philippine Sea plate along Philippine Trench is very large. Affected by the subduction of the northwest Eurasian plate, generally it is vertically downward to the transition zone whose depth is 400-660 km, then it gets rid of the impact of Eurasian plate and extends to the west. The change of subduction angle indirectly proves that from north to south subduction rate becomes smaller gradually as a whole. |
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