| Crustal stress field plays a key role in simulation of the tectonic plate motion, interpretation of earthquake mechanism. In this study, an algorithm to determine the stress tensor and its confidence range from focal mechanism data by using grid search method is provided. The experimental verification by using artificial focal mechanism data, shows that the precision of the estimated stress tensor based on this algorithm is greatly improved compared with the traditional algorithms. The confidence range of the determined stress tensor can be given by using F-test. We apply this algorithm in the Capital Region of C hina, and the Tonga-Kermadec subduction zone.1、Dense seismic network has been employed in the Capital Region of China in recent years, so focal mechanisms of small earthquakes can be determined. The focal mechanisms of strong earthquakes in last fifty years and small or moderate earthquakes between 2002 and 2004 are collected. By weighing the focal mechanism data with magnitude of earthquakes, we calculate the present tectonic stress field in the Capital Region of China by the grid search method. Result shows that: the azmuth of the maximum principle compressive stress axes is N43° ~ 86°E in Beijing-Zhangjiakou-Datong region, N38°~86°E in Tangshan region, N79°~81°E in Xingtai region, respectively. The similarities of our results with the previous ones reflect the correctness of our approach. The results reveal both uniformness and sub-region difference of stress field in Capital Region of China.2、The Tonga-Kermadec subduction zone which is a dynamic active region related to subducting from the Pacific Plate to the Australian Plate, is a key region to the global geodynamic study of subduction zone. Firstly, by using the global seismicity EHB catalogs, we investigate the geometry of the Tonga-Kermadec subduction zone(18.5°S~28.5°S) by plane fitting. The result indicates that the strike of slab is ~196° and the dip of slab is ~48° between 18.5°S and 28.5°S. Secondly, we inversed the present-day stress tensor for different sub-areas and depth intervals of study region based on focal mechanisms from Global Centroid Moment Tensor, and obtained stress image of different sub-areas with high resolution. The stress inversion results indicate that the stress regime has strong heterogeneity with coexistence of strike-slip, normal, transition regime in shallow part(60~300km), and the maximum principal compression stress axes rotate at the point of ~24°S, which is associated with the subducting Louisville Seamount C hain. Strike-slip stress regime presents in intermediate part(300~500km), with continuously rotation from north to south of the maximum and minimum principal stress axes, which is probably associated with dynamic action reduction of north-south mantle flow for slab. The normal and transition type of stress regime emerge in deep part(500~700km) with down dip of the compressive stress axes. Different stress regimes between main subd uction zone and the western offset slab are discovered in our study, which confirms the offset slab separate from main subduction zone.In this study, we build an algorithm to determine the stress tensor with high accuracy from focal mechanism data by using grid search method. We test the correctness of the method with synthetic data. By using this algorithm, we determined the stress image of the Capital Region of C hina, and the Tonga-Kermadec subduction zone, which laid a solid foundation for earthquake mechanism explanation of the Capital Region and geodynamics research of the Tonga-Kermadec subduction zone. |
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