Numerical Semntatfdtis Research On Subduction Of Pacific Plate To Northeast Chini About The Seismogenic Mechanism Of Deep And Shallow Focus Earthquakes

The Pacific Plate dived into Northeast China up to660km deep, leading to a series of deep earthquakes in Hunchun zones, Heilongjiang province, which was noticed by seismologists at home and abroad for a long time. As the only deep-earthquake belt in China, its occurrence-time, locations and magnitudes are closely associated with Japan trench earthquakes and shallow earthquakes in Northeast China. Until now, seismologists at home and abroad attached great importance to this phenomenon and reaearched it in various related aspects. On the one hand, the research results confirmed the phenomenon, on the other hand, by using inversion, tomographic, statistical analysis and other methods, masses of important results were studied, including the structure of the Pacific subducting plate, lithosphere structure in Northeast China, the earthquake focal mechanism, seismic dynamic mechanism about the deep and shallow earthquakes, the dynamics environment in northeast region. But until now, there is seldom systematic research about the relationship among Pacific plate, deep and shallow earthquakes using numerical simulation method. Therefore, this paper will adopt this method to study their relationships, and further to explore the tectonic stress field and dynamics environment in Northeast China.Based on the above ideas, using finite element numerical simulation method, the following work will be studied with the characteristics of seismic activity in Northeast China in this paper:(1) When the Pacific plate dived to Northeast China with different angles, the stress fields distribution were affected differently. We built two2D vertical models according to the Pacific plate subducting to660km deep in northeast area with different subduction angles, to discuss the role of subduction angles affecting stress field and seismogenic mechanism about deep and shallow earthquakes. In addition, base on the stress fields, we can analysis the relationship between the Pacific plate subduction and the seismogenic mechanism about deep and shallow earthquakes, reseaching different stress fields were caused by different angles respectively.(2) Three group typical sections were built up including the elastic and viscoelastic models to study the stress field distribution of Pacific plate diving to Northeast China,. Based on the actual angle,we built three group models with typical structure sections, such as the northern of Tanlu faults, Changbaishan volcanic zone, Haichenghe fault. Firstly we discussed the stress field distribution of elastic models, then the viscoelastic models, with time scale were also discussed. Through the different stress fields, stress change mechanisms and seismogenic mechanism could be revealed.(3) Take the northern part of Tanlu fault for example, the regional stress field and displacement field were analyzed by numerical simulation analysis. To make the shallow earthquake mechanism clearly, we selected the northern part of Tanlu fault, Dunhua-Mishan fault and Yilan-Yitong fault as the research object to research their stress fields and displacement fields using numerical analysis results, discussed the relationship between structure of fault zones、tectonic environment and shallow earthquakes.Through numerical simulation and comprehensive analysis, the conclusions were as follows:(1) The Pacific plate diving into Northeastern China is the main dynamic resource causing a series of deep-focal and shallow-focal earthquakes, which had the united tectonic stress fields;(2) The stress field numerical simulation results showed that there were two chief areas of stress concentration zone under the subduction of pacific plate, where was the seismogenic zone of deep and shallow earthquakes. So it could be certificated that the relationship about the seismogenic mechanism between deep and shallow earthquakes, which would provide much information about dynamic mechanism about Northeast China.(3) The results of three groups sections with elasticity and viscoelasticity showed that there were some similarity to a certain degree between the two kinds model, which appeared stress concentration effect. The elastic models proved that the deep earthquakes trigger the shallows during each seismic period; And the viscoelastic models proved that the time scale build-up effect of stress field and volcanic stress field are existed.(4) In order to research the regional stress field and displacement field characteristic, we take the north part of Tanlu faults for example, including Dunhua-Mishan fault and Yilan-Yitong fault. The displacement field and deformation field numerical results were given. The results showed that the displacement field and stress field were controlled by the Pacific plate and the GPS observation data. The south part of Dunhua-Mishan fault was the potential danger area, where the tectonic activity was strong and should be furthly monitored. Furthermore, the mantle convection had an effect on the bottom of lithosphere, and then affected the faults. Some low velocity medium existing in the middle crust layer leaded to the occurrence of the shallow earthquakes, which was lied in between7-17km in depth. Meanwhile, these stress field and displacement fields would provide much useful information to study the seismogenic mechanism of shallow earthquakes and analyse the trend of shallow seismic activity.