| The earthquakes which are the Violent release pattern of energy in the Earth, give a huge disaster and economic losses to the persons. The Wenchuan earthquake occurred on May 12, 2008, the Haiti earthquake occurred on Jan. 12, 2010 and the Chile earthquake occurred on Feb. 17, 2010 all give tremendous losses and casualties. it has long been a goal of seismologists to really understand the mechanism of the earthquake. There is the chinese largest oil field-Daqing Oilfield in the Northeast china, and there are a large number of industrial and character-intensive areas here,but the Northeast China is located in the Eurasian Plate and the Pacific plate collision zone, near one of the world's two major seismic zone, Central Pacific seismic zone, seismic activity has its particularity. To further study the causes of earthquakes as well as the relationship between crustal structure, the Northeast China is chosen to be the study area.We selected seismological travel time data from 2001 to 2008 in the Northeast seismological stations net and seismological travel time and waveform data from 2007 to 2009, in the 16 stations we laid in the Northeast China. We did the re-positioning, tomography and focal mechanisms to the earthquakes in the Northeast China.to try to explain the cause of the earthquakes and its relationship with the crust-mantle structure and stress field in the Northeast China.For the inversion of geophysical work, the well posed solution is very important, including wellposedness existence, uniqueness and stability. For the inversion problem, the establishment of an appropriate model is very important. The earthquake relocation, tomography and the inversion of focal mechanisms, are based on one-dimensional velocity model. So we first determine the one-dimensional velocity model. The one-dimensional Global velocity model has four kinds. We go through the average standard deviation of the residuals of the theoretical and actual travel time to determine the one-dimensional velocity model. When the average standard deviation is smaller, it illustrate the difference between the theoretical travel time and the actual travel time is smaller and the model is more similar to the actual formation.After the establishment of one-dimensional velocity model, we take the travel time data in the Northeast China to relocate the earthquakes source to analyze the northeast China's spatial distribution of earthquakes, the depth layer of the earthquake based on the re-positioning results,Using the travel time data, we carry on the tomographic imaging of the crust and mantle in the Northeast China using the pseudo-bending ray tracing to study the crust and mantle in Northeast China , the earthquake distribution and its relationship with the crust-mantle structure and the structural characteristics of the Changbai volcano in Northeast China.We carry on the inversion of focal mechanisms with moment tensor inversion in Northeast China to determine the principal stress axis, advocated force axis. The very important issue in moment tensor inversion is the synthetic theory of wave. The Green's function in the waveform synthesis problem is calculated to use the coefficient matrix and the wave number integral method, then according to the synthetic theory waveform fit with the observed waveform , the best focal mechanism is found.According to the relocation research, the earthquakes in the Northeast China can be roughly divided into four area, which are mainly distributed in the fracture areas, mostly in uplifts surrounding the basin area, rarely in the sedimentary basin; In the spatial distribution, the depth of the earthquakes in southern area is deeper and the depth of the earthquakes in northern area is shallower. There are significantly differences between the north and south sides separated by the latitude 44°, which separately has two active earthquake layers.Based on the tomographic result, the horizontal sections can be divided into 3 bands of speed anomaly with nearly NE-SW direction. A band anomalies shows low anomaly in the Northeast, high-speed anomalies in the central, the major changes in the southwest is that the high-speed anomaly changes into the low-speed anomaly from 2km section to 5km section , which changes into high-speed anomaly untile 30km section; B band anomaly always shows low-speed anomaly in the southwest, whose area increases continuously, and it shows low-speed anomaly in the Northeast and quickly disappears and change into high-speed anomaly, take on the low-speed anomaly in the 30km section, and in the central of the section it shows high-speed anomaly and only performance low speed abnormal in the 30km section. C band anomaly performance the low-speed anomalies in the northeast and high-speed anomalies in southwest, and both areas are expanding. From the NE and NW vertical profile sections, the North China plate is composed of "low - high - low - high" four velocity anomalies which is inclining to NW to form a "dome" structure; The northern accretion zone of North China Plate takes on"low-high-low"three velocity anomalies; The Ergun-Xingan masiff presents"low-high-low"three velocity anomalies which is inclining to NE; The SW part of the Songnen masiff which crosses the Songliao basin shows"high-low-high"three velocity anomalies, and the NE part of the Songnen masiff takes on the nearly horizontal"low-high-low-high"four velocity anomalies, and the top layer is thin; The Xingkai masiff and the Jiamusi masiff both show"low-high-low"three velocity anomalies, and compared with the Songnen masiff the stratum has good continuity, but the same characteristic anomaly in the Xingkai masiff and the Jiamusi masiff is shallower than in the Songnen masiff..The Changbai volcano has low velocity body below, ,which is consistent with previous research work which is a volcanic arc plate. The locations of the earthquakes source are mainly in the high anomaly areas or near 0 velocity disturbance line, ie, high-speed anomaly and low-speed anomaly intersection region, which shows that the earthquake occurrence has close relationship with the high-speed anomaly. The earthquakes should occur in the high-speed area or anomalies transition zones. The individual earthquakes in North China plate are in the low velocity zone, which has close relation with the water.The focal mechanism can be divied into two types based on the focal mechanism solutions. Deep earthquake focal mechanism solutions is the typical result of plate subduction, the main plane in the Japan Sea is nearly horizontal and the secondary plane is nearly vertical, so the dive angle is small,and near the northeast, the subduction angle increases; A pair of plane of focal mechanism solutions of the shallow earthquakes in the land is nearly vertical ,which indicate the dextral strike-slip and sinistral strike-slip fault caused with the massif slip movement.Through the focal relocation, tomography and focal mechanism studies, we can see that the Northeast region is influenced by the subduction of the Pacific plate whose subduction angle increases from SE to NW and velocity structural abnormalities among the masiff, make the blocks strike-slip movement to form four major earthquake zone in the deep fault belt.
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