Xiangjiaba Reservoir is currently China's third largest reservoir and began impounding water at the end of 2012. After the impoundment, the water level rose to 71m, while seismic activity near the dam was not significantly increased. At the end of June 2013, the reservoir began impounding water again, the water level continued to rise and flooded the tail region of the reservoir. In the reservoir area and the Xiluodu reservoir area in the upstream, a reservoir seismic network including 35 seismic stations was set up which can roundly record earthquakes in this area. According to the records of the reservoir seismic network from September 2007 to June 2013, only 38 earthquakes with M_L≥1.0 occurred, 0.66 times a month on average, while in July-September 2013, 186 earthquakes with M_L≥1.0 occurred, with an average of 62 events a month, nearly 100 times of that in the past. So, most of the earthquakes are induced earthquakes. At the same time 553 earthquakes with M_L≤1.0 were also recorded in this area. A large number of small earthquakes occurring in the strong earthquake background area have caused a big stir. The source location of these earthquakes are rechecked based on 3D velocity model, 94% of the rechecked focal depth is less than 5km. Based on observations of the reservoir seismic network and vertical P- and S-wave's maximum amplitude ratio method, we inversed 9 focal mechanisms before the impoundment and 69 focal mechanisms after the impoundment in the tail region of the reservoir. Using these focal mechanisms, the stress field of the northern part and southern part of the study area is calculated in order to analyze the characteristic and cause of the induced earthquakes. The results indicate that most of the 69 focal mechanisms are strike-slip type, there is more transitional type, and less normal type and thrust type. The focal mechanisms spatial orientation is complex, fracture types are diverse, which may indicate that the stress state is uneven and the control of regional stress field to small earthquakes is weak. The stress field in the south and north is quite different and not consistent with regional stress field. The north shows compressive stress state while the south shows a state of weak extension. The Yaziba Fault, which passes through the tail region of reservoir, is an active fault, but does not control the induced seismicity, which may indicate that the reservoir storage inhibits the reverse fault activity. Carbonate rocks, limestone and karst cave are developed in the tail region. Analysis believes that reservoir water flows into the caves, penetrates into cracks and joints, leading to increased pore pressure, reducing the frictional strength and fracture strength and increasing reservoir water load which cause elastic deformation, so, it is believed that the combined action of all the above factors is the cause for the induced earthquakes.

There are carbonate rock, limestone and caves in the reservoir head area of Xiluodu Reservoir, which is the third largest reservoir in the world. After the impoundment, the water level has risen to about 140 meters, and consequently, more than 6 000 micro-earthquakes occurred on the reservoir head region, with magnitude of the vast majority being less than 1 and the maximum magnitude M_L3. These micro-earthquakes concentrated within an area of 10km in width from the reservoir banks, 5km in depth, and 40km in length along the reservoir basin. These earthquakes did not affect the safety of the reservoir and dam. We inverted 700 focal mechanisms by using the waveforms recorded by the reservoir's digital seismic network before and after the impoundment, and further inverted the stress field of the whole reservoir head region and the sub-regions. The results show a complex orientation of focal mechanism, different rupture types, and uneven and unstable stress state, which is not in consistency with other regional stress fields obtained by a lot of natural earthquakes, indicating the reservoir induced seismicity is not strictly controlled by the regional stress field. According to the analysis, the reservoir water flows into caves, penetrating into cracks and joints, leading to increase of pore pressure, reducing the friction and fracture strength of rocks, and generating elastic deformation caused by the increased load of reservoir water. The joint actions of these may be the cause of the earthquakes. The accumulated regional stress and local stress were released first, then, the additional stress produced by the reservoir water loading was dominating. There are no major active faults in the reservoir head area. Reservoir water level will rise again by tens of meters in 2014. With the penetration of cracks, the adjustment of stress field, and the backflow of water which will inundate the upstream region of the reservoir basin, the possibility of occurrence of moderate earthquakes cannot be ruled out. The seismic fortification criteria are high for the dam of Xiluodu Reservoir, so these earthquakes will not cause safety problems. We suggest carrying out detailed hydro-geological, geophysical explorations during the continuous active period of the reservoir-induced seismicity to obtain accurate scientific data for determining the causes of induced seismicity and searching for the technical approaches for controlling the induced seismicity. These measurements will mitigate the impact of emergencies and play an exemplary role for the other similar reservoirs.

The precisely located earthquake catalogue is important to seismicity, seismic tomography and crustal stress inversion studies. It also has great application value in rapid report of an earthquake that just occurred. By considering the arrival time uncertainty, and the constraints on station elevation and seismic depth, we propose a relatively accurate method to estimate hypocentral location and its uncertainty based on inversion theory. Our method can combine the arrival times of Pg wave, Sg wave, Pn wave and Sn wave in hypocenter location, so it increases the location accuracy by involving more data; and it can be also used in local and regional earthquake location simultaneously. In order to test our location method, we located earthquakes by using the simulated data with different uncertainty of Pg,Sg,Pn,Sn arrivals. The result shows that the location determined by using our method is more accurate than that by using other method. We apply it to earthquakes occurring in the period from 2001 to 2008 in Sichuan area, and obtained a more clustered hypocentral distribution convergent to the fault zones. The result provides a solid foundation for studies of seismicity, geometry of the active faults and seismic tomography in Sichuan region. It is also helpful to study the seismicity precursors before the Wenchuan earthquake.

In this paper, Sichuan-Yunnan and its adjacent areas are divided into Yajiang, Central Yunnan, Sichuan-Qinghai, Central Sichuan, and Myitkyina-West Yunnan blocks. The mode and rates of motion of these blocks and their boundaries are studied separately. The predominant direction of the principal compressive stress axes within the blocks is analyzed in term of focal mechanism solutions of 442 moderate strong earthquakes. The models of focal faulting are determined through the rake angles λ obtained from source parameters of 771 events with magnitudes about 3 or more, and they are evidenced by focal faulting or slip modes derived from the distributions of N axis plunges of P-wave first motion solutions for moderate-strong earthquakes. A comparison is made between the moment tensor rates for moderate-strong earthquake and the calculated annual average slip rates of each seismotectonic zones within the blocks. Based on regular resurveying of across fault short level lines and baselines during the period of 1980-2001 along the border of the Sichuan-Qinghai, Yajiang and Central Yunnan blocks, the annual average rates of horizontal and vertical deformation for each site are analyzed. The motion rates of the Sichuan-Yunnan Block are inhomogeneous. The Sichuan-Qinghai Block moves toward SEE direction, and the predominant direction of the principal compressive stress axis falls in the range of N100°～130°E. The motion rate of this block is smaller than that of the Yajiang block, implying a delayed motion relative to that of the later. As a result, the Xianshuihe Fault between these two blocks displays a strong left lateral displacement. In addition to earthquakes of strike-slip type, the earthquakes of reverse dip-slip type are also predominant in the Sichuan-Qinghai Block. The predominant direction of the principal compressive stress axes in the Yajiang Block is within the range of N160°～170°E, i.e. SSE. The motion rate of this block is larger than that of the Sichuan-Qinghai Block, and the percentage of strike-slip type earthquakes in this block is larger then that in the other blocks. The motion direction of the Central Yunnan Block is nearly the same as that of the Yajiang Block, leaning slightly to the east. The predominant direction of the principal compressive stress axes in this block is in the range of N150°～160°E, and there are various types of earthquake failure including strike-slip, normal dip-slip, and reverse dip-slip. The direction of the principal compressive stress axes in the Central Sichuan Block falls in the range of N110°～140°E, approximately the same as the motion direction of the South China Block. The percentage of strike-slip type events in this block is relatively low, and there is a portion of events belonging to normal dip-slip or oblique dip-slip types. The motion rates at the northeast boundary between the Yajiang and Central Yunnan Blocks are greater than those at the west and south boundaries. The SSE-directed motion of the blocks is associated with right lateral component. Similarly, as the motion rates of the eastern boundary of the Sichuan-Qinghai Block is greater than that of the Longmenshan belt to the southeast of the block, the SSE-directed motion of the Sichuan-Qinghai Block is also associated with right lateral component. Two predominant directions of the principal compressive stress axes in the Myitkyina-West Yunnan Block indicate the NE-directed squeezing and the SE-directed escaping of the block, while the motion rate of the block is larger than that of the Yajiang and Central Yunnan Blocks. This might be attributed to the dynamic mechanism of the formation of collision and squeezing belts due to the direct action of the Asam Wedge while Indian Plate moving northward.