On 16^th September 2013, an M5.1 earthquake occurred in Badong County, Hubei Province, which is the biggest one since the first water impounding in 2003 in the head region of the Three Gorges Reservoir area. The crustal velocity information is needed to determine the earthquake location and focal mechanism. By comparison, the 1-D velocity structure model from Zhao was adopted in this study. Double difference location method was applied to determine the precise locations of the M5.1 earthquake sequence. Relocation results show that the dominant distribution of this sequence is along NEE direction. In order to understand its seismogenic structure, focal depth profiles were made. Profile AA' was along the sequence distribution, and the earthquake sequence extended about 12km. Focal depth of mainshock is deeper than that of aftershocks, and earthquake rupture propagated laterally southwestward. The seismic profile BB' and CC' were perpendicular to profile AA', which represent the dip direction. Both profiles show that the focal depth becomes deeper toward southeast, and dip angle is about 50°. It means that the possible seismogenic fault strikes NEE and dips southeast. Focal mechanism could provide more information for judging the seismogenic structures. Many methods could obtain the focal mechanism, such as P-wave first motion method, CAP method, and some other moment tensor methods. In this paper, moment tensor inversion program made by Yagi Y is adopted. 12 regional seismic stations ranging from 100~400km are picked up, and before the inversion, we removed the mean and trend. The seismic waveforms were band pass filtered between 0.05 and 0.2Hz, and then integrated into displacement. Green's functions were calculated using the discrete wavenumber method developed by Kohketsu. The focal mechanism of the M5.1 mainshock manifests that the NEE-striking fault plane probably is the possible seismogenic fault, which is consistent with the analysis of focal depth profiles. The focal mechanisms of the M_L≥2.0 aftershocks are retrieved by P-wave first motion method, and the nodal plane I is in accordance with the earthquake sequence distribution and the fault plane of the mainshock. FMSI program was adopted to inverse the stress field in the earthquake area, and the results show that the earthquake sequence is under the control of the regional stress field. The earthquake sequence occurred on the stage of slow water unloading, and ETAS model was introduced to testify the influences of water level fluctuations on earthquakes. The results denote that the reservoir played a triggering role in the earthquake, however, the NEE-striking seismogenic fault is the controlling factor.

On 27^th and 30^th March 2014, an M4.2 and M4.5 earthquake sequence occurred in Zigui County, Hubei Province, and the earthquake sequence type is double seismic type. The two earthquake sequences occurred at the water unloading stage of the 175m trial impounding, and G-R relations showed the similar characteristics with that of the tectonic earthquakes. In order to verify the influences of dam reservoir on earthquake triggering, ETAS model was introduced, the results showed that the slow water level changes had little impact on the occurrence of earthquake. Double difference precision relocation results indicated that the two earthquake sequences occurred at the intersection part of a NE-striking fault and the NNW-striking Xiannvshan Fault, and the preferred direction of aftershock distribution was separately NE and NNW. Moment tensor inversion method and P wave initial motion method were used to determine the focal mechanisms of the two earthquakes, and the results indicated that the two earthquakes were controlled by the regional tectonic stress field and were of reverse-slip type. Comprehensive analysis showed that the M4.2 earthquake was caused by a small-scale fault striking NE with a big dip angle. From the hypocenter profile, it can be seen that the M4.2 earthquake sequence was restrained by an east-dip fault, and the M4.5 earthquake sequence was the product under the conjugate action of the NE-striking fault and the NNW-striking Xiannvshan fault.

On 13, April, 2010, a great earthquake of M_W7.0 occurred in Yushu County, Qinghai Province, which is another big one in China since 2008 Wenchuan earthquake. And with seismic wave data, InSAR data and field investigations, many researchers studied the focal mechanism and source rupture process of this earthquake and many valuable results were obtained. However, there are some arguments on the rupture velocity. Some think that this earthquake is a super-shear rupture event, and some insist on opposite opinion. In order to explore whether it is a super-shear rupture event or not, this study chooses the teleseismic wave data recorded by 33 seismic stations with epicentral distances between 30～90 degrees, good azimuth coverage and high signal-noise ratio to reexamine the rupture process using Yagi's program. By comparison of different given rupture velocities in the range of 2.5～5.5km/s, it is found that rupture velocity of 4.7km/s yields the smallest normalized misfit between the observed and synthetic waveforms. And the inversion result is more in accordance with field observation. The relationship between subfault dimension, rise time and rupture velocity is discussed, which shows that the rupture velocity is not so dependent on the two parameters. And by teleseisemic analyses using an envelope deconvolution method with an empirical Green's function, the location and timing of the high-frequency event also show a rupture velocity of 4.7 to 5.8km/s, which is apparently greater than the shear wave velocity in this region. By comprehensive analyses, it can be concluded that the super-shear rupture exists in this earthquake. According to our inversion result, the strike, dip, and rake angle of this earthquake separately is 300, 88 and 4. Beach ball shows the seismogenic fault is of strike-slip type, which is consistent with the Ganzi-Yushu Fault. And the rupture extended to the surface on the northwest and southeast segments of the Yushu Fault with the length of 19km and 31km. Due to the existence of pull-apart Longbao Basin, the central part where the epicenter is did not rupture. By comprehensive analysis, super shear rupture is one of the main reasons that caused serious damage to Yushu County.

Ruichang-Yangxin earthquake is another moderate-size earthquake in the Yangxin-Jiujiang area since the M5.7 Jiujiang-Ruichang earthquake in 2005.In order to have a better understanding of the seismic activities in this area,we investigate the moment tensor solution and the seismogenic structure of Ruichang-Yangxin earthquake. Precise earthquake relocation shows that the main shock occurred on the southwestern part of the NE-trending fault and aftershocks are distributed along both NNE and NW directions. By comprehensive analysis of the earthquake distribution,isoseismal features,focal mechanism,and regional structure characteristics,it is inferred that the this earthquake is caused by the southern segment of the NNE-trending Tanlu Fault(TLF).In addition,it has close relationship with the conjugated NW-trending fault as well.

Accurate earthquake location is of primary importance in studying the seismicity of reservoir area. The well known program Velest was used to inverse the minimum 1-D velocity,hypocenter location and station correction simultaneously.We selected 529 earthquakes,which were recorded at least by 10 stations each and whose max azimuth gap was less than 180?,to re-determine the velocity model and station corrections with three different initial models used by different researchers before.Using the 1-D minimum velocity model and station correction,we relocated 2138 earthquakes in the Three Gorges area.The average focal depth changes remarkably in different areas,as 5.6km in the area near Xiannvshan Fault,2.9km in the Xietan area and 2.6km in the Badong area.Divided by the depth of 5km,the b values are different,as 1.07,1.16 and 0.90 for all,shallower and deeper events respectively. The frequency of shallower or deeper events per 15 days increases with impounding.But that of the shallower ones drops quickly and the deeper attenuates slowly when water lever keeps stable.

There are many kinds of earthquakes in the Three Gorges area,such as tectonic earthquake,mine earthquake,reservoir induced seismicity and so on.And different earthquakes have different characteristics. But as far as this area is concerned,the majority of the earthquakes belong to micro-earthquakes and small earthquakes,so it is comparatively more difficult to identify their causes and types. The Three Gorges seismic network is a very good platform,which offers us very good earthquake data to do some research on the identification.In this paper,time-frequency spectrum method is used to analyze the difference between tectonic earthquake and mine earthquake,and this is the first step.By comparison,it is found there is obvious difference between them.Firstly,the main frequency of mine earthquake wave is lower than that of tectonic earthquake.And the main frequency of the former mainly centers around 2.2Hz,however the latter is around 5.5Hz.Secondly,the frequency component of tectonic earthquake wave is more abundant than that of mine earthquake,the former's energy density spectrum spreads more broadly along frequency and time axes,being 4 times broader than the latter. And it has many frequency segments with intense energy.Thirdly,the maximum value of energy intensity of mine earthquake wave occurs in all wave train earlier than that of tectonic earthquake and the energy decays quickly.