Small earthquakes have been recorded in Yibin area, Sichuan Province since 1970, the frequency and intensity of seismicity have shown an increasing trend in recent ten years, and the earthquakes are distributed mainly in Changning, Gongxian and Junlian areas. Based on the seismic data from January 2008 to May 2015 recorded by Sichuan and Yunnan regional networks and Yibin local network, seismicity analysis, precise location and velocity structure inversion for earthquakes in Yibin area are carried out, the three-dimensional spatial distribution of seismic activity and the velocity structure at different depths in this region are investigated, trying to analyze the seismic activity law and seismogenic mechanism in Yibin area.
The earthquake relocation result shows that the spatial cluster distribution of earthquakes is more obvious in Yinbin area, the earthquakes are concentrated in Changning-Gongxian and Gongxian-Junlian regions. The seismic activity presents two dominant directions of NW and NE in Changning-Gongxian region, and shows asymmetric conjugate distribution, the long axes of NW-trending and NE-trending seismic concentration area are about 30km and 12km respectively, and the short axes are about 5km. There is a seismic sparse segment near Gongxian, the frequency and intensity of seismicity in the southeast side are obviously higher than that in the northwest side, and the earthquakes with larger magnitude are relatively deep, the focal depth is gradually shallower with the distance away from Gongxian. Seismic activity is sparse in the west and dense in the east in Gongxian-Junlian region, the predominant direction of earthquakes in the seismic dense area of the eastern segment is NE. Seismic activity extends in opposite direction in the easternmost part of the two earthquake concentrated area.
The P-wave velocity structure at different depths in the study area is obtained using joint inversion method of source and velocity structure. In view of the predominant focal depth in this region, this paper mainly analyzes the velocity structure of the upper crust within 10km. Within this study area, the P-wave velocity of earthquake concentration areas is relatively high within 10km of the predominant focal depth, especially in the northwest of Gongxian and eastern Junlian area, the P-wave velocity on the southeast of Gongxian increases gradually with depth, especially at 6km depth. These high-velocity zones are generally related to brittle and hard rocks, where the stress is often concentrated.
Comparing earthquake distribution and velocity structure, seismic activity in this area mainly occurs in high-low velocity transition areas, the inhomogeneity of velocity structure may be one of the factors controlling earthquake distribution. The transition zone of high and low velocity anomalies is not only the place where stress concentrates, but also the place where the medium is relatively fragile, such environment has the medium condition of accumulating a large amount of strain energy and is prone to fracture and release stress.

The Ganzi-Yushu Fault, the boundary of Bayan Har active tectonic block, Qiantang active tectonic block and Sichuan-Yunan active tectonic block, is a sinistral strike-slip fault zone with intensive Holocene activity. Thus, the study of activity characteristics and rupture behavior of paleoearthquakes in the late Quaternary on the Ganzi-Yushu Fault is of fundamental importance for understanding the future seismic risk of this fault. The southeast section of Ganzi-Yushu Fault is made up of three segments of Ganzi, Manigange and Dengke, where a M_S7.3 earthquake in 1866, a M_S7.7 earthquake in 1854 and a M_S7.3 in 1896 occurred, respectively. There is still lack of in-depth study on the active features and the cascading rupture possibility of these segments, which hindered the evaluation of seismic risk for the southeast section of Ganzi-Yushu Fault. By the means of field geological survey and micro topography measurement, this paper studied the geological and geomorphological features of the southeast section of the Ganzi-Yushu Fault. The results show that the Ganzi and Dengke segments show obvious extension movement, in addition to the left-lateral movement. For Manigange segment, the characteristics of the movement are mainly left-lateral strike-slip and thrusting, and the maximum vertical displacement of the Holocene strata is greater than 2m. In part areas, the movement is normal faulting, which perhaps relates to the left stepping zone in the local stress environment. Therefore, combining the research results such as the fracture distribution in different motion characteristics, rupture behavior of paleoearthquakes, and the distribution of historical earthquake surface ruptures, we divide the southeast section of Ganzi Yushu Fault into Ganzi, Manigange and Dengke segment, and consider the Yakou and the Dengke Basin as the stepovers and the segments' boundaries. As the small scale of impermanent barriers including Dengke Basin and the ridge near Yakou, of which the width is about 1~2km, they may be broken through in great earthquake rupture in future. A trench was excavated in Zhuqing township to investigate the paleoearthquakes on the Manigange segment, radiocarbon dating was employed and 3 paleoseismic events were revealed in the Zhuqing trench, which are the seismic events occurring respectively at 3875~3455BC, after 775BC, and the latest one that ruptured the surface. Compared with the previous results of paleoseismology in the southeast section of Ganzi-Yushu Fault, it is found that the paleoseismic events in the Manigange segment are obviously different with that in Ganzi segment and Dengke segment. Due to the lack of sufficient data on the southeast section of the Ganzi-Yushu Fault, it still needs further discussion whether the cascade-rupturing between these segments exists.

The M_S7.0 Jiuzhaigou earthquake in Sichuan Province of 8 August 2017 triggered a large number of landslides. A comprehensive and objective panorama of these landslides is of great significance for understanding the mechanism, intensity, spatial pattern and law of these coseismic landslides, recovery and reconstruction of earthquake affected area, as well as prevention and mitigation of landslide hazard. In this paper, we use the trinity method of space, sky and earth to create a panorama of the landslides triggered by this event. There are 4 roads in the distribution area of the coseismic landslides. The Jinglinghai-Xiamo and Jiudaoguai-Jiuzhaitiantang road sections register the most serious coseismic landslides. The landslides are mainly of moderate-and small-scales, and also with a few large landslides and avalanches. A detailed visual interpretation of the coseismic landslides is performed in two areas of Wuhuahai(11.84km²) and Zharusi-Shangsizhai village(47.07km²), respectively. The results show the overall intensity of landsliding(1088 landslides, a total area 1.514km²) in the Wuhuahai area is much higher than those in the Zharusi-Shangsizhai village area(528 landslides, a total area 0.415km²). On the basis of a scene of post-earthquake Geoeye -1 satellite images, we delineate more than 4 800 coseismic landslides with a total occupation area 9.6km². The spatial pattern of these landslides is well related with the locations of the inferred seismogenic fault and aftershocks. Widely distributed earthquake-affected weakened slopes, residual loose materials staying at high-position slopes and in valleys have greater possibilities to fail again and generate new landslides or debris flows under the conditions of strong aftershocks or heavy rainfalls in the future. Geological hazard from these events will become one of the most serious problems in the recovery and reconstruction of the earthquake-affected area which should receive much attention.

Surface ruptures of the M_S 8.0 Wenchuan earthquake are distributed mainly on the Longmenshan central fault and front-range fault,extending 235km and 72km,respectively.The ruptures exhibit complicated characters in geometry and kinematics.On the riverbed of Baisha river,a backthrust scarp was formed in the southwest of the major fault that comprises four right-step sub-faults,and precise topographical measurement shows a kinematic character of fault block and surface tilting in the rupture process;In Shiyan village,the master fault and the secondary fault formed an imbricate structure,and show as flexures and earthquake swells on ground surface.We analyzed the ruptures at Tongmakan village and Shiyan village,both locating on the central fault,by geometry and kinematics,and the results indicate that the earthquake rupturing along the central fault is mainly characterized by thrusting associated with right-lateral strike-slipping.This result is consistent with the focal-mechanism solutions which are promulgated by U.S.Geological Survey,Harvard University and China Earthquake Networks Center.In addition,the profiles of Tongmakan and Shiyan display different tilting directions,and the reason is that the former locates at the trailing edge of the fault,while the latter in the leading edge of fault.

The geodatabase of scientific investigation of Wenchuan earthquake stores many field investigation data,such as the data of field geological survey sites,the surface rupture belts,the Quaternary faults in the earthquake region,paleoseismological trenches,folds,etc.Moreover,it includes some collection data,such as strata data,the catalog of the Wenchuan mainshock and aftershocks,the relocation catalog of aftershocks of the Wenchuan earthquake,the catalog of historical strong earthquake.The field data are recorded,edited,analysed,mapped and output with ArcGis,a powerful function in Gis software.The geodatabase of scientific investigation of M_S8.0 Wenchuan earthquake was initially established,which includes the basic seismological information and realizes the integrated management of spatial location and attribute information.Data can be queried,analyzed and processed for the need of the correlation analysis of the data.Seismic tectonic map and surface rupture map can be drawn with the geodatabase,which can provide the basis for the determination of safety distance from the risky seismic rupture zones in post-disaster reconstruction and for the construction of the Wenchuan earthquake geographic information system.

At 14:28,May 12,2008,a great earthquake of M_S8.0 occurred in Wenchuan County,Sichuan Province.It produced two ruptures along the Longmenshan Thrust Belt,with the major one of ca.240km along the central fault zone and minor one of ca.70km along the mountain front fault zone,respectively.Baishahe rupture section,located at the southern end of the rupture of Wenchuan earthquake,extends 14km along Baishahe River,11km north of Dujiangyan City.This rupture section presents complicated geometric features,and is composed of several segments.Almost each segment has different strike from others(from 0 to 90°),but the section has an average strike of 50°.Co-seismic slip along the rupture section also shows complexity and multiformity.Up-thrust of the northwest wall is the main vertical co-seismic slip,and the maximum height of fault flexure scarp is 6.5m,but local backward flexure scarps exist there;the right-slip is the main co-seismic strike slip,and the maximum right-slip offset is 4.8m,but local left-slip offset also appears.A rupture plane of 40°/NW∠76° crops out at the northern end of the rupture section.Two groups of striations present on the plane,75°SW and 80°SW.This indicates not only that reverse faulting is major,and strike-slip faulting is minor,but also two rupture events in the earthquake.Based on the analysis of the rupture geometric features and co-seismic slip distribution,we have found the following knowledge: 1)The gradual change of the rupture plane from low angle at source depth to high angle at surface compels the horizontal shortening to transform into uplifting,causing directly the ratio of strike-slip to horizontal shortening approaching to 1,and forming backward fault flexure scarp behind the main rupture zone;2)The direction and amount of strike slip are determined by three factors of strike of rupture,azimuth of reference and gravity;3)The thickness of the overlying unconsolidated deposits affects the scope and style of rupture at the surface.

On the 12th of May,2008,a devastating M_S 8.0 earthquake struck Wenchuan County,Sichuan Province.The earthquake rupture extends about 200km,and the maximum co-seismic strike-slip displacement and vertical displacement are(4.8±0.2)m and(5±0.2)m,respectively.Only one fault plane(N32°E/NW∠76°)has been found on the southern section of the rupture.The high dip of the fault plane and high angle stria on the fault plane show that the fault is dominated by reverse motion,with a small amount of right-lateral strike-slip component in this segment.