The relationship between large-scale landslides and active faults has attracted much attention. From the point of view of active tectonics and disaster geology, the late Quaternary activity of the Jinsha River fault zone is investigated and studied, and the relationship between large-scale landslides and activity of the Jinsha River fault zone is emphatically analyzed. The Jinsha River fault zone was formed during the closure of the Paleotethys Ocean. According to the distribution of the 5km-wide ophiolitic melange zone, the ultramafic rock zone, and the local migmatization and progressive metamorphism around the Variscan intermediate acid intrusive rock mass distributed along the fault, it is inferred that the fault zone was once a strongly active superlithospheric fault zone with obvious compressive properties. The Jinsha River fault zone is a large-scale, long-term active suture structure, with many branches, forming a 50km wide structural fracture zone. Affected by the eastward compression of the Tibet Plateau, it has changed into a strike-slip fault zone characterized by dextral shear since Pliocene. In the study area, the fault landforms are clear along the Zengdatong, Xulong, Nizhong, Lifu-riyu, Langzhong and Guxue faults, which are mainly manifested as straight fault trough, linear ridge, fault scarp, and directional aligned fault facets. Results of field geological and geomorphological investigation and chronology show that the late Pleistocene and Holocene deposits are faulted, indicating the faults are active during the late Quaternary and dominated by dextral strike-slip with an average horizontal slip rate of 3.5~4.3mm/a in Holocene. The study area is located in the middle and north of the world-famous Jinsha River suture of the north-south structural belt in Sichuan, Yunnan and Tibet, and the geological structural conditions are very complex. The main structural line is distributed in NS direction, interwoven with NE and NW faults and fold axes in network shape, and the structure is complex. Strong neotectonic movement, huge topographic elevation difference, steep mountains, dry-hot valleys microclimate and other factors have caused serious internal dynamic geological disasters on both banks of Jinsha River. The landslide in the area has the characteristics of high frequency, large scale and serious damage. There are 23 large-scale and super large-scale landslides in the main stream and its tributaries of Jinsha River within the 38km-long section from Narong to Rongxue. Most of them are super large-scale landslides with a volume of more than 10 million cubic meters, even have a volume of more than 100 million cubic meters. Most of the landslides are located within 1km on both sides of faults, and many of them are developed on the fault zone. The occurrence of these large-scale landslides is closely related to the long-term activity, evolution history and complex structure of Jinsha River fault zone along the river, as a result, the rock mass structure gets fragmented and the continuous tectonic activity becomes the main cause of landslides. Active faulting is the fundamental controlling factor for the occurrence of large landslides along the river, especially for large landslides, and is an important internal dynamic condition for the formation of landslides. Further analysis of the fault structure shows that landslide is closely related to the movement evolution history of Jinsha River fault zone. Special structural combination parts(mechanical mechanism)such as closely adjacent faults, acute angle area of fault intersection, right turning parts of the faults and the intersection area between the main faults and the transverse faults are the key sites where the tectonic stress is easy to concentrate, thus conducive to generating large-scale landslides. Many large landslides occur in these structural parts. The controlling effect of active faults on landslides is not only embodied in the process of large earthquakes, but also can lead to the intensive occurrence of large and super large landslides in a natural state(non seismic action). This research has positive scientific significance for understanding the formation mechanism and development law of landslides on both sides of Jinsha River, and for understanding the relationship between fault activities and large landslides.

The Weixi-Qiaohou Fault is located in the west boundary of Sichuan-Yunnan rhombic block, and also the north extension segment of active Red River fault zone. Strengthening the research on the late Quaternary activity of Weixi-Qiaohou Fault is of great theoretical and practical significance for further understanding the seismogeological background in northwest Yunnan and the structural deformation mechanism of the boundary of Sichuan-Yunnan block. Based on the 1︰50 000 active fault mapping and the research results of the National Natural Science Fund project, this paper mainly elaborates the latest active times of the fault and paleoseismic events along it revealed by exploration trenches at Matoushui, Shiyan, and Yushichang. Matoushui trench revealed three faults developed in late Pleistocene and Holocene pluvial fan accumulation, and the latest ages of faulted strata are(638±40)a BP and(1 335±23)a BP, respectively. The Shiyan trench revealed six faults, three in the western section and three in the eastern section. The three faults in the western section dislocated the late Pleistocene and Holocene accumulation, and the ¹⁴C ages of the latest faulted strata are(4 383±60)a BP, (4 337±52)a BP and(4 274±70)a BP, respectively; the other three faults revealed in the eastern part of the trench offset the Holocene fluvial facies accumulation, the ¹⁴C age of the latest faulted strata in the footwall of the main fault is(9 049±30)a BP, and the ¹⁴C ages of two sets of faulted sag pond deposits in the hanging wall are(1 473±41)a BP and(133±79)a BP, separately. Five active faults are revealed in Yushichang trench. Among them, the F₁ and F₂ dislocated the gray-white gravelly clay layer and the black peat soil layer. The ¹⁴C age of the gray-white gravelly clay layer is(1 490±30)a BP, and ¹⁴C ages of the upper and lower part of the black peat soil layer are(1 390±30)a BP and(1 190±30)a BP, respectively. The F₃ and F₄ faults offset the gray-white gravelly clay layer, the black peat soil layer and the brown yellow sand bearing clay, and the OSL age of brown yellow sand bearing clay is(0.6±0.2)ka. The F₅ fault dislocated the gray-white gravelly clay layer, its ¹⁴C age is(1 490±30)a BP. According to the relationship between strata and the analysis of dating data, the Yushichang trench revealed two seismic events, the first one occurred at(1 490±30)~(1 390±30)a BP, as typified by the faulting of F₅, the second paleoseismic event is represented by the faulting of F₁, F₂, F₃ and F₄.The F₁ and F₂ faulted the gray-white gravelly clay layer and the black peat soil. Fault F₃ and F₄ dislocated the gravelly clay, the peat soil and the sandy clay, and a seismic wedge is developed between fault F₃ and F₄, which is filled with the brownish yellow sandy clay. The OSL dating result of the brownish yellow sandy clay layer is(0.6±0.2)ka. Judging from the contact relationship between strata and faults, F₃ and F₄may also faulted the upper brownish yellow sandy clay layer, but the layer was eroded due to later denudation. Therefore, fault F₁, F₂, F₃ and F₄ represent the second event. Combined with the analysis of fault scarps with a height of 2~2.5m and clear valley landform in the slope near the fault, it is estimated that the time of the second paleoearthquake event is about 600 years ago, and the magnitude could reach 7. The trench at Gaichang reveals that the seismic wedge, soft sedimentary structure deformation and the medium fine sand uplift(sand vein)and other ancient seismic phenomena are well developed near the fault scarp. All these phenomena are just developed below the fault scarp. The vertical dislocation of the strata on both sides of the seismic wedge is 35cm, and ¹⁴C ages of the misinterpreted peat clay are(36 900±350)a BP and(28 330±160)a BP, respectively, so, the occurrence time of this earthquake event is estimated to be about 28 000a BP. If the fault scarp with a height of 2m was formed during this ancient earthquake, and considering the 0.35m vertical offset revealed by the trench, the magnitude of this ancient earthquake could reach 7.The Matoushui trench revealed three faults, which not only indicated the obvious activity of the faults in late Pleistocene to Holocene, but also revealed two paleoseismic events. Among them, the OSL age of the faulted sand layer by fault F₁ is(21.54±1.33)ka, which represents a paleoearthquake event of 20 000 years ago. The faulted strata by fault F₂ and F₃ are similar, which represent another earthquake event. The ¹⁴C dating results show that the age of the latest faulted strata is(638±40)Cal a BP, accordingly, it is estimated that the second earthquake time is about 600 years ago. A clear and straight fault trough with a width of several ten meters and a length of 4km is developed from Meiciping to Matoushui. Within the fault trough, there are fault scarps with different heights and good continuity, the height of which is generally 3~5m, the lowest is 2~3m, and the highest is 8~10m. Tracing south along this line, the eastern margin of Yueliangping Basin shows a fault scarp about 5m high. After that, it extends to Luoguoqing, and again appears as a straight and clear fault scarp several meters high. In addition, in the 2km long foothills between Hongxing and Luoguoping, there are huge rolling stones with diameters of 2~5m scattered everywhere, the maximum diameter of which is about 10m, implying a huge earthquake collapse occurred here. According to the length, height, width and dislocation of the rupture zone, and combined with the experience of Yiliang M≥7 earthquake and Myanmar Dongxu M7.3 earthquake, this earthquake magnitude is considered to be ≥7.

The Sudian Fault extends in nearly NS direction and crosses the border between China and Myanmar, with a length of about 100km. Historically, many earthquakes have occurred along the fault. However, restricted by traffic, climate and other factors, there has been little research on the late Quaternary activity of the fault for a long time. On the basis of results of field geological and geomorphological investigation, trenching and geochronology, the movement characteristics of the fault in late Quaternary, the latest active age and sliding rate are analyzed in this paper. The Neotectonic activity of the Sudian Fault is obvious. Beaded Quaternary basins in areas of Sudian, Mengdian, Huangcaoba and Longzhong have developed along the fault. Many boiling springs and gas springs are distributed linearly in the area of Humeng in the south section of the fault. The fault controls the Lama River and Zhanda River obviously. Fault landforms are mainly characterized by clear fault scarps, straight linear ridges and fault valleys. Mengdian pull apart basin is developed in the middle segment of Sudian Fault. In the Zuojiapo area of the western margin of the basin, there is a clear linear ridge about 1.7km long and a parallel fault valley which is close to the west side of the linear ridge. Trench excavation was carried out in this fault valley(24.97°N, 97.93°E). Zuojiapo trench reveals that three faults have developed in Quaternary deposits. At the position of 2~3m(from west to east)on the S wall of the trench, a fault dislocated all the strata(unit②~unit⑥)below the modern loam layer(unit①). These strata are obviously offset and some of them are cut off. The ¹⁴C age of the displaced unit ④(tested by BETA laboratory, USA)is(7 680±30)a, two ¹⁴C ages of the displaced unit ③are(6 970±30)a and(5 860±30)a, and the ¹⁴C age of the displaced unit ②is(1 260±30)a. The fault developed at 21m in the east section of S-wall of the trench has offset the lower bedrock(unit⑧), the middle gravel layer(unit⑤and unit⑦), the upper dark gray gravelly clay layer(unit④)and the peat interlayer(unit④'). In the peat interlayer(unit④'), there is obvious structural deflection deformation, and its ¹⁴C age is(350±30)a. There is another fault developed at 26~27m in the east section of S-wall of this trench, which cuts off the light yellow and light gray gravelly clay(unit ②), gray black gravelly clay(unit③), gray white sandy gravel(unit⑤), yellow gravelly silty clay(unit ⑥), yellow clay gravel(unit ⑦)and hornblende schist and quartz schist of Gaoligongshan group(unit ⑧). The fault shows obvious normal fault property, and the maximum offset is 1.3m. A 10cm wide schistosity zone is developed and gravels are arranged along the fault plane. The ¹⁴C ages of the faulted upper stratum(unit ③)are(1 100±30)a and(870±30)a. The N-wall also reveals the existence of faults, corresponding to the S-wall of the trench. These faults and dislocated strata fully indicate that the fault was active during the Holocene. According to field investigation, the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. For example, in Mengnong tea field, obvious synchronous dextral displacement occurred in three gullies along the fault. From south to north, the displacements of the three gullies are 40m, 42m and 45m, respectively. Shutter ridge landform is developed at the gully mouth. In the lower part of the northernmost gully, there is a pluvial fan, and the ¹⁴C age of the bottom of the pluvial fan is(13 560±40)a, which is less than the formation age of the gully, but roughly represents the formation age of the gully, indicating that the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. In Sudian area, the Mengga River is right-laterally offset 1 050~1 100m by the fault. At 1.7km north of Sudian, a diluvial fan is right-laterally offset 18~22m. There are fault scarps with a height of 1~1.5m developed on the alluvial fan, Quaternary faults and bedrock fault scarps with a height of about 8m developed on its extension line. The three points of the scarps, Quaternary faults and bedrock scarps are in a straight line, which absolutely shows the reliability of the dislocation of the alluvial fan. An organic carbon sample is obtained 1.8m below the alluvial fan, and its ¹⁴C test age is (6 210±30)a. This age should be close to the formation age of the pluvial fan, indicating that the fault underwent obvious horizontal dextral strike-slip movement during the Holocene. In the Sadung Basin, Myanmar, a river is offset about 380m right-laterally, forming a hairpin bending landform. Due to the continuous collision between the Indian plate and the Eurasian plate, the Indosinian block in the southeastern margin of the Tibet Plateau around the Eastern Himalayan Syntaxis escaped southerly, and the western Yunnan became the most intense part of the south extrusion. During the southerly escapement of the Indosinian block, the right-lateral strike-slip movement of Sudian Fault and other faults striking near SN plays a role in adjusting and absorbing the block strain. Under the action of current NNE tectonic stress field, the intersection of the dextral strike-slip Sudian Fault striking NS and the sinistral strike-slip Dayingjiang Fault striking NE is the key part of tectonic stress concentration, which will be the seismic risk area to be focused in the future. The research result of late Quaternary activity of the fault is of great practical significance for the correct understanding and reasonable assessment of the medium to long-term strong earthquake risk in this area, and for the mitigation and prevention of the earthquake disaster in the border area.

The 2014 Jinggu M6.6 earthquake attacked the Jinggu area where few historical earthquakes had occurred and little study has been conducted on active tectonics. The lack of detailed field investigation on active faults and seismicity restricts the assessment of seismic risk of this area and leads to divergent view points with respect to the seismotectonics of this earthquake, so relevant research needs to be strengthened urgently. In particular, some studies suggest that this earthquake triggered the activity of the NE-trending faults which have not yet been studied. By the approaches of remote sensing image interpretation, structural geomorphology investigation and trench excavation, we studied the late Quaternary activity of the faults in the epicenter area, which are the eastern margin fault of Yongping Basin and the Yixiang-Zhaojiacun Fault, and drew the conclusions as follows:
(1)The eastern margin fault of Yongping Basin originates around the Naguai village in the southeastern margin of Yongping Basin,extending northward across the Qiandong, Tianfang, and ending in the north of Tiantou. The fault is about 43km long, striking near SN. The linear characteristic of the fault is obvious in remote sensing images. Structural geomorphological phenomena, such as fault troughs, linear ridges and gully dislocations, have developed along the faults. There are several dextral-dislocated gullies near Naguai village, with displacements of 300m, 220m, 146m, 120m and 73m, respectively, indicating that the fault is a dextral strike-slip fault with long-term activity. In order to further study the activity of the fault, a trench was excavated in the fault trough, the Naguai trench. The trench reveals many faults, and the youngest strata offseted by the faults are Holocene, with ¹⁴C ages of(1 197±51)a and(1 900±35)a, respectively. All those suggest that it is a Holocene active fault.
(2)The Yixiang-Zhaojiacun Fault starts at the southeast of the Jinggu Basin, passes through Xiangyan, Yixiang, Chahe, and terminates at the Zhaojiacun. The total length of the fault is about 60km, and is a large-scale NE-trending fault in the Wuliangshan fault zone. Four gullies are synchronously sinistrally dislocated at Yixiang village, with the displacements of 340m, 260m, 240m and 240m, indicating that the fault is a long-term active sinistral strike-slip fault. A trench was excavated in a fault trough in Yixiang village. The trench reveals a small sag pond and a fault. The fault offsets several strata with clear dislocation and linear characteristic. The thickness of strata between the two walls of fault does not match, and the gravels are oriented along fault plane. The offset strata have the ¹⁴C age of(2 296±56)a, (3 009±51)a, and(4 924±45)a, respectively, and another two strata have the OSL age of(1.8±0.1)ka, (8.6±0.5)ka respectively, by which we constrained the latest paleoearthquake between(1.8±0.1)ka(OSL-Y01)and(378±48)a BP(CY-07). This again provides further evidence that the fault is a Holocene fault with long-term activity.
(3)Based on the distribution of aftershocks and the predecessor research results, the 2014 Jinggu M6.6 earthquake and the M5.8, M5.9 strong aftershocks are regarded as being caused by the eastern margin fault of Yongping Basin, which is part of the Wuliangshan fault zone. The seismogenic mechanism is that the stress has been locked, concentrated and accumulated to give rise to the quakes in the wedge-shaped area near the intersection of the SN and NE striking faults, which is similar to the seismogenic mechanism in the southwest of Yunnan Province.

The 3 August 2014 Ludian, Yunnan M_S6.5 earthquake has spawned more than 1, 000 landslides which are from several tens to several millions and over ten millions of cubic meters in volumes. Among them, the Hongshiya and Ganjiazai landslides are the biggest two with volumes over 1 000×10⁴m³. The Hongshiya and Ganjiazai landslides are two typical landslides, the former belongs to tremendous rock avalanche, and the latter belongs to unconsolidated werthering deposit landslide developed in concave mountain slope. Based on field investigations, causes and formation mechanism of the two landslides are discussed in this study. The neotectonic movement in the area maintains sustainable uplifting violently all the time since Cenozoic. The landform process accompanied with the regional tectonic uplifting is the violent downward erosion along the Jinshajiang River and its tributary, forming landforms of high mountains and canyons, deeply cut valleys, with great height difference. The regional seismo-tectonics situation suggests that:Ludian earthquake region is situated on the southern frontier boundary of Daliangshan secondary active block, and is seismically the strongest active area with one earthquake of magnitude greater than M5.0 occurring every 6 years. Frequent and strong seismicity produces accumulated effects on the ground rock to gradually lower the mechanical strength of slopes and their stability, which is the basis condition to generate large-scale collapse and landslide at Hongshiyan and Ganjiazhai. The occurring of Hongshiyan special large rock avalanche is associated with the large terrain height difference, steep slope, soft interlayer structure and unloading fissures and high-angle joints. The formation mechanism of Hongshiyan rock avalanche may have three stages as follows:Stage 1, when P wave arriving, under the situation of free surface, rocks shake violently, the pre-existent joints(in red)parallel to and normal to the river and unloading cracks are opened and connected. Stage 2, on the basis of the first stage, when S wave arriving, the ground movement aggravates. Joints(in green)along beds develop further, resulting in rock masses intersecting each other. Stage 3, rock masses lose stability, sliding downward, collapsing, and moving over a short distance along the sliding surface to the inside of the valley, blocking the river to form the dammed lake. The special large landslide at Ganjiazhai is a weathering layer landslide occurring in the middle-lower of a large concave slope. Its formation process may have two stages as follows:Firstly, under strong ground shaking and gravity, the ground rock-soil body around moves and assembles to the lower of the central axis of the large concave slope, which suffers the largest earthquake inertia force and firstly yields plastic damage to generate compression-expansion deformation, because of the largest water content and volume-weight within the loose soil of it. Secondly, in view of the steep slope, along with the compression, the plastic deformation area enlarges further in the lower of slope, giving rise to a tensional stress area along the middle of the slope. As soon as the tensional stress exceeds the tensile strength of the weathering layer, a tensional fracture will occur and the landslide rolls away immediately making use of momentum. This two large landslides are the basic typical ones triggered by the M_S6.5 Ludian earthquake, and their causes and mechanism have a certain popular implication for the landslides occurring in this earthquake region.