5.1 Faulting characteristics
At the intersection of LFZ and the Jinsha River, a large number of signs of tectonic activity are exposed in the Quaternary strata on both sides of the river. The surface deformation of fluvial terrace and the records of fault activity preserved in sediments are typical in the vicinity of Laojie in Qiaojia segment of the Jinsha River. The records of structural faults in each typical profile are as follows.
5.1.1 The profile to east of Laojie
The profile is located in the quarry on the third terrace on the east side of Laojie terrace, which is a manual excavation profile. At the outcrop of the fault, the profile is about 9m high and can be divided into four layers (Fig. 4). The fault is developed in the gravel layer in the profile, and its strike is NW, and its occurrence is 45°∠63 °. The gravels are oriented along the fault surface, cemented by carbonate under the action of precipitation leaching and deposition, forming a dense thin layer with a thickness of about 40cm. The top boundary of the gravel layer on the profile shows the hanging wall thrust of the fault, which indicates that the fault has obvious thrust property.
5.1.2 The profile near the confluence of Niulan river
The profile is located at the second terrace 1.7km southwest of Niulan River estuary on the right bank of the Jinsha River (Yunnan side), which is a manual excavation profile. The profile features are shown in Fig. 5a, and the structure is shown in Fig. 5C. The strike of the fault is NE, and the occurrence of the fault plane is 292 °∠78 °.The fault contact relationship between the dolomite block in the middle of the profile and the gravel layer on both sides shows that the fault has the characteristics of flower like structure. According to the scratch marks on the dolomite, the fault is characterized by thrusting and dextral strike slip (Fig. 5b), and the sediment in the upper part of the dolomite is disturbed (Fig. 5C), indicating that the fault was active after the formation of the accumulation.
5.1.3 The profile to southwest of Shanjiang Township
The profile is located in the first terrace accumulation 1.2km southwest of Shanjiang Township on the left bank of the Jinsha River (Fig.6). The exposed stratum of the profile is mainly gravel layer, which is arranged directionally and the bedding of fine sand layer is clear; There are many positive cycle rhythmic layers from bottom to top. The vertical fault distance is about 18cm. The strike of the fault is NE, the apparent dip angle is small, and the apparent dip is SW. It is a normal fault. The OSL age of the fine sand layer directly overlying the latest faulted formation is 7.1±8 ka (Table1), indicating that the latest fault activity occurred in the early-middle Holocene.
5.1.4 The profile to Southwest of Changpingzi
The fault occurs in the accumulation of the second terrace 0.8km southwest of Changpingzi, and is composed of a group of NE trending faults. The cap layer at the top of the profile is gravel. The middle and lower parts are mainly faulted strata, with fine sand layers on both sides; The middle gravel layer has coarse bedding, and its occurrence is nearly horizontal. There is little fine material in the gravel layer, which is supported by gravel, the south-east side thrusts over the fine-grained sediments(Fig. 7), the whole has extrusion characteristics. Several secondary faults in the profile have a fault core composed of oriented gravel (Fig. 7b, Fig. 7c), showing strong structural deformation; Precipitation leaches and deposits calcium carbonate along the profile, causing the profile to be lighter than both sides, is light gray. Most of the faults are steeply dipping, and the faulting surface tends to be opposite to the topographic slope; The one to the northwest is slightly gentler, with an occurrence of 344°∠55°. The OSL samples collected from the fine-grained sediments of the faulty strata on the hanging wall of the fault are dated to 8.7 ± 1.3ka, and the OSL age of the overlying sandy sediments covering the faulty strata is 1.3 ± 0.1ka (Table 1), which shows that the latest active time of the fault is from middle Holocene to Late Holocene.
5.2 Characteristics of sand veins
Sand veins are widely found in Quaternary deposits in the study area. Typical profiles are as follows:
5.2.1 The profile to southwest of Laojie
The sand vein is generally located in the inner part of LFZ. The exposed profile is formed by manual sand mining and is located in the sand gravel layer above the accumulation of the fourth stage platform in the southwest of Laojie (Fig. 8). The original sedimentary strata are slightly stratified. The veins are in various forms, mainly composed of fine sand, and the degree of cementation is higher than that of the strata on both sides. The width of vein body is about 3-10cm; The boundary is clear; Most of the veins cross the strata and intersect with the horizontal bedding at a large angle. The direction is variable, mainly in vertical or oblique direction (Fig. 8b). Most of the veins are cemented with CaCO3 caused by precipitation leaching, and the whole veins are white or light gray (Fig. 8d).
5.2.2 The profile near Xiaoniulan River bridge
The profile is located in the accumulation at the top of the second terrace near the Xiaoniulan River bridge (Fig. 9), and is distributed 800m to the east of LFZ. It is a manual excavation profile. The height of the profile is about 40m. The profile deposits is more complex. The upper left part of the profile is gravelly sand deposit with small angle bedding and low gravel content. In the middle and upper part, there are sand and gravel deposits with well-developed oblique bedding and mixed accumulation, and the bedding is less and less obvious in the lower part. From the top to the bottom, a small amount of medium gravel, boulder and medium gravel accumulated, and almost horizontal bedding gravel bearing sand layer appeared at the bottom. Sand veins mainly exist in the upper part of the profile (Fig. 9a). The main features are that the veins are straight and slender, about 3 ~ 5cm wide, with clear boundary. The sand veins are obliquely intersected with the primary bedding, and the apparent dip of the sand veins in the upper left and middle upper part of the profile is opposite. The color of the sand vein is lighter than that of the strata on both sides, which may be due to carbonate deposition caused by precipitation leaching..
The response of loose surface sediments to ground motions is mostly related to the liquidization mechanism(Li et al. 2021), and the liquefaction mechanisms with important sedimentary significance include liquefaction and fluidization(Li et al. 2021). There are many mechanisms of sediment liquefaction(Bridge and Demicco 2008): inverse density gradients is a common driving mechanism, and the liquefaction caused by this mechanism is mainly distributed along the interface (usually bedding) with the largest change of reverse density gradient, and is mainly characterized by curly bag shape; At present, there are many uncertainties about the contribution of fluid shear stress to sand liquefaction(Bridge and Demicco 2008). However, if such liquefaction exists, it will be distributed along the interface between fluid and sediment, and has obvious directionality. Under the action of gravity, the loose sediments will also liquefy when they are dumped along the slope (Allen 1985; Horowitz 2010; Owen 2010). This liquefaction is limited in the sliding boundary zone with a certain thickness, which has the properties of brittle and plastic transition, and has obvious shovel shaped characteristics on the profile. Fluidization is mainly related to the rising velocity of pore fluid. The strata of Laojie southwest profile (g) and Xiaoniulan River bridge profile (h) are mainly composed of sand and gravel, which are difficult to liquefy. The vein is mainly composed of fine sand, which is mainly interpenetrated in the middle and upper part of the sand gravel profile, and does not have the profile structure characteristics of the above mechanism. Therefore, the possible explanation mechanism is that the instantaneous vibration deformation caused by earthquake increases the pore water pressure in the sand body. When the pore water pressure rises to zero, the sand body is suspended in water, The total loss of strength and bearing capacity (Zhang et al. 2009), fluid and sediment escape rapidly upward through cracks or pipelines, sediment flows and migrates to form a series of sand veins. Based on the material composition and assemblage characteristics of the profile, the shape, occurrence and transgression characteristics of sand veins and their occurrence horizon, the authors believe that these sand veins are the products of post sedimentary seismic activities and one of the seismic records in Quaternary sediments.