Introduction

The Tibetan Plateau is slowly uplifting due to the northward subduction of the Indian plate to the Eurasian plate (Wang et al. 2008). Material on the eastern margin of the Tibetan Plateau rotates clockwise around the eastern Himalayan tectonic junction, and a series of large-scale active fault zones is distributed in the Tibetan Plateau and adjacent areas (He and Oguchi 2008). Most of the active faults are distributed along the eastern margin of the Tibetan Plateau (Tang et al. 2016), and these faults are characterized by the most frequent seismic activity, the most complex geological environment, and a high incidence of geological disasters (Wen et al. 2008; Qu et al. 2018; Ye et al. 2021). Recently, the eastern margin of the Tibetan Plateau has been of great concern to the local government, scholars, and engineers in regard to the planning and construction of the Sichuan-Tibet railway and Sichuan-Tibet expressway (Wu et al. 2020; Chen et al. 2020).

The Anninghe fault is located on the southeastern margin of the Tibetan Plateau (Fig. 1). It is an important active fault along the eastern margin of the Tibetan Plateau (Ran et al. 2008; Ren et al. 2010; Ren ( 2014) and is also an earthquake-generating fault zone of Southwest China (He and Ikeda 2007). Many destructive earthquakes have occurred along this fault zone, denoting high activity (Wang et al. 2014, 2017). However, cities and towns are distributed along the Anninghe fault zone. In addition, numerous key projects have been planned, are under construction, or have been completed, including the G5 Beijing-Kunming expressway, Chengdu-Kunming railway, Xichang satellite launch center, and Xichang Qingshan airport. The geological conditions of the Anninghe fault zone have been the research focus of numerous engineering and seismic geologists (Cheng et al. 2021).

Fig. 1
figure 1

Location of the study area and main infrastructures in this area

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Generally, a large-scale fault zone consists of some secondary faults of different stages, properties, and scales. The activity of a fault exhibits notable heterogeneity across time, space, and intensity throughout the long-term geological evolution process (Li et al. 1998). The geometric structure, activity time, activity mode, moving velocity, and seismic activity of a fault zone also vary (Ahmad 2021). The Anninghe fault zone is a representative reflection of the nonuniformity of segmental activity (Yi et al. 2004).

Fault zones play a key role in influencing and controlling the evolution and distribution of landslides (Osmundsen et al. 2009). On the one hand, fault zones can destroy the rock mass structure and integrity of a slope. Loose soil or rock slopes are prone to fail under rainfall, and a higher distribution density of landslides is recorded on both sides of fault zones (Scheingross et al. 2013). On the other hand, the sudden movements of fault zones (earthquakes) can trigger numerous types of coseismic landslides and secondary disasters (Xu et al. 2020), leading to numerous casualties and significant property losses (Ho et al. 2020; Zhao et al. 2021). In addition, geological structure, especially that of fault zones, also plays a key role in controlling the stability of landslides (Stead and Wolter 2015; Vick et al. 2020). Unfortunately, few studies have been conducted on the present-day activity of fault zones or on the relationship between active segmentation and the distribution of large-scale landslides along the Anninghe fault zone, as most researchers have focused on surface rupture characteristics, historical activity, and earthquake prediction and evaluation (He and Oguchi 2008; Ran et al. 2008; Ren 2014).

A 5-year engineering geological investigation and study of the Anninghe fault zone was carried out and involved data collection, geological surveys, remote sensing image interpretation, high-precision GPS monitoring, scanning electron microscopy (SEM) tests of the fault gouge, and radon tests of the fault zone. The spatial distribution characteristics, historical earthquakes, activity modes, and activity of the Anninghe fault zone were found to be notable. The present-day activity characteristics of different sections of the Anninghe fault zone are thoroughly considered in this paper. According to the evolution and distribution of large-scale landslides, fault-control sliding modes are summarized, and the relationship between the active section and evolution characteristics of large-scale landslides along the Anninghe fault zone is analyzed. The presented results are of great significance to the planning and construction of large-scale infrastructures and disaster prevention and mitigation in Southwest China.

Geological setting

The Anninghe fault zone is located in southwestern China at the intersection of the southeastern margin of the Tibetan Plateau and the Yunnan-Guizhou Plateau. The zone is an important part of the Y-shaped fault system in China. The Anninghe fault zone is characterized by long-term activity, multiple activities, high-intensity activity, and a high sliding rate. The activity time of the Anninghe fault zone runs from the Mid-Late Pleistocene to the Holocene. According to geological surveys and remote sensing interpretations, the authors further subdivided the Anninghe fault zone into 17 secondary faults of different sizes, shapes, and properties (Table 1).

Table 1 Development characteristics of secondary faults of the Anninghe active fault zone
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The Anninghe fault zone originates in the town of Tianwan, Shimian County, in the north; connects to the southern end of the Xianshuihe fault; and divides into two nearly parallel north–south strike faults after passing through the towns of Xieluo, Zimakua, Yihai, and Xiaoyanjing. The fault zone is a single fault zone consisting of a series of secondary faults north of the town of Xiaoyanjing, while two parallel oblique fault zones consist of a series of secondary faults south of the town of Xiaoyanjing (Fig. 2), i.e., an east branch fault distributed from Xiaoyanjing and Houshan to Luji and a west branch fault distribution from Xiaoyanjing, Linli, Lugu, Yuehua, and Lizhou to Xichang.

Fig. 2
figure 2

Planar distribution of secondary faults in the Anninghe active fault zone: a the northern section, b the middle section, and c the southern section. F1, Zimakua fault; F2, Paisigedi fault; F3, Yejidong fault; F4, Yihai fault; F5, Mixiluogou fault; F6, Shawan fault; F7, Linlicun fault; F8, Shilong fault; F9, Shaguoshu fault; F10, Lugu fault; F11, Yijiahaizi fault; F12, Yangfushan fault; F13, Shejinuo fault; F14, Hongshanzui fault; F15, Shayema fault; F16, Dapingzi fault; F17, Luji fault

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Active characteristics of the Anninghe fault zone

Activity mode of the Anninghe fault zone

The mode of fault activity is an important component of engineering geological surveys of fault zones, and it is also an important reference measure for dividing potential seismic areas. Faults can be divided into creep slip faults and stick slip faults based on the nature of activity. Generally, moderate to minor earthquakes are induced by creep slip faults due to long-term, relatively slow displacement acting as an inhibitory factor of strain energy accumulation. By contrast, the dislocation displacement of a stick slip fault occurs suddenly, usually triggering destructive earthquakes. Therefore, studying the mode of fault activity is not only helpful for predicting the range of fault-induced earthquakes but also advantageous for analyzing the genetic mechanism and distribution characteristics of geological disasters along faults.

Research results have shown that the stress microshape characteristics of quartz particles in fault gouges can reflect the mode of fault activity (Zhang et al. 1994, 2002; Kanaori et al. 1985), and this characteristic reflects the process of fault activity and its dynamic characteristics. The stress microshape represents the stick slip movements of a fault, including wedge pit, conchoidal fracture, river pattern, wallner line, dense cleavage, triangular pit, linear slickensides, and linear microfault. Their edges and peaks have sharp apices though a little blunt at each end (Fig. 4b, c, h), and they are characterized by simple and smooth faces (Fig. 4a, g). Sometimes, minute cracks can be observed at the surface with the length of 50 to 100 um (Fig. 4d). Moreover, some stress microshapes have well-defined round shapes with considerably undulated surfaces (Fig. 4e). By contrast, fatigue striations, arc slickensides, tension fractures, spoon-shaped grinding pits, spoon-shaped slickensides, and orange peel-shaped spherulites denote a predominantly creep slip fracture. The surfaces of quartz particles of creep slip type are generally smooth, though slightly undulated (Fig. 3d, e), or approximate rounded (Fig. 3f). The quartz particles are characterized by small angular shapes (Fig. 3a) and nearly smooth surfaces with small undulations (Fig. 3b) or irregular small holes of 20 to 100 um in length (Fig. 3c). In addition, cleavage steps, oval pits, and irregular pits are regarded as transition types (Kanaori et al. 1980).

Fig. 3
figure 3

Surface micromorphology of quartz debris in the fault gouge of the Anninghe active fault (creep slip). a Fatigue striations; b arc slickensides; c tension fracture; d spoon-shaped grinding pit; e spoon-shaped slickensides; f orange peel-shaped spherulite

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According to the above results, 21 fault gouge samples were taken from the seventeen Anninghe secondary faults, and the sampling locations are shown in Fig. 2. The stress microshape characteristics of quartz particles in the fault gouge were observed under a Hitachi S-4800 scanning electron microscope (SEM). To ensure that the observed suspicious particles were quartz particles, the particles were analyzed by energy spectroscopy (Figs. 3 and 4). Modes of fault activity were distinguished as follows:

  • A microshape is mainly characterized by stick slip movement with less creep slip, denoting a stick slip fault gouge.

  • A microshape is mainly characterized by creep slip movement with less stick slip, denoting a creep slip fault gouge.

  • The presence of both stick and creep slip characteristics with more of the latter denotes a creep-stick slip fault gouge. When there are more stick slip characteristics, this denotes a stick-creep slip fault gouge.

Fig. 4
figure 4

Surface micromorphology of quartz debris in the fault gouge of the Anninghe active fault (stick slip). a River pattern; b Conchoidal fracture, Wallner line; c Wedge pit, Wallner line, Conchoidal fractrure; d Linear micro-fault, Wallner line, Wedge pit; e Conchoidal fracture, Cleavage steps, Wedge pit; f Dense cleavage; g Conchoidal fracture, River pattern, Triangular pit, Wallner line; h Conchoidal fracture, Wedge pit, River pattern; i Conchoidal fracture, Linear slickensides

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According to the test results for the stress microshape characteristics of quartz particles in the fault gouge, the activity modes of the 17 secondary faults of the Anninghe fault zone were obtained by combining the classification and frequency distribution of the SEM corrosion structure on the quartz surface of the fault gouge (Xu et al. 1987) (Table 2). The F1 fault in the northern section of the Anninghe fault is mainly a creep slip fault with an active age of the Mid-Late Pleistocene to the Holocene and strong activity in the Mid-Late Pleistocene. The activity modes of the F2 to F17 faults are mainly stick slip modes, while the F3, F4, F7, F9, F13, F14, F16, and F17 faults also have creep slip characteristics. In addition, the activity mode of No. 16 (southern section of F14) is stick-creep slip, dominated by creep slip. The activity modes of the eastern and western branches of the Anninghe fault differ slightly from those of the Xiaoyanjing to Luji sections. Among all 17 secondary faults, the F2, F3, F5, F8, F10, and F16 were the most active during Late Pleistocene age; the F9, F12, and F15 were the most active during the Holocene age; the F1, F4, F6, and F7 were the most active during the Mid and Late Pleistocene age; the F14 was the most active during the Early and Middle Pleistocene age; and the F13 was the most active during the Early Pleistocene age. The faults with strong activity during the Holocene age are mainly of the stick slip activity mode, e.g., the F9, F10, F11, F12, and F15 faults. However, the activities of the F6, F7, F8, F16, and F17 faults are weak during the Holocene age, and the faults are of the stick-creep slip activity mode. The F13 and F14 faults in the southern section of the Anninghe fault are mainly of the stick slip activity mode and exhibit the creep slip activity mode with strong activity in the Early-Middle Pleistocene in some sections.

Table 2 Age and nature of secondary fault activity identified by SEM testing of the fault gouge of the Anninghe fault zone
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Activity of the Anninghe fault zone

Radon, a radioactive gas, has stable chemical properties and strong migration ability. It can continuously migrate and release into the Earth’s surface along the fracture zone of a fault and its vicinity. Radon is often used as an indicator element of deep information below the Earth’s surface. Research results show abnormally high levels of radon along active faults (Yang et al. 2018). Consequently, the spatial locations of faults below the surface can be detected, and the activity of faults can be evaluated by conducting radon tests. In this paper, three radon test profiles are deployed on the Zimakua fault (F1), Shawan fault (F6), and Hongshanzui fault (F14). The profile locations are shown in Fig. 2, and the corresponding test results are shown in Fig. 5.

Fig. 5
figure 5

Radon test location and results of secondary faults of the Anninghe active fault zone, a radon test result of F1, 1–1′ profile of Fig. 2; b radon test result of F6, 2–2′ profile of Fig. 2; c radon test result of F14, 3–3′ profile of Fig. 2

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The average radon concentration of the Zimakua fault (F1) profile is 2869 Bq/m3, while the average radon concentration is 4137 Bq/m3 near the fault fracture, which is 1.44 times that of the whole profile. The maximal radon concentration of the fault fracture is 7211.9 Bq/m3, which is 2.51 times that of the whole profile.

The Shawan fault (F6) is located in a landslide area, which may have a negative influence on the test results. The average radon concentration is 3222 Bq/m3 along the profile, and the maximal radon concentration is 9016.7 Bq/m3 near the fault fracture, which is 2.80 times that of the whole profile.

The average radon concentration of the profile of the Hongshanzui fault (F14) is 4598 Bq/m3 and the average radon concentration is 7964 Bq/m3 near the fault fracture, which is 1.73 times that of along the profile. The maximal radon concentration is 10629.2 Bq/m3 near the fault fracture, which is 2.3 times that of the whole profile.

The maximum radon concentration near the fault fracture is 2.3–2.8 times the average value along the fault profile. These test results indicate that the Anninghe fault zone is an active fault, and the activity of the middle section is stronger than that of the northern section.

Historical earthquakes along the Anninghe fault zone

The number and magnitude of historical earthquakes along a fault and its adjacent areas are important indices for evaluating fault activity. According to the National Earthquake Data Center (https://data.earthquake.cn/), relevant county annals, and literature, the timing, epicenter location, magnitude, and epicenter intensity of destructive earthquakes with magnitudes of greater than Ms 4.7 along the Anninghe fault zone and its vicinity are shown in Fig. 6 and Table 3.

Fig. 6
figure 6

Epicenter location of Ms ≥ 4.7 earthquakes and section along the Anninghe fault zone

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Table 3 Characteristics of Ms ≥ 4.7 earthquakes along the Anninghe fault zone and its vicinity (N: 27°30′ ~ 29°30′; E: 101°30′ ~ 102°50′)
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According to the epicenter location, frequency, and intensity of historical earthquakes, the Anninghe fault zone has a significant controlling effect on seismic activity in the study area. Most of the earthquake epicenters are located along the Anninghe fault zone. A total of 31 destructive earthquakes (Ms ≥ 4.7) have occurred in the study area, including 7 earthquakes with magnitudes of Ms 4.7 ~ 4.9, 11 with magnitudes of Ms 5.0 ~ 5.9, 11 with magnitudes of Ms 6.0 ~ 6.9, and 2 with magnitudes of Ms 7.5, for which the epicenter intensity was X degrees and the epicenters were located at the cities of Lizhou and Tianba, Xichang.

Many historical earthquakes are distributed along the Tianwan-Zimakua section, and the number of earthquakes occurring accounts for more than 70% of those of the whole Anninghe fault zone (Xu 1988); however, only one fault (F1) is located in this area. In addition, the magnitudes of these earthquakes were less than Ms 5.0 and mainly recorded Ms values of 2.0 ~ 4.0. This phenomenon is mainly determined by the creep slip nature of a fault.

Many historical earthquakes (Ms ≥ 6.0) have been distributed along the Zimakua-Lizhou section of the Anninghe fault zone; e.g., the Xiaoyanjing Ms 6.0 earthquake occurred on August 19, 1913, and the Shilong Ms 6.75 earthquake occurred on September 30, 1952. This section includes the F2 ~ F12, F16, and F17 faults. Such phenomena are related to the stick-creep slip activity mode of the faults.

Some historical earthquakes of high magnitudes occurred along the Lizhou-Xichang section. This section is the most active of the whole Anninghe fault zone, including the F13 and F14 faults. Two strong earthquakes (Ms 7.5) that occurred on March 29, 1536, and September 12, 1850, were distributed in this section. This is the case because the fault belongs to the stick slip activity mode, and the section is located at the intersection of the Anninghe and Zemuhe faults. There have been few historical earthquakes of low magnitudes in the southern Xichang area, denoting weak fault activity.

Segment characteristics of the Anninghe fault zone

The Anninghe fault zone is a complex fault zone consisting of 17 secondary faults. The activity characteristics of the Anninghe fault are segmented due to differences in spatial distribution characteristics, activity modes, and tectonic stress environments of the secondary faults. In this study, the Anninghe fault zone is divided into three sections (northern, middle, and southern sections) with different activity characteristics on the basis of spatial geometrical shapes, structures, activity modes, and active intensity and historical seismic activity levels (Fig. 6, Table 4).

  1. 1.

    Northern section (I): from Tianwan to Zimakua

    This area is approximately 50 km long, only includes the Zimakua fault (F1), and has a simple structure. The section is a single fault zone composed of multiple secondary small faults, and the head and tail of the small faults are not connected to each other. The activity and movement speed of the fault are moderate, and the structure is moderately complex in this section. As the fault belongs to the creep slip activity mode, only small- and moderate-magnitude earthquakes (> Ms 7.0) have occurred in this section.

  2. 2.

    Middle section (II): from Zimakua to the city of Xichang

    This fault is very complex in structure with strong activity and a high movement speed. Historical strong earthquakes have mainly occurred in this section due to the stick slip nature of the fault. According to the geometric shape, activity mode, and active intensity of the fault zone, the middle section of the Anninghe fault zone is further divided into three subsections, i.e., the Mid-North section, Mid-Middle, and Mid-South sections. Mid-North section (II-1): from Zimakua to Xiaoyanjin This area is approximately 40 km long and has a simple plane structure. This section includes the Paisgedi fault (F2), Yejidong fault (F3), and Yihai fault (F4). Generally, the fault movement speed is relatively low and of the stick-creep slip activity mode. Earthquake intensity levels are of moderate magnitude in this section. Mid-Middle section (II-2): from Xiaoyanjin to Lizhou This area is approximately 55 km long and composed of two nearly parallel secondary fault zones. The eastern fault includes 8 faults, i.e., the Mixiluogou fault (F5), Shawan fault (F6), Linlicun fault (F7), Shilong fault (F8), Shaguoshu fault (F9), Lugu fault (F10), Yijiahaizi fault (F11), Yangfushan fault (F12), and Shejinuo fault (F13). The western fault includes the Shayema fault (F15), Dapingzi fault (F16), and Luji fault (F17). The plane structure of this section is complex with a high movement speed and strong activity. Historically, mainly strong earthquakes have occurred in this section, as the fault is of the stick slip activity mode (Tang et al. 2016). Mid-South section (II-3): from Lizhou to Xichang This area is approximately 15 km long and only includes the Hongshanzui fault (F14). The plane structure of the fault is simple with a high movement speed. Due to presence of the stick slip activity mode, many earthquake epicenters are distributed across this section. Two Ms 7.5 earthquakes occurred near the northern and southern ends of the F14 fault in the study area.

  3. 3.

    South section (III): the southern area of the city of Xichang

    This section includes a slow-moving, low-activity fault. There are only minor, sporadic earthquakes, and the fault in this section largely belongs to the creep slip activity mode.

Table 4 Displacement rates (2008–2016) and fault properties of the Anninghe fault
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Large-scale landslide distribution along the Anninghe fault zone

Recent studies have revealed that more than 80% of landslides in the study area are distributed within 3 km of the Anninghe fault rupture zone (Zhou et al. 2021a, b). However, the existing research does not explain the piecewise difference in the distribution density of large-scale landslides along the Anninghe fault zone. According to the distribution characteristics of large-scale landslides in the study area, the corresponding relationship between the active section of the Anninghe fault zone and the distribution of large-scale landslides is analyzed.

Evolution characteristics and sliding mode of large-scale landslides

The evolution of a large-scale landslide is a complex geological process. The occurrence of a landslide is closely related to the geological conditions and human activity. In particular, the formation and evolution of a large-scale landslide are closely related to the spatial distribution of the regional active faults and the corresponding adjacent areas. Hovius and Meunier (2012) analyzed the relationship between the large-scale faults and the landslide evolution by using the examples of Wenchuan in Sichuan of China, Chi Chi in Taiwan, Niigata in Japan, Finisterre in Spain, and Northridge in the USA, and they pointed out that the mountain uplift caused by active faults was mainly balanced and coordinated by collapse, landslide, or denudation. Strong neotectonic movement has formed the landform of high mountains and valleys, with a large elevation difference and a strong potential energy, which easily lead to the formation of large-scale landslides (Fig. 7).

Fig. 7
figure 7

Schematic change of fault–uplift–denudation (rockslide)

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Remote sensing interpretation and geological surveys were conducted, and 57 large-scale landslides (> 1.0 × 106 m3) were found to be distributed within 20 km of both sides of the Anninghe fault zone, including 45 landslides (1 ~ 10 × 106 m3) and 12 landslides (> 10 × 106 m3) (Table 5). The investigation results show that 57 landslides are controlled by faults to varying degrees, and these landslides are called “fault-controlled landslides (FCLs)” in this paper. The main sliding modes of large-scale landslides controlled by the fault are summarized as follows.

  1. 1.

    Landslide-entirety fault-controlled type (LEFT): fault rock constituting the whole landslide body

    As a large-scale regional fault zone, the width of the Anninghe fault fracture zone can reach hundreds of meters. A landslide can occur in the fault fracture zone, and the rock mass in the fault zone directly constitutes the sliding body due to favorable topographic conditions, such as those of the Zhaobishan landslide (Fig. 8a). The Mixiluogou fault (F5), with a 350 ~ 400 m wide fracture zone, passes through the middle of the failure slope. Continuous large-scale deformation and sliding have occurred in the fracture zone under rainfall conditions due to its poor stability.

  2. 2.

    Landslide-toe fault-controlled type (LTFT): fault controlling the toe of a landslide

    In our field investigation, the authors found that faults passing through the foot of the slope play an important role in influencing the stability of the slope and the formation and evolution of landslides, especially the effect of superimposed earthquake. There are many destructive earthquakes along the Anninghe fault zone and its adjacent areas. Dabacha landslide, a typical example, was controlled by a fault passing through the toe of the slope and was triggered by a strong earthquake. The sliding direction runs from SW230° to EW270° and then to NW310° due to the constraint of the downward terrain (Fig. 8b).

  3. 3.

    Landslide-crown fault-controlled type (LCFT): fault controlling the crown of a landslide

    Compared to that of intact rock, the rock mass in this fault has poor mechanical strength. The mechanical strength of the fault, which passes through the middle and upper parts of the slope, can significantly influence and control the stability of the slope. The sliding-shearing deformation of the slope is controlled by the fault fracture zone. Then, large-scale failure occurs, leading to a landslide after continuous deformation (e.g., the Luonaige #1 landslide (Fig. 8c)). The slope has suffered multiple slides to the free face due to the Hongshanzui fault (F14), which passes through the crown of the landslide.

  4. 4.

    Landslide-side fault-controlled type (LSFT): fault controlling the lateral boundary of a landslide

    Faults can not only control the crown of a landslide but can also control the lateral boundary of a landslide, which is the case of the Yangcaigou landslide. This landslide is controlled by a buried fault on the east side and is controlled by the Shawan fault (F6) on the west side (Fig. 8d). The Yangcaigou landslide is classified into three zones, i.e., landslides #1, #2, and #3. Landslide #1 is mainly located in the Shawan fault fracture zone and has developed between two faults.

Table 5 Characteristics of fault-controlled landslides in the Shimian to Xichang sections of the Anninghe fault zone
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Fig. 8
figure 8

Typical fault-controlled landslides, a LEFT (Zhaobishan landslide, No. 6); b LTFT (Dabaca landslide, No. 29); c LCFT (Luonaige landslide 1#, No. 14); d LSFT (Yangcaigou landslide, No. 48)

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Sectional characteristics of the density of large-scale landslides

Figure 9 shows that the Anninghe fault zone plays a prominent role in controlling the distribution of large-scale landslides. At farther distances to the Anninghe fault, the distribution density of large-scale landslides in the east–west direction is lower. However, the distribution density of large-scale landslides is uneven and can be divided into three sections in the north–south direction.

  1. 1.

    Low distribution quantity and density section

    This section is approximately 70-km long and runs from Tianwan to Tuowu. There are 7 large-scale landslides, and only 2 landslides are within 3 km of the fault rupture zone. The distribution density of the landslides is 0.02 ~ 0.15 per 10 km2, indicating that the influencing and controlling effects of the Anninghe fault are weak.

  2. 2.

    High distribution quantity and density section

    This section is approximately 35-km long and runs from Tuowu to Mianning. There are a total of 20 large-scale landslides, and 19 are within 3 km of the fault rupture zone. Notably, the distribution density of large-scale landslides is 0.4 ~ 0.6 per 10 km2 near Caogu and Huian, denoting the highest distribution density of large-scale fault-controlled landslides along the Anninghe fault zone.

  3. 3.

    Higher distribution quantity and density section

    This section is approximately 70-km long and runs from Mianning to Xichang. There are 30 large-scale landslides, and 26 are within 3 km of the fault rupture zone. The length of the fault is only 30 km from the Lugu to Anning sections; however, there are 24 large-scale landslides near the fault rupture zone, and the distribution density of large-scale landslides is 0.3 ~ 0.6 per 10 km2.

Fig. 9
figure 9

Density diagram of fault-controlled landslides in the Tianwan to Xichang sections of the Anninghe fault zone: a Tianwan to Huian sections; b Huian to Xichang sections

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Relationship between large-scale landslides and the active section of the Anninghe fault zone

In general, the stronger present-day fault activity is, the higher the distribution density of landslides is according to the comprehensive analyses presented in Figs. 6 and 9, and Table 4. There is a good relationship between the sectional activity of the Anninghe fault zone and the distribution density of large-scale fault-controlled landslides. The Anninghe fault zone can be further divided into four sections based on the present-day displacement rate, sliding mode, fault earthquake activity, and distribution density of large-scale landslides.

  1. 1.

    Section from Tianwan to Xiaoyanjing

    The present-day displacement rate of the Anninghe fault is 1.2 ~ 1.66 ± 1.66 mm/a in this section with weak activity. The fault mainly belongs to the creep slip activity mode, and the stick slip activity mode is found in one section. Many earthquakes have occurred here over thousands of years; however, these earthquakes have been low or moderate in magnitude. The distribution density of large-scale landslides is less than 0.15 per 10 km2.

  2. 2.

    Section from Xiaoyanjing to Mianning

    The present-day displacement rate of the Anninghe fault zone is 2.98 ± 1.73 mm/a in this section with stronger activity. The fault belongs to the stick slip activity mode. Many earthquakes of moderate and large magnitudes have occurred in this section. The distribution density of landslides is 0.3 ~ 0.6 per 10 km2 in this section.

  3. 3.

    Section from Mianning to Lugu

    Similar to that of the section from Xiaoyanjing to Mianning, the present-day displacement rate is 2.98 ± 1.73 mm/a in this section with stronger activity. The fault mainly belongs to the stick slip activity mode. Most earthquakes have been of moderate magnitude in this section. The distribution density of landslides is 0.1 ~ 0.3 per 10 km2 in this section.

  4. 4.

    Section from Lugu to Xichang

    The present-day displacement rate of the Anninghe fault zone is 8.15 ± 1.31 mm/a in this section with strong activity. The fault belongs to the stick slip activity mode. There have been many earthquakes of high magnitude in this section. Due to the numerous large-scale landslides that have developed in this section, the distribution density of landslides is 0.3 ~ 0.6 per 10 km2.

Discussion

From engineering geological investigations of the Anninghe fault zone, Zhou et al. (2021a, b) found large-scale landslides distributed in a belt along the fault zone. In addition, the distribution density of large-scale landslides is closely related to the sectional activity of the fault zone. Therefore, the relationship between the present-day sectional activity of the Anninghe fault zone and the distribution density of large-scale fault-controlled landslides was analyzed using a variety of technical methods. The results show that the stronger the present-day activity of a fault is, the larger the distribution density of large-scale landslides is. There is thus a positive correlation between present-day fault activity and fault-controlled landslide distribution density along the Anninghe fault zone.

There are two main causes of this phenomenon. During the slow creep of a fault with strong activity, gaps and tensile cracks form in the slope that the fault passes through. This phenomenon can result in the subsidence deformation or local stress concentration of the slope rock mass, causing or aggravating landslide disasters (Zhang et al. 2015, 2017). In addition, a rock mass around a fault often causes extrusion or tension deformation and failure due to the creep deformation of the fault. This phenomenon can result in tension cracks and the disintegration of the slope rock mass, inducing landslide disasters and reducing the stability of a rock mass slope (Yao et al. 2017). Such a fault exhibits a very low creep deformation speed or almost no creep deformation; however, the degree of tensile deformation or stress concentration of the slope rock mass around the fault is low. There are relatively few landslides along a fault with weak activity or no activity, and they are mainly found the sliding control boundaries of large-scale landslides.

The stronger the activity of a fault zone is, the higher the frequency and magnitude of an earthquake will be. There are more large-scale landslides closer to epicenters and under greater earthquake intensity (Tolga et al. 2011). Compared to those caused by faults with weak activity, more landslides are triggered by major faults with strong activity.

It should be noted that there is no absolute positive quantitative correlation between the present-day displacement rate of a fault and the distribution density of large-scale fault-controlled landslides near the Anninghe fault zone. The distribution density of large-scale landslides is significantly different, with almost 2 times the value found in the Xiaoyanjing to Mianning sections and Mianning to Lugu sections of the Anninghe fault; however, the present-day displacement rate of the fault is the same based on GPS measurement results. This is the case because the number and scale of landslides are related to the spatial location, the active characteristics of faults, rainfall, and human engineering activities. These values are also related to the accuracy of geological surveys and the density of GPS monitoring stations.

In addition, only landslides of more than 1 million m3 were considered in this study, and landslides of less than 1 million m3 were not included in the statistical analysis. This variable may also impact the quantitative value of the distribution density of fault-controlled landslides. However, it should not significantly influence the results of our long-term investigation and substantial work. Recently, some new GPS monitoring stations have been established. We will continue to carry out remote sensing image interpretation and geological surveys to illustrate the relationship between the distribution characteristics of small- and medium-scale landslides and the present-day sectional activity of the Anninghe fault zone.

The occurrence of a landslide is related to many factors, such as landform, lithology, rock mass structure, active fault, rainfall, human activity, and earthquake. Through the 5-year engineering geological survey, however, we found that most landslides locate at the Anninghe active fault zone and its adjacent areas (Zhou et al. 2021a, b). Statistical results show that more than 80% of large-scale landslides were within 2–3 km of the Anninghe fault rupture zone. The distribution of these large-scale landslides has an aggregation effect. The density of landslides per million square meter is positively related to the fault activity along the north–south direction. It should be noted that the 57 landslides in Table 5 refer to the large-scale fault-controlled landslides controlled by the Anninghe active fault. The small-scale landslides caused by rainfall and human activities are excluded in this paper.

Conclusions

The Anninghe fault zone is an important regional boundary fault at the eastern margin of the Tibetan Plateau. According to geological survey results, the Anninghe fault zone is further divided into 17 Holocene secondary faults. By spatial geometric shape, structure, activity mode, intensity, and seismic activity, the Anninghe fault can be divided into three sections, i.e., northern, middle, and southern sections. The northern section runs from Tianwan to Zimakua, its activity and displacement rate are moderate, and its structure is moderately complex. The earthquakes that have occurred in this section are mainly low- and moderate-magnitude earthquakes occurring due to the creep slip activity mode of the fault. The southern section is located in the southern area of the city of Xichang and is characterized by weak activity and a low displacement rate. Only sporadic, small earthquakes have occurred here, and the fault belongs to the creep slip activity mode. The middle section runs from Zimakua to Xichang and has a complex fault structure and strong activity. Destructive earthquakes have mainly occurred in this section, the displacement rate is high, and the fault belongs to the stick slip activity mode. The middle section can be further divided into three subsections, i.e., Zimakua to Xiaoyanjing, Xiaoyanjing to Lizhou, and Lizhou to Xichang. Generally, the activity of the middle section is the strongest, followed by that of the northern section and then that of the southern section.

Four sliding modes of large-scale landslides controlled by the Anninghe fault zone are summarized, i.e., the landslide-entirety fault-controlled type (LEFT), landslide-crown fault-controlled type (LCFT), landslide-side fault-controlled type (LSFT), and landslide-toe fault-controlled type (LTFT). In relation to typical landslides, the characteristics and mechanisms of the four sliding modes were analyzed and expounded.

The activity of the Anninghe fault shows a positive correlation with the number and density of large-scale fault-controlled landslides. The present-day displacement rate is 1.2 ~ 1.66 ± 0.66 mm/a in the section from Tianwan to Xiaoyanjing, and the distribution density of large-scale landslides is less than 0.15 per 10 km2 in this low-activity area. The present-day displacement rate is 2.98 ± 2 mm/a in the sections from Xiaoyanjing to Mianning and Mianning to Lugu, and the distribution densities of large-scale landslides in the area with stronger activity are 0.3 ~ 0.6 per 10 km2 and 0.1 ~ 0.3 per 10 km2, respectively. The present-day displacement rate is 8.15 ± 8 mm/a in the section from Lugu to Xichang, and the distribution density of large-scale landslides in the area with strong activity is 0.3 ~ 0.6 per 10 km2.