Surface deformation and damage of 2022 (M 6.8) Luding earthquake in China and its tectonic implications

Baixu Chen; Zhongyuan Yu; Luwei Li; Rongying Zheng; Chuanyong Wu

doi:10.1515/geo-2022-0490

Article Open Access

Surface deformation and damage of 2022 (M 6.8) Luding earthquake in China and its tectonic implications

Baixu Chen , Zhongyuan Yu , Luwei Li , Rongying Zheng and Chuanyong Wu

Published/Copyright: June 12, 2023

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From the journal Open Geosciences Volume 15 Issue 1

Abstract

The 2022 (M 6.8) Luding earthquake on the Xianshuihe Fault Zone (XFZ) caused severe casualties and property losses, and surface deformation and damage of which is crucial for studying the earthquake hazard assessment. However, few intensive scientific understanding has obtained to date because of widespread coronavirus transmission, strong vegetation coverage, and post-earthquake paralyzed traffic. By integrating high-resolution satellite images, large-scale geomorphic mapping, and UAV surveys, we constrain coseismic fractures and ruptures along an NW-SE-trending surface deformation zone, with discontinuous geomorphic scarps, en echelon cracks, and bulges concentrated in the areas of Yanzigou, Moxi, Menghugang, and Xingfu villages near the epicenter. Field observation also shows that the zone extends nearly parallel to the pre-existing XFZ with a length of ∼35 km with variable widths and a maximum vertical displacement of ∼100 ± 10 cm. The earthquake-induced surface coseismic effects, such as landslides, rock falls, and collapses, caused damage to the area. The amplification effect of the topography and the improper aseismic design and poor constructions may be responsible for the spatial distribution of MM Intensity IX, which is larger than other previous earthquakes that occurred in the surrounding area with a similar tectonic setting.

Keywords: coseismic surface deformation; surface faulting; 2022 Luding earthquake; China

1 Introduction

According to the report of China Earthquake Networks, a M 6.8 strong earthquake occurred on September 5, 2022, at a depth of 16 km in Luding County, Sichuan Province, China [1]. The epicenter was located in an mountainous area of high intensity (IX degree) (Figures 1 and 2). The earthquake caused 93 deaths, 25 missing, more than 110,000 people affected, and more than 50,000 houses damaged [2]. It also triggered landslides, which also caused a large number of casualties and building and infrastructure damage. The intensity distribution of the earthquake was determined by referring to the fault structure background, instrument intensity, aftershock distribution, focal mechanism, and UAV remote-sensing interpretation. The highest intensity of the earthquake was IX degrees, and a long axis of the isoseismic line oriented NW. Based on the earthquake intensity map released by the China Earthquake Administration, Ministry of Emergency Management, the distribution area (≥VI degree) was approximately 19,089 km², and the IX degree area was ∼280 km² [3]. The main affected areas included Moxi town, Yanzigou town in Luding County, Garze Tibetan Autonomous Prefecture, and other regions in Shimian County, with seven towns. According to the 3D fault model characterized by the aftershock distribution, it activated the seismogenic fault was the Moxi section at the southern end of the Xianshuihe Fault Zone (XFZ) [4]. The maximum slip was about 184 cm near the epicenter, and the rupture duration time was ∼18 s [5]. Both seismological and geological observations suggest that the 2022 M 6.8 Luding earthquake was triggered by the latest activity of the Moxi segment along the XFZ, which recorded at least 14 destructive earthquakes (M ≥ 7) (Figure 1) during its recent sinistral strike-slip activity.

Figure 1

Sketch map of earthquake distribution and geological background of the study area. (a) Topographic relief of the Tibetan Plateau. (b) Seismotectonic map of the XFZ and the surrounding area. Red thick lines in the map represent the generalized XFZ structure, and the thin red lines stand for the active faults. Blue hollow circles represent the destructive earthquake epicenters (Ms ≥ 4.7), and the red solid lines represent the traces of regional active faults [1,36,77,78]. (c) Light yellow rectangle on the southeastern side of the image represents the main study region. Insert to the right of (a) shows the abbreviation of regional active faults illustrated in the map, and the histogram in the northeastern corner in (b) shows the topographic relief height. (d) Topographic Profile AA′ (location of the profile marked in (b) is illustrated by a black dashed line, the axes not in scale) shows that three regional strike-slip faults, the JCFZ, XFZ, and EKLFZ from southwest to northeast, constitute the boundary structures of different blocks within the plateau interior and have obvious control over topographic variations.

Figure 2

Detailed characteristics of the seismotectonic setting of the study area. Large-scale seismic structure map of the 2022 (M 6.8) Luding earthquake and its surrounding area in Sichuan Province, China. The red five-pointed star in the map represents the epicenter of the AD 2022 (M 6.8) Luding earthquake, and the focal mechanism of the earthquake shows that the mechanism is dominated by strike slip (https://www.ief.ac.cn/kydts/info/2022/69585.html). Red lines and blue hollow circles in the map represent the main active faults and destructive earthquakes, respectively [1,36,77,78]. The earthquake catalog used in the map shows all destructive seismic events (M ≥ 4.7) prior to September 30, 2022.

Detailed field investigation of surface deformation and devastation after the strong earthquakes (M ≥ 6) is crucial not only for understanding the seismic rupture behavior of structures in the middle and upper crust [5,6,7,8,9,10,11,12,13,14] but also for evaluating the long-term seismic hazards of specific seismogenic faults and assessing regional seismic hazard risks [15,16,17,18,19]. These surface deformations and devastation usually expressed as surface ruptures along the seismogenic faults or discontinuous deformation characteristics, such as secondary faulting, block rotation, surface warping, and/or microcracking and extrusive bulges [8,20,21,22,23,24,25,26,27].

Although the 2022 (M 6.8) Luding earthquake occurred at a previously predicted location, its magnitude was smaller than that expected by earlier studies [28,29,30]. However, the published Macroseismic area (IX degree area, ∼280 km²) was unusual larger than other earthquakes with greater magnitudes, such as the 2017 M 7.0 Jiuzhaigou earthquake (IX degree area, ∼140 km²) and 2013 M 7.0 Lushan earthquake (IX degree area, ∼208 km²), that occurred in the surrounding area [3]. Furthermore, according to the emergency post-earthquake investigation at the scene, the main cause of death and injury in the event was not building collapse [3]. This phenomenon was different from what observed in the previous earthquakes [28,29]. Thus, what can we see about the surface deformation characteristics of the 2022 (M 6.8) Luding earthquake and coseismic secondary effects? What features do building structures show in particular geological conditions in the meizoseismic area when they experience a large earthquake? These questions directly influence the understanding of the seismic rupture behavior along the Moxi segment of the XFZ, and the seismic potential of the area. However, limited by the spread of coronavirus, strong vegetation coverage, and damage to infrastructures, the above-mentioned questions remain unanswered.

Answering these questions is particularly important because the ∼1,400-km-long XFZ, which is the seismogenic structure of the 2022 M 6.8 Luding earthquake, which extends from southern Yunnan Province northwest through Sichuan Province into Qinghai Province on the central Tibetan Plateau, is probably the most active intracontinental strike-slip fault within China [31,32,33,34,35,36,37,38] (Figure 1). Large-scale intracontinental strike-slip faults have played important roles in the accommodation of tectonic deformation that results from the collision between the Indian Plate and Eurasian Plate since 45 ± 5 Ma [34,39,40,41,42,43,44], although their geodynamics is still debated. In particular, the XFZ constantly releases strain energy with an average left-lateral slip rate of 8–10 mm/year or less [38,45,46,47,48,49,50,51] or with a temporal and spatial variable slip rate [52,53,54] and has experienced multiple cycles of stress accumulation and release since the late Quaternary (Figures 1 and 2). Although many scientific achievements along the XFZ have been attained by geologists in recent years, the active tectonics and seismogenic behavior along the whole portion of the XFZ remain disputed [35,36,39,55,56,57,58,59,60,61,62].

2 Tectonic setting

The 2022 (M 6.8) Luding earthquake occurred on the Moxi segment of the XFZ, which is a distinguished NW–SE-trending seismogenic structure with strong earthquakes within the Tibetan Plateau and has triggered at least 60 strong earthquakes (M ≥ 6) since 1500 CE [29,33,38,39,45,48,49,51,57,60,62,63,64] (Figure 1). The XFZ includes many subsections from southeast to northwest; they are named the Zemuhe Fault Zone, the Anninghe Fault Zone (AFZ), the Xianshuihe Fault Zone (XFZ, in the narrow sense), the Ganzi-Yushu Fault Zone (GYFZ), and the Dangjiang Fault Zone (DFZ) to the northwest (Figure 1). In general, the XFZ varies in trend from WNW‒ESE in its northwestern section in the DFZ to NNW‒SSE in its southeastern section in the Xiaojiang Fault Zone, having the overall geometry of an arc projecting northeastward [51,65]. Previous studies have shown that the activity time, evolutionary processes, tectonic deformation, and geomorphic expressions in the late Quaternary and the rupture behavior of strong earthquakes along these subsections are different.

Except for the generalized XFZ structure, there are also some other important active faults distributed throughout the whole plateau area, including the Litang-Dewu fault zone, East Kunlun fault zone (EKLFZ), Puduhe fault zone, Nujiang fault zone, Longmenshan fault zone (LFZ), Lancangjiang fault zone, Jiali-Chayu fault zone (JCFZ), Mopanshan fault zone, and Longquanshan fault zone (Figure 1b). Topographic Profile AA′ (location of the profile marked in Figure 1b is illustrated by the black dashed line) obviously shows that three regional strike-slip faults, the JCFZ, XFZ, and EKLFZ from southwest to northeast, may constitute the main boundary fractures of different active blocks within the plateau interior and have direct control over regional topographic variations. In addition, both the spatial distribution images of destructive earthquakes (Ms ≥ 4.7) and the active tectonic studies in recent years show that these regional faults have undergone strong tectonic deformation since the late Quaternary [1,33,38,48,51,62].

Our study area is located in the northern section of the NS-trending tectonic belt at the junction of the Sichuan-Yunnan fault block and Liangshan fault block bounded by the Xianshuihe-Aninghe-Xiaojiang fault blocks (Figure 2). The main outcrop beds are pre-Mesozoic, Paleogene, Neogene and Quaternary in age, with a small amount of Indosinian-Yanshan volcanic rock mass [30,65]. Proterozoic granites such as potassium feldspar granite, monzogranite, and hybrid granite can be seen in the pre-Mesozoic units [30]. The Permian unit contains quartzite and amphibolite schist. The Triassic unit contains sandstone and siltstone. The Quaternary sediments are mostly alluvial, sloped clay, sand, and gravels, and a small amount is distributed on the terraces on both sides of the Dadu River valley [66] (Figure 2).

The Moxi segment of the XFZ, as the seismogenic structure of the 2022 (M 6.8) Luding earthquake, has experienced multiple strong strike-slip earthquakes since the Holocene. The Moxi fault intersects with the NE-trending LFZ and the NW-trending AFZ to form the known “Y”-shaped fault zone in the western Sichuan area. As a whole, these faults play an important role in the tectonic deformation and strain partition of the eastern Tibetan Plateau and its adjacent areas. The XFZ overlaps the SE end of the GYFZ with a left en-echelon stepover of approximately 40 km (Figure 1). In the narrow sense (Figure 2), the XFZ starts from Ganzi in the northwest, passes through Kangding, Luding, and Moxi in the southeast, gradually weakens its active trace, disappears in the vicinity of Shimian county, and finally continues to divide into several branch faults in the southeast. The fault intersects the main body of the Songpan-Ganzi orogenic belt and was the product of intracontinental deformation in the late orogenic period. The fault zone is characterized by large scale, strong deformation, and high frequency of large earthquakes. Since earthquake events started to be recorded in 1725, there have been at least 24 earthquakes of magnitude 6 or above [37,38]. The surface ruptures of moderate and strong earthquakes cover almost the whole fault zone in spatial distribution [29]. The long-term activity of these faults in the study area directly resulted in unique tectonic landforms and large topographic and geomorphological differences. For example, the height difference in topographic relief between the highest Gongga Mountains and Chengdu Basin is larger than 5,000 m (Figure 2). From the perspective of the crustal velocity structure in the focal area, the 2022 (M 6.8) Luding earthquake occurred in a transition zone from high velocity to low velocity, which reflects the controlling effect of the crustal structural difference between the eastern Tibetan Plateau and the Sichuan Basin on the regional stress accumulation [67]. The distribution of destructive earthquakes in the study area further reflects this variation.

On June 1, 1786, a M 7¾ earthquake occurred in the area between Kangding city and Moxi town, and earthquake-induced landslides and floods resulted from the blockage of the Dadu River caused approximately 100,000 victims, making it the deadliest earthquake in Sichuan Province and the most catastrophic flood caused by China’s recorded earthquakes [68,69] concluded that the recurrence interval of strong earthquakes in the Moxi section of the XFZ structure was approximately 300 years based on the paleoseismological data. Wen et al. revealed that the coseismic horizontal dislocation of the 1786 CE M 7¾ earthquake was ∼4.5 ± 0.5 m, and they also suggested that the recurrence interval of strong earthquakes in the Moxi section was 400–500 years based on an estimated average slip rate of 8 ± 2 mm/year of the Moxi fault. Combined with the geometric structure and tectonic environment of the southeastern section of the XFZ [29,70], Chen et al. revealed that the Kangding-Tianwan section of the XFZ was the seismogenic structure of the 1786 CE M 7¾ earthquake [30] and that its coseismic rupture length of this earthquake was approximately 80 km. The tectonic deformation along the Moxi fault is intensive, which has been expressed as a high horizontal slip rate (∼9 mm/year) since the late Quaternary and short recurrence intervals of strong earthquakes.

3 Methods and data

In this article, we adopted three survey methods: remote sensing interpretation, drone photography, and detailed field surveys. First, the existing data, such as 1:50,000 geological maps and published literatures [66,30,71] are collected and analyzed. Then, high-resolution satellite data, including web-based Google Earth images and 30 m resolution SRTM (Shuttle Radar Topography Mission) data were interpreted to digitally map individual structures and tectonic features. The offset alluvial fans, linear fault scarp alignments, displaced gullies, and possible sites of coseismic surface effects were interpreted for the identification of the trace and preparation of field campaigns (Figures 3–7). These analyses were helpful for the precise fault mapping. Finally, to constrain the surface deformation features, low-altitude unmanned aerial vehicle (UAV) Structure-from-Motion photogrammetry [13,72,73] was conducted in October 2022 after field reconnaissance to obtain high-resolution topographic ground models for two sites along the Moxi segment of the XFZ (Figures 4 and 7) and other geo-hazard observation sites (Figures 8 and 9). For each site, at least 180 hundred aerial photographs were captured with a DJI Spirit 4RTK professional multirotor UAV equipped with 18.3-mm focal length camera. The photographs were processed using Agisoft Photoscan software indoors to generate 3-D topographic models and subsequently orthorectified image mosaics [72]. The high-resolution DEMs also helpful for precisely locating the coseismic surface ruptures and earthquake-induced landslides in alpine canyon landforms [73,74,75,76].

$Figure 3 Surface deformation zone of the 2022 (M 6.8) Luding earthquake in Sichuan Province, China. (a) Google image showing field investigation sites along the deformation belt of the earthquake. The red triangles indicate the spatial distribution of the Moxi segment along the XFZ. (b) Color relief map with topographic contours showing that the Moxi segment of the XFZ is distributed in a step oblique pattern in the plane and that the structure is developed along a linear tectonic valley. The numbers on the topographic contour lines show the range of topographic relief height variation. The orange region along the structure approximately represents the distribution of the surface rupture zone of the 2022 (M 6.8) Luding earthquake in Sichuan Province, China. The short solid lines in the orange area represent fractures or cracks observed in the surface. In general, the surface rupture zone is approximately 35 km long (nearly north from Nanmenguan village, south to Xingfu village) and 200–800 m wide based on the field investigation, and it seems that the surface rupture zone is jointly controlled by the fault zone structure, modern river system, topography relief, etc.$

Figure 3

Surface deformation zone of the 2022 (M 6.8) Luding earthquake in Sichuan Province, China. (a) Google image showing field investigation sites along the deformation belt of the earthquake. The red triangles indicate the spatial distribution of the Moxi segment along the XFZ. (b) Color relief map with topographic contours showing that the Moxi segment of the XFZ is distributed in a step oblique pattern in the plane and that the structure is developed along a linear tectonic valley. The numbers on the topographic contour lines show the range of topographic relief height variation. The orange region along the structure approximately represents the distribution of the surface rupture zone of the 2022 (M 6.8) Luding earthquake in Sichuan Province, China. The short solid lines in the orange area represent fractures or cracks observed in the surface. In general, the surface rupture zone is approximately 35 km long (nearly north from Nanmenguan village, south to Xingfu village) and 200–800 m wide based on the field investigation, and it seems that the surface rupture zone is jointly controlled by the fault zone structure, modern river system, topography relief, etc.

Figure 4

Surface deformation zone of the 2022 (M 6.8) Luding earthquake in Sichuan Province, China, observed at the Yanzigou site. (a) DEM data from drone flights; the red lines indicate the distribution of the surface cracks and scarps. The larger the crack is, the thicker the line. The distribution pattern of the surface rupture lines shows obvious sinistral strike-slip characteristics. (b) Orthophoto image, and four representative field survey sites were concentrated in the map range and the adjacent areas. The numbers and positions in (c–f) are the same as those shown in (b). (c) Tensional fissures and small scarps with a vertical displacement of approximately 11 cm. (d) Surface fault scarp with a vertical displacement of approximately 100 cm. Note the person in the map for scale. (e) Extrusion bulges and small scarps with a vertical displacement of approximately 15 cm were observed. (f) Surface fault scarp with a vertical displacement of ∼100 cm. The fresh surface rupture indicates that these deformation features were formed during the latest strong earthquake.

$Figure 5 Surface deformation features of the 2022 (M 6.8) Luding earthquake, China, observed along the line from Yuejinping village to Ertaizi village (corresponding geographic distribution is shown in Figure 3). (a) Newly developed surface tensile cracks in a cornfield. (b–e) Extensive extrusion uplifts and tensional cracks developed on the concrete-poured pavement, and the paved road bricks suffered obvious damage. (f) The tensional fractures developed in the surface show the fault kinematic properties of sinistral strike slip. (g) The roadbed and the guardrail were obviously broken, and the falling stones from the collapse further damaged the highway. The extrusion bulges can also be observed continuously near this observation site. (h) A stone ridge was dislocated, and tensional cracks developed in the ridge.$

Figure 5

Surface deformation features of the 2022 (M 6.8) Luding earthquake, China, observed along the line from Yuejinping village to Ertaizi village (corresponding geographic distribution is shown in Figure 3). (a) Newly developed surface tensile cracks in a cornfield. (b–e) Extensive extrusion uplifts and tensional cracks developed on the concrete-poured pavement, and the paved road bricks suffered obvious damage. (f) The tensional fractures developed in the surface show the fault kinematic properties of sinistral strike slip. (g) The roadbed and the guardrail were obviously broken, and the falling stones from the collapse further damaged the highway. The extrusion bulges can also be observed continuously near this observation site. (h) A stone ridge was dislocated, and tensional cracks developed in the ridge.

Figure 6

Surface deformation zone of the 2022 (M 6.8) Luding earthquake in Sichuan Province, China, observed at the Menghugang Pass site (corresponding geographic distribution is shown in Figure 3). Red arrows in the map represent the surface deformation features. (a) DEM data from drone flights; the red lines in the map indicate the surface cracks. The width of the cracks is illustrated by the thickness of red lines, and the distribution pattern of surface rupture lines shows obvious sinistral strike-slip motion. (b) Orthophoto image, and six representative field survey sites were concentrated in the map range and adjacent areas. The numbers and positions in Figure 7(c–h) are the same as those shown in Figure 7b. (c) Tensional fissures and small scarps with a vertical displacement of approximately 11 cm, and fresh landslides can also be observed. (d–g) Surface tensional cracks with small vertical displacements. Note the person in the map for scale. The fault kinematic properties of sinistral strike-slip are delineated based on the plane geometric characteristics of surface fissures. (h) Surface cracks are approximately 60 cm wide and 90 cm deep. The fresh rupture plane hints that these features were formed during the latest faulting event.

Figure 7

Surface deformation features of the 2022 (M 6.8) Luding earthquake, China, observed along the line from the Menghugang Pass to Aiguo village (corresponding geographic distribution is shown in Figure 3). (a) Newly developed surface tensile cracks and extrusion bulges resulted in the serious destruction of the highway. Note the field book in the map for scale. (b–h) The great earthquake caused extensive development of landslides, tensional fissures, and extrusive bulges, which blocked river channels and faulted highways and damaged the local ecological environment.

Figure 8

Widely developed landslide and collapsed geological hazards that resulted from the 2022 (M 6.8) Luding earthquake in Sichuan Province, China, are distributed throughout almost the whole investigation region. (a and b) Aerial view from a drone shows the massive landslide in which the steep terrain and loose sediments along both sides of the river were hit hard by landslides after the earthquake. (c–h) Regional ecological environment, such as vegetation, highways, bridges, river systems, etc., suffered serious damage during the 2022 M 6.8 Luding strong earthquake.

$Figure 9 Typical seismic devastation and damage to the buildings in the Moxi ancient town in the earthquake (some photos and descriptions refer to the published results of [79], courtesy of Dr. Qu Zhe at the IEM). (a) Scientific research building, Gongga Mountain Ecosystem Observation and Experimental Station, Chinese Academy of Sciences, a masonry structure, lost its entire first story during the strong earthquake. (b) Tremendous shear failure and severe damage to the pillar ends were observed at the local Thangka folk museum under construction. (c) Shear failure of column ends and beam-to-column joints in concrete moment frames were observed in a modern private residential community during the strong earthquake. (d) The roofs, columns and beams of traditional stone buildings suffer from serious shear failure, although their overall performance remains relatively good. (e) The first story of an owner-built hotel was destroyed when the strong earthquake struck, and one person died from the damage to the ground floor of the building [79]. (f) The tower of the ancient Catholic Church suffered slight shear fracture during the latest faulting event. The columns were dislocated, and the walls suffered shear damage.$

Figure 9

Typical seismic devastation and damage to the buildings in the Moxi ancient town in the earthquake (some photos and descriptions refer to the published results of [79], courtesy of Dr. Qu Zhe at the IEM). (a) Scientific research building, Gongga Mountain Ecosystem Observation and Experimental Station, Chinese Academy of Sciences, a masonry structure, lost its entire first story during the strong earthquake. (b) Tremendous shear failure and severe damage to the pillar ends were observed at the local Thangka folk museum under construction. (c) Shear failure of column ends and beam-to-column joints in concrete moment frames were observed in a modern private residential community during the strong earthquake. (d) The roofs, columns and beams of traditional stone buildings suffer from serious shear failure, although their overall performance remains relatively good. (e) The first story of an owner-built hotel was destroyed when the strong earthquake struck, and one person died from the damage to the ground floor of the building [79]. (f) The tower of the ancient Catholic Church suffered slight shear fracture during the latest faulting event. The columns were dislocated, and the walls suffered shear damage.

All earthquake records and active faults used in this research were collected from the China Earthquake Networks Center and the Editorial Board of Annals of Sichuan Province as well as previous studies [1,36,77,78]. Previous typical research data also provided appropriate references for this study [30,50,66,71,79].

4 Results

In this study, the surface deformation of the 2022 (M 6.8) Luding earthquake was first mapped through integrating data from satellite images (Figures 2 and 3), and previous studies [30,66], and our field observations. Field investigation revealed that the strike of the Moxi fault of the XFZ belt) is oriented WN and can be traced for ∼80 km (Figures 2 and 3). The structure is located on the western side of the Dadu River and the eastern side of the Gongga Mountains (Figures 1–3). High-resolution satellite images revealed that the Moxi section starts from Kangding city in the north, passes Moxi town in the middle, and reaches Anshun town in the south (Figures 2 and 3). The elevation of the northern end along the fault zone near the Kangding city is approximately 3,900 m and decreases to ∼1,200 m southward to Anshunchang town (Figure 2b). Previous research has shown that the average horizontal slip rate of the Moxi section is 6.0–9.9 mm/a [30] and that the sinistral strike-slip deformation of this section triggered the 1786 CE M 7¾ earthquake [77] (Figure 2). At its southeastern extremity, the Moxi fault connects with the DFZ (Daduhe Fault Zone) and then extends into the complex Chuan-Dian Block region [33,37,38,46,80] (Figure 1).

The main surface deformation was expressed as discontinuous scarps, en-echelon ruptures and bulges concentrated in the areas of Yanzigou village, Moxi town, Menghugang Pass, and Xingfu village near the epicenter, and the whole zone extends along the XFZ with a length of approximately 40 km and a maximum seismic-shaking vertical displacement of ∼100 ± 10 cm. Among all field observation sites, the surface rupture phenomena at the Yanzigou site and Menghugang Pass site are the most typical; thus, we provide a detailed introduction to these two points in this paper (Figures 4 and 7). Other surface deformation features observed between the area from Yuejingping village to Ertaizi village are also illustrated (Figure 5). Specifically, the typical coseismic effects and damage to the building structures in Moxi town, as one of the worst-hit meizoseismal areas (IX degree) (Figure 6), and widely developed geological hazards such as landslides, rock falls, and collapses (Figures 8 and 9), are carefully selected and highlighted in this paper.

4.1 Surface deformation at the Yanzigou site

The surface extensional fractures developed in cornfields included two main groups (Figure 4a). One group, made up of two larger fractures, was distributed in a nearly N‒S direction and was distributed in a left-stepped pattern. The other group consisted of a great number of small fractures, which were mainly distributed in a NW direction and intersected with large-scale main fractures at an acute angle (Figure 4a). This kind of distinctive geometric pattern also proved the kinematics of the structures suggested by previous studies. Based on the field investigation, four representative survey points were concentrated at the Yanzigou site (Figure 4b and c). A series of fault scarps, troughs, and depressions could be observed within the unconsolidated alluvial and pluvial sediments, and the heights of the scarps varied from 100 ± 10 to 15 ± 1.0 cm (Figure 4c–f). In addition, fresh landslides were also observed at this site, and the extensional direction was consistent with that of the main rupture zone on the surface (Figure 4d).

Similar surface rupture features could also be observed from Yuejinping village to Ertaizi village (corresponding geographic distribution is shown in Figure 3) (Figure 5). For example, extensive extrusion uplifts and tensional cracks developed on the concrete-poured pavement at Nanmenguan village, and the paved road bricks at Moxi town suffered obvious damage (Figure 5b–e). In addition, the roadbed and guardrail were obviously broken, and the falling stones from the seismic-shaking further damaged the highway near the Mozigou observation point (Figure 5g). In addition, a stone ridge was dislocated, and tensional cracks developed in a ridge near Ertaizi village (Figure 5h).

In general, these fresh ruptures indicate that they were formed during the latest strong earthquake that occurred in 2022. The kinematic properties of the fault were dominated by the left-lateral motion, which was evidenced by a spatial combination morphology of the surface tensile fractures and fault scarps observed in the field.

4.2 Surface deformation at the Menghugang pass site

Typical surface deformation during the 2022 (M 6.8) Luding earthquake was also observed at the Menghugang pass site. Fresh surface ruptures are delineated and indicated with red lines in Figure 6a and b based on field observations and low-altitude UAV detection, and six representative survey locations were distributed at the Menghugang pass and adjacent areas (Figure 6c–h). Discontinuous scarps, en-echelon cracks and small bulges were observed in Figure 6c–g. The fault kinematic properties of sinistral strike-slip were delineated based on the plane geometric distribution characteristics of surface fissures. The earthquake, resulting in a surface fracture zone approximately 60 cm wide and 90 cm deep, which extended intermittently approximately for 500 m (Figure 6h), cracked the township road paved with stones.

To the southeast, from the Menghugang Pass to Aiguo village, (corresponding geographic distribution is shown in Figure 3); it was observed that the newly developed surface tensile cracks and extrusion bulges resulted in serious destruction of the highway (Figure 7a). Meanwhile, the strong earthquake also resulted in the blockage of river channels, the destruction of highways, and damage to the local ecological environment such as destruction of vegetation (Figure 7d–h). The field investigation found no evidence of new surface faulting effects south of Aiguo village.

4.3 Surface coseismic effects

Widely developed coseismic effects such as landslides, triggered by the 2022 (M 6.8) Luding earthquake, were distributed throughout almost the whole investigation region. For example, the aerial view from a drone showed that landslides hit the steep terrain and loose sediments along both sides of the Dadu River hard after the earthquake (Figure 8a and b). In addition, local infrastructures systems, such as highways, bridges, and river embankment, suffered serious destruction during the strong earthquake (Figure 8c–h). These new landslides destroyed villages and were the main cause of death and injury during the earthquake. Studies show that the number of human casualties caused by geological secondary effects was significantly higher than the number of those resulting from the collapse of urban buildings [3].

Geometrically, the landslide area was divided into an eastern part and a western part, and the overall distribution was greatly affected by the spatial distribution of the fault zone and Dadu River system (Figures 7 and 8). The overall direction was NNW trending and extended approximately 40 km parallel to the western section of the XFZ structure. The eastern landslide area was mainly distributed from approximately 5 km north of Detuo town to the southwestern end of Luding County, and the landslide spread NE along both sides of the Dadu River, extending for approximately 10 km. The reasons for this spatial distribution were the geometries of the fault zone and the Dadu River system, which may have also been affected by a variety of physical factors, such as the rock type as well as the mechanical properties of the rock type cropping out in the area.

Based on the seismic intensity map published by the Ministry of Emergency Management of China on September 11 [3], the maximum seismic intensity of the 2022 M 6.8 Luding earthquake was IX degrees on a modified Mercalli (MM) scale with a total region of ∼280 km² based on an extensive field survey [79,81]. Moxi town was one of four typical towns with a seismic intensity of IX degrees. During our fieldwork, we also investigated the typical seismic damage to the local building structures in Moxi town under the special geological conditions in the meizoseismal area, and the survey results of approximately six typical building structures that are presented here.

Most of the stone buildings and concrete buildings have suffered obvious earthquake damage, and the site of the damage, the degree of deformation and the impact of the earthquake show unique characteristics. As shown in Figure 9a, a typical masonry structure lost its entire first story during the strong earthquake and killed one graduate student in college, although the building was redecorated and reinforced in 2019 with a brand new steel roof (Figure 9a). The building suffered significant damage to its columns, windows, load-bearing walls, and the adjacent steel staircase (Figure 9a). The local Thangka folk art museum under construction also suffered tremendous shear failure and severe damage to pillar ends [79] (Figure 9b), and the earthquake almost completely destroyed this multistory reinforced concrete (RC) frame structure (Figure 9b).

Modern private residential buildings with RC frame structures with masonry infills in Moxi town also suffered from seismic damage, whether from cracking of infills or shear rupture. As shown in Figure 9c and e, the column ends and beam-to-column joints in concrete moment frames in a modern private residential community are destroyed during the strong earthquake (Figure 9c), and one person died from damage to the ground floor of the building [79] (Figure 9e).

The ancient building structures also underwent the influence of the 2022 (M 6.8) Luding earthquake, and the seismic damage they suffered was no less severe than what the modern buildings suffered from the earthquake. For example, the Priest House, constructed in 1918, the roofs and columns and beams suffered from obvious shear failure, although their overall performance remained relatively good (Figure 9d and f, photos courtesy of Qu Zhe at the IEM). Traditional wooden structures show relatively light seismic damage, which hints that the relatively good seismic performance of these traditional stone buildings resulted from their special design and characteristics of building materials.

In addition to the six typical building structures in Moxi town introduced above, there were several typical seismic types of damage to the structures observed in Detuo town and Aiguo village. As we can see from Figure 10a–d, located between Ertaizi village and Tianwan village, both a pillar foot and bridge suffered shear dislocation with a horizontal offset of ∼7–10 cm. Most load-bearing walls of the building structure suffered X-type shear failures (Figure 10c–g), and parts of the roof structure collapsed and/or were deformed (Figure 10f and h).

Figure 10

Seismic damage characteristics from the 2022 (M 6.8) Luding earthquake in Sichuan Province, China, at a seismic intensity of Degree IX, to local buildings, such as the Moxi ethnic style ancient town and Detuo town. (a–e) The earthquake caused shear rupture of walls, pillars, and bridges of different building material types. (f–h) Many buildings were badly damaged and rendered uninhabitable by the latest faulting event in 2022.

5 Discussion

5.1 Surface deformation produced by the 2022 (M 6.8) Luding earthquake

The surface deformation features resulting from strong earthquakes along regional strike-slip faults are crucial for understanding the activity behavior of seismogenic faults. The left-lateral, strike-slip Luding earthquake is aligned along an NW‒SE belt (Figure 3). The deformation belt included discontinuous scarps, en-echelon ruptures, and bulges concentrated in the areas of Yanzigou village, Moxi town, Menghugang Pass, and Xingfu village, and the whole zone extended along the preexisting XFZ with a length of ∼35 km. The width of the earthquake surface rupture zone was in the widest part ∼1.2 km and in the narrowest part ∼20 m. This difference was driven by the fault geometry (Figure 3). Survey data indicate that the maximum surface vertical displacement is ∼100 ± 10 cm at the Yanzigou site, and the horizontal displacement is less than 20 cm.

Two possible formation mechanisms may need to be considered when comparing and analyzing these displacements. The first one is faulting-related cracks and scarps, which directly resulted from the movement of the active fault. The second one is seismic-induced or related to the unstable foundations and slopes, which belongs to the gravity instability effect of slope subjected to seismic vibration. Although part of those NW-trending fractures should directly related to the latest deformation of XFZ, the strike of most fractures and scarps intersects the strike of the XFZ at a large angle. That is to say, most of the cracks and fissures observed at the surface should be classified as seismic-shacking induced deformation features, rather than faulting related (Figures 4–7). Unfortunately, overdeveloped landslides may conceal a possible horizontal displacement marker, and this may affect the reliability of the survey data. In addition, due to their rugged and steep local terrain conditions and high vegetation coverage, and some special areas have remained inaccessible for investigation long after the earthquake (Figure 3). For example, the area between Ertaizi village and Wandong village still has no accurate survey data because it is too difficult to access given the high and steep mountains and the destroyed infrastructures. To these few unreached places need to be given special attention in future work.

5.2 Damage to the buildings in the meizoseismal area (IX-degree area)

The earthquake caused a wide range of geological secondary effects, such as landslides, rock falls, and collapses, which directly caused serious casualties and property losses. One particular phenomenon of damage caused by this earthquake needs to be focused on here. The given meizoseismal area (IX degree area of seismic intensity, ∼280 km²) was obviously larger than that of other earthquakes with greater magnitudes, such as the 2017 M 7.0 Jiuzhaigou earthquake (IX degree area of seismic intensity, ∼140 km²) and 2013 M 7.0 Lushan earthquake (IX degree area of seismic intensity, ∼208 km²) that occurred in the surrounding area with a similar tectonic setting. Earthquake-induced landslides, rock falls, and collapses cause the majority of fatalities during the earthquake. Our field observations also confirmed this phenomenon. Based on the field investigation as well as other studies [30,79], two possible reasons are suggested to explain this special phenomenon.

First, local terraces may significantly amplify the ground motion of seismic waves [79]. The terrain in this area varies greatly in elevation, representing typical mountain and canyon landforms, which have directly resulted from the strong crustal uplift and long-term sinistral shear deformation of active faults. For example, the Gongga Snow Mountains on the western side of the fault has a peak elevation of 7,556 m, which towers above the ∼4,000 m-high plateau (Figure 11a) and is believed to be a direct product of regional tectonic deformation. The cooling history of the Gongga batholith has been used to study the kinematics of the XFZ and the SE Tibetan area [82,83]. That is, the mountain terrain can significantly amplify the acceleration of ground motion, the amplification effect is positively correlated with the slope angle of the mountain, and the strike of the mountain will affect the difference in the horizontal component of ground motion [84,85]. This significant amplification effect was clearly expressed in previous great events such as the 2008 M 8.0 Wenchuan earthquake and 2013 M 7.0 Lushan earthquake [84,86]. The field investigation showed that almost all of the worst-hit buildings, such as the Gongga Mountain ecosystem observation station and the Thangka folk museum (Figures 9 and 10), were either on or next to loosely stacked Quaternary river terraces with heights of more than 100 m or on steep mountainsides that rise more than 2,000 m above sea level. In this case, the amplification effect of the high mountain topography and unstable slopes or foundations played an important role in the abnormal increase in the area of the high-intensity region (IX degree).

Figure 11

The controlling effect of the XFZ on regional geomorphology and the suggestion for future strong earthquake risk areas along the structure. (a) Tectonic geomorphological map of the XFZ and its adjacent areas. Note that the Gongga Snow Mountain and Chengdu Plain are the two most significant geomorphic units in this region. The yellow arrow represents the direction of the regional principal compressive stress, while the red lines represent the fault structures. (b) Geometrics of the main active fault zone and the strong earthquake distribution around the Bayanhar Block [53,52,45,36,47,61,48,89,51]. High-risk areas for potential strong earthquakes are suggested in yellow based on previously published research and field investigations, perhaps deserving more attention and detailed investigation in the near future.

Second, Qu et al. found that improper design and poor construction quality in remote area such as Moxi town also resulted in heavy devastation and damage to local building structures [79], which directly affected the high value (IX degree) of the seismic intensity investigation. For example, both the owner-built hotel (Figure 9f) and the private structures (Figure 10) destroyed in the earthquake had professional design and construction and detailed seismic measures that were very important for the seismic fortification of the buildings [79]. Otherwise, these inherently “flawed buildings” suffer great damage when hit by strong earthquakes. Qu et al. further argued that buildings that collapsed and claimed lives, concentrated in unengineered rural houses in mountainous villages near the epicenter, should receive more in-depth analysis and research [79]. From this point of view, the seismic design and reinforcement of construction in remote mountainous areas with high local intensity levels, such as Moxi town and Detuo town in Sichuan Province with a 0.4 g peak ground acceleration in the design for basic earthquakes, should be further strengthened [79].

5.3 Implications for hazards along the boundaries of the BayanHar Block

The distribution of historical earthquakes is of great importance in assessing the seismic hazard of a special region. More than ten strong earthquakes (Ms ≥ 7) have occurred along the boundaries of the BayanHar Block on the northern Qinghai–Tibetan Plateau (Figure 11b). This earthquake distribution suggests that the tectonic strain caused by the collision of the Indian Plate and Eurasian Plate is mainly concentrated on the boundaries of the block, with minor deformation in the block interior [87]. In particular, the XFZ and GYFZ, which are viewed as the main structures along the southern boundary of the block, have resulted in numerous destructive historical earthquakes, such as the 1320 CE Ms 8.0 in the Maligange area, the 1738 CE Dangjiang Ms 7.6, the 1845 CE Ms 7.3 in the Ganzi step zone, the 1854 CE Ms 7.7 in the Maligange area, the 1896 CE Dengke Ms 7.5, and the 2010 CE Ms 7.1 in Yushu City (Figure 11b). The 2022 (M 6.8) Luding earthquake was also a strong faulting event caused by the latest activity of the XFZ structure.

The activity of the Moxi fault caused the 1786 CE M 7¾ earthquake with ∼4.5 ± 0.5 m coseismic horizontal displacement, and the recurrence interval of strong earthquakes in the Moxi section is 300∼500 years based on an estimated average slip rate of 8 ± 2 mm/year on the Moxi fault [30,88]. Thus, the slip deficit on the Moxi segment of the XFZ reaches ∼2.3 m (since 1786, the elapsed time of the last strong earthquake was ∼236 years), which hints at a potential earthquake with a magnitude of M 7.4 at present (M = 7.04 + 0.89 × log[D]; [15]. This theoretical magnitude of ∼M 7.4 estimated by the empirical relationships is clearly higher than the magnitude M 6.8 of the occurred earthquake, which may indicate that the 2022 (M 6.8) Luding earthquake did not fully release the accumulated strain since the 1786 CE M 7¾ earthquake. If this is the case, the available data suggest that the risk of strong earthquakes along the adjacent area at the northern and southern ends of the Moxi segment, such as the Daofu section, Selaha fault, and Shimian segment, cannot be ignored, in which the elapsed time is far beyond the repeated time of strong earthquakes or the seismic gap of strong earthquake events [89] (Figure 11b). For example, the last faulting event on the Selaha segment along the XFZ structure occurred in the past ∼450 years and corresponds to the 1786 CE M 7¾ earthquake [88], while the surface rupture of the 2022 M 6.8 Luding earthquake apparently did not propagate to the Selaha segment of the XFZ. This piece of evidence reminds us of the high risk for strong seismic activity along the Selaha fault if the average recurrence interval of ∼300 years in the past millennium is consistent with the long-term recurrence interval of events along the fault zone.

Structural analogy analysis of activity habits focused on seismogenic tectonic conditions of strong earthquakes along the XFZ suggested that the recurrence probability of a strong earthquake along the Dangjiang segment (Figure 1), as the northwestern continuation of the XFZ, cannot be underestimated given that its ∼284 years elapsed time of the last great faulting event in the 1738 CE is nearly close to the ∼350 ± 41 years recurrence interval of major earthquakes [51,52,53] or even slightly beyond its repeated time of ∼274 ± 30 years [47]. In addition, previous studies also show that the risk of the Maqin-Maqu section (east of the Amne Machin extrusion step area shown in Figure 11b), the middle-eastern section of the northern boundary belt of the BayanHara Block, cannot be ignored, if the jumping characteristics of the strong earthquake activity around the BayanHara Block in the past 20 years are considered from the perspective of integrity [89] (Figure 11b).

In summary, three potential high-risk areas of strong earthquake activity around the boundary belts of the BayanHara Block are suggested based on previous studies and this study, including the Maqin-Maqu section of the northern boundary belt of the BayanHara Block, the adjacent northern and southern areas of the Moxi section, and the Dangjiang segment at the northwestern continuation of the XFZ (Figure 11b).

6 Conclusions

Based on the high-resolution satellite images interpretation, large-scale geomorphic mapping, and detailed field investigation, we conclude that (1) the 2022 (M 6.8) Luding earthquake produced an ∼35 km long surface deformation zone that includes distinctive features such as discontinuous scarps, en echelon cracks and bulges. (2) The earthquake induced severe geological secondary effects, such as landslides, rock falls, and collapses, which causes the majority of fatalities and property losses. The amplification effect of the high mountain topography and the improper seismic design and poor construction quality in remote rural area may jointly contribute to the abnormally large spatial area distribution of MM Intensity IX of this earthquake. (3) Three potential high-risk areas of strong earthquakes around the boundaries of the BayanHara Block deserve more attention.

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Acknowledgments

We are grateful to Professors Wei Min, Zhe Qu, and Shi-yuan Wang for their great assistance in this research. Some of the basic data in this article refer to the relevant research achievements and post-earthquake scientific surveys of the China Earthquake Administration. Therefore, we would like to express our heartfelt thanks and give great tribute to the related organizations and individuals for their help and support. This article also expresses our deep condolences to the compatriots who lost their lives in the Luding (M 6.8) earthquake on 5 September 2022. The Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (2019QZKK0901) sponsored this work. We would like to especially acknowledge the editors and anonymous reviewers for their constructive comments and suggestions, which greatly improved this article.

Author Contributions: Baixu Chen: data preparation and field investigation; Zhongyuan Yu: conceptualization, financial support, map drawing and writing; Luwei Li:visualization and field investigation; Rongying Zheng: field investigation and logistics support; Chuanyong Wu: writing-reviewing and supervision.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: All data used in this article came from published sources listed in the references.

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Received: 2023-04-03

Revised: 2023-04-30

Accepted: 2023-05-08

Published Online: 2023-06-12

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0490

Keywords for this article

coseismic surface deformation; surface faulting; 2022 Luding earthquake; China

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