The coseismic surface ruptures associated with large earthquakes contribute to severe damage to near-fault buildings through fault deformation. However, previous studies simplified geological conditions and were based mainly on numerical and physical simulations. In other words, the scarcity of large earthquakes, especially for active thrust faults, limits the understanding of the mechanisms of building damage near active faults. Herein, the study selected the 2008 Mw7.9 Wenchuan earthquake as an example. Based on compass measurements, unmanned aerial vehicle data, ground-based lidar mapping, and numerical simulation, the study showed that twelve buildings within the two sides of the fault were damaged by tilted deformation. The study suggests that the closer the buildings are to the fault, the greater the tilted deformation of the buildings. Moreover, the degree of tilted deformation of the buildings on the hanging wall is significantly greater than that on the footwall, indicating an asymmetric characteristic. Furthermore, the azimuth of the tilted deformation of all buildings is consistent toward the northwest and nearly perpendicular to the strike of the coseismic surface rupture, which indicates that the tilted damage to the buildings might be primarily influenced by the thrust deformation. In addition, the simulation results reveal that parameters such as the distance from the fault, angle of the fault, thickness and strength of the sediment can affect the tilted deformation of near-fault buildings. The results enable us to better understand the influence mechanisms of the tilted damage of near-fault buildings and provide a scientific basis for the avoidance of active thrust faults.

Numerous studies have shown that large earthquakes are produced by active faults, which are characterized by coseismic surface ruptures that pose seismic hazards [1-10]. For example, the 1999 Chi-Chi, Taiwan earthquake (Mw7.6) caused significant casualties and economic losses due to the seismogenic Chelungpu Fault crossing several important facilities and urban areas [11-13]. Typical examples of destruction, such as the collapse of large structures such as the Pi Feng Bridge and the Shi Gang Dam, were offset by coseismic surface ruptures [14-16]. A four-story building in Feng'an City Central Park was severely tilted and experienced rotational shear deformation associated with coseismic offset [17]. Therefore, to effectively reduce the casualties and economic losses caused by large earthquakes, many countries or regions have introduced relevant laws for seismic prevention and disaster reduction planning and have required buildings (structures) to avoid active faults as much as possible (such as the Alquist-Priolo Earthquake Zoning Act in the United States, Eurocode 8 in Europe, the Active Fault Law in Japan, China’s Code for Seismic Design of Buildings (GB 50011-2010), and Avoidance of Active Faults). However, to scientifically and reasonably avoid the influence of active faults, several key scientific questions need to be answered. How do active faults cause the destruction and deformation of near-fault buildings? What are the key parameters that control the damage variability of near-fault buildings? Is the damage to buildings on the two sides of active faults consistent? Answering these scientific questions has significant practical implications for guiding seismic mitigation and disaster reduction during large earthquakes, which also helps us better understand the deformational mechanisms of active faults and their associated seismic effects.

Due to the scarcity of large earthquakes, previous studies have focused primarily on numerical or physical simulations to reveal the rupture propagation behavior of active faults in shallow loose sediments and the deformational response of structures on different types of active faults [11, 16, 18-33]. However, the aforementioned studies usually simplify the true complex geological conditions, and more large earthquake cases are needed to test the results. Therefore, it is important to further quantify the damage patterns of near-fault buildings and the associated deformational characteristics of coseismic surface ruptures. The typical cases of large earthquakes related to thrust faults are relatively rare compared to those of normal faults and strike-slip faults, and coseismic surface ruptures due to thrust deformation, such as the 1999 Chi-Chi Mw7.6 earthquake, the 2008 Wenchuan Mw7.9 earthquake, and the 2011 Tohoku Mw9.0 earthquake, are more complex [8, 12, 34-37]. Moreover, revealing more cases of large earthquakes related to thrust active faults can help us improve the theoretical analysis of the scientific avoidance of active faults.

The 2008 Wenchuan Mw7.9 earthquake occurred on the Longmen Shan fault zone along the eastern margin of the Tibetan Plateau (Figure 1) and resulted in the deaths of approximately 80,000 people and nearly $87 billion in economic losses, attracting widespread attention from global geoscience researchers [4]. The earthquake provided a rare opportunity to explore how coseismic surface ruptures influence the damage patterns of near-fault buildings due to thrust faulting. Herein, the study focused on the Shenxigou section along the coseismic surface ruptures for the following reasons. First, the Shenxigou section shows a large coseismic vertical displacement, which facilitates the identification of the geometric distribution characteristics and deformational style of the fault. Second, numerous buildings were built on both the hanging wall and footwall of the fault before the earthquake, which can be used to compare the differences in damage patterns of the buildings on the two sides. Third, the results could be helpful for providing a scientific basis for earthquake prevention and disaster reduction work in similar continental collision orogenic belts.

Since the late Cenozoic, the Indian Plate has continuously been subducting northwards and colliding with the Eurasian Plate, leading to the rapid uplift of the Tibetan Plateau and its gradual outward expansion [8, 38, 39]. To the east, the relatively rigid Sichuan Basin has blocked the eastwards movement of the Tibetan Plateau, which is characterized by intense compressional uplift of the Longmen Shan thrust fault, forming a steep topographic change zone with an elevation difference of approximately 3000 m within a range of approximately 10 km [40]. The Longmen Shan thrust fault mainly consists of three nearly parallel branch faults that extend in a northeast direction—the Maoxian–Wenchuan fault, Yingxiu–Beichuan fault, and Jiangyou–Guanxian fault—from the plateau to the basin [39] (Figure 1).

On May 12, 2008, the Wenchuan Mw7.9 earthquake occurred on the Longmen Shan fault, which produced coseismic surface ruptures exceeding 300 km in length. The length of the coseismic surface rupture along the Yingxiu–Beichuan fault is approximately 240 km, which shows thrust deformation south of Beichuan, and the right-lateral strike-slip component gradually increases north of Beichuan. Along this rupture zone, the maximum coseismic vertical displacement is approximately 9 m, whereas the maximum right-lateral strike-slip displacement is approximately 4.9 m [8, 39, 41, 42]. The length of the coseismic surface rupture along the Jiangyou–Guanxian fault is approximately 72 km and is primarily characterized by thrust deformation, with a maximum coseismic vertical displacement of approximately 3.5 m [8]. Additionally, a northwest-oriented coseismic surface rupture approximately 7 km in length has developed along the Xiaoyudong fault, which primarily shows thrust deformation with a left-lateral strike-slip component [3, 4, 8, 43].

The Shengxigou section is located in the southwest part of the Yingxiu–Beichuan fault. The coseismic surface rupture at the site is dominated by thrust deformation, with a coseismic vertical displacement of approximately 3–5 m. The fault strikes northeastward at approximately 53° based on an unmanned aerial vehicle (UAV) survey and is continuously and stably distributed. The coseismic surface rupture passed through a series of buildings and roads that were severely damaged and collapsed (Figure 2). Geomorphologically, this section features high mountain and canyon topography and is covered by alluvium deposits primarily consisting of poorly sorted gravels of varying sizes. Lithologically, the strata on the hanging wall are mainly composed of Triassic Xujiahe Formation (T3x) sandstone, whereas the strata on the footwall are primarily composed of Devonian Guanwushan Formation (D2g) limestone.

3.1. UAV Survey and Measurement of Tilted Buildings via a Compass

To obtain the deformational parameters of the buildings, such as the azimuth and angle of tilt deformation and the distance between the building and the fault, the study carried out detailed field investigations on the Shengxigou section. Landform surveys were conducted based on drone mapping technology. High-resolution photographs were obtained using DJI’s Phantom 4 RTK aerial camera, ensuring an aerial photo overlap rate of more than 70%. Using structure-from-motion digital photogrammetry techniques, the study obtained orthophotographs and digital elevation model data with centimeter-level precision to analyze the geometric patterns of the coseismic surface rupture and the distribution characteristics of the buildings [44].

In the selection of near-fault buildings for measurement and statistics, priority should be given to walls with complete structures and few shear cracks to avoid collapsed walls or walls with strong shear deformation. When the compass was used to measure the tilt azimuth and angle of the wall, the study measured multiple sets of data each time and carried out statistical analysis to ensure the reliability of the measured data. Considering that the plane shapes of buildings are different, there might be significant differences when measuring the distance between the building and the fault. For better comparison, the study used the nearest point of the building to the fault as the measurement point and suggested the perpendicular distance from the point to the fault as the statistical distance between the building and the fault (Figure 2(b)).

3.2. Measurement of Tilted Buildings by Lidar

Considering that some areas at the site have dense vegetation, the study conducted land-based lidar scanning to supplement the deficiency of UAV topographic surveys and obtained more accurately distributed geometric patterns of coseismic surface ruptures. Moreover, land-based lidar mapping is useful for obtaining the three-dimensional, high-resolution real structures of buildings, which facilitates the calculation of the tilt deformation parameters of buildings through point cloud data. Specifically, the study used the VZ-400i developed by RIEGL to collect data from the Shengxigou section. The basic principle is to calculate the distance between the laser radar and the target point by emitting pulsed laser measurements and receiving the interval of the pulse signal. In combination with the Global Positioning System and Inertial Navigation System, the equipment can achieve real-time perception of the environment and obtain spatial position information and distance information about objects [45]. Data collection should ensure that the placement of sites forms an irregular triangle, and the distance between adjacent sites should not be too great, ensuring that each site has a large overlapping area with the previous site and ensuring successful data stitching. The measurement system shows a maximum range of 800 m with a laser emission frequency of 1200 kHz and a scanning accuracy with a resolution of approximately 6 mm. After the scanning was completed, the study exported the LAS file and processed it in RISCAN and Cloud Compare software. After filtering out vegetation and miscellaneous points, the study used the cutting profile and measurement tools in the Cloud Compare software to measure and statistically analyze the data. Specifically, using the built-in “Compass” measurement tool, the study selected the point data within a plane (corresponding to the measured surface of the buildings) with a minimum radius of 0.8 m. These data were then used to calculate the most accurate azimuth and dipping angles of the plane, ensuring a standard deviation of greater than 0.9. The measurements include the tilt azimuth and angle of the buildings and the distance from the fault, and the study extracted three sets of data from each wall and calculated their average values to reduce randomness.

3.3. Numerical Simulation

The study used the discrete element simulation software PFC3D 6.0 to reveal the influence mechanism of coseismic displacement on the deformation of near-fault buildings, which has been well applied in other large earthquake cases [17, 46-51]. Specifically, loose sediments can be represented by discrete particles, and the strength and cohesion between particles are described using a linear model. The contact force between two particles is determined according to the size of the particle sphere to characterize the torque caused by the normal and tangential components of the contact force.

The buildings in the model are characterized by rigid blocks [17, 50, 52]. The dimensions of the model box are set to 0.6 m × 0.5 m × 0.4 m. The size of the discrete element particles is based on standard quartz sand, with a diameter set to 2.2 mm. The porosity is 0.45, and other corresponding physical parameters are shown in Table 1 [50]. The simulation must ensure that the particles reach an internal force equilibrium under gravity conditions. Then, the study placed a rigid block to keep the entire model in a state of balance. The study applied velocity at the bottom of the model to simulate fault movement and further analyzed the deformation of the particles and rigid blocks.

4.1. Tilted Deformational Patterns of Buildings

4.1.1. Buildings on the Footwall

The tilted deformation of the representative buildings on the footwall is S1–S4. Building S1 is made of brick and concrete with a relatively complete structure. The wall did not show obvious shear fissures during the earthquake. The maximum tilt of the building is located on the southeast wall, which shows a tilt azimuth and an angle of 315°–321° and 1°–2°, respectively, based on compass measurements. The distance between the building and the fault is approximately 15.1 m (Table 2, Figures 3(a) and 3(b)). Correspondingly, the study further used the lidar data to calculate that the tilt azimuth and angle of the northwest side of the wall are 312° and 2.3°, respectively, whereas the distance from the fault is approximately 13.7 m (Table 3, Figure 3 (d)).

Building S2 is constructed of steel-reinforced concrete with a relatively intact structure. The walls did not exhibit significant shear cracking during the earthquake. The maximum tilt of the building is located on the northwest side of the wall, with a tilt azimuth and an angle of 296°–332° and 1°, respectively. The building was measured to be approximately 24.8 m from the fault (Table 2, Figures 4(a) and 4(b)). Correspondingly, the study further used the lidar data to calculate that the tilt azimuth and angle of the northwest sidewall are 303° and 1.7°, respectively, whereas the distance from the fault is approximately 23.4 m (Table 3, Figure 4(d)).

Building S3 is constructed of brick with a basically intact overall structure. There are shear cracks in the walls, and the maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 300° and 0°, respectively. The distance from the building to the fault is approximately 26.6 m (Table 2, Figures 4(a) and 4(b)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 305° and 1.3°, respectively, whereas the distance from the fault is approximately 26.5 m (Table 3, Figure 4(f)).

Building S4 is constructed of brick with a generally poor overall structure. The roof has collapsed, leaving walls that exhibit several shear fractures. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 304°–306° and 1°–2°, respectively. The distance from the building to the fault is approximately 26.9 m (Table 2, Figures 4(a) and 4(b)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 310° and 1.0°, respectively, whereas the distance from the fault is approximately 27.4 m (Table 3, Figure 4(h)).

Therefore, the study suggests that the tilt parameters and the distance from the fault obtained by the field measurements and lidar cloud-point data for buildings S1–S4 are basically consistent. Moreover, the overall degree of tilting of the buildings on the footwall appears to be relatively weak.

4.1.2. Buildings on the Hanging Wall

The representative buildings from S5 to S12 on the hanging wall of the fault also show tilted deformation. Building S5 is constructed of brick, and its wall partially collapses, but the noncollapsed parts remain intact with several shear cracks. The maximum tilted deformation of the building occurs at the northwest sidewall, with a tilt azimuth and an angle of 305°–337° and 5°–6°, respectively. The building is approximately 8.4 m from the fault (Table 2, Figures 5(a) and 5(b)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 345° and 5.7°, respectively, whereas the distance from the fault is approximately 7.8 m (Table 3, Figure 5(d)).

Building S6 is constructed of brick and concrete with partial wall collapse on the southeast side, whereas the northwest sidewalls are relatively well preserved with some shear cracks. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 310° and 5°, respectively. The building is measured to be approximately 12.4 m from the fault (Table 2, Figure 6(a)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 310° and 4.3°, respectively, whereas the distance from the fault is approximately 13.9 m (Table 3, Figure 6(c)).

Building S7 is constructed of brick with an overall relatively complete structure characterized by small shear cracks. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 317°–318° and 0°–2°, respectively. The building is approximately 27.3 m from the fault (Table 2, Figure 6(a)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 317° and 2.0°, respectively, whereas the distance from the fault is approximately 29.3 m (Table 3, Figure 6(c)).

Building S8 is constructed of brick and concrete with an overall relatively good structure characterized by minor shear cracks. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 325° and 2°, respectively. The distance from the building to the fault is approximately 24.7 m (Table 2, Figure 6(a)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 310° and 2.0°, respectively, whereas the distance from the fault is approximately 25.7 m (Table 3, Figure 6(d)).

Building S9 is constructed of brick with a collapsed roof and shear cracks on the walls. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 349° and 1.0°, respectively. The distance from the building to the fault is approximately 28.6 m (Table 2, Figures 7(a) and 7(b)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 349° and 1.7°, respectively, whereas the distance from the fault is approximately 26.6 m (Table 3, Figure 7(e)).

Building S10, which is constructed of brick, collapsed, but part of the wall is well preserved with shear cracks. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 314° and 4°, respectively. The distance from the building to the fault is approximately 17.5 m (Table 2, Figures 7(a) and 7(c)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall are 312° and 4.7°, respectively, whereas the distance from the fault is approximately 15.6 m (Table 3, Figure 7(f)).

Building S11 is constructed of brick with a collapsed roof, and only part of the wall remains characterized by shear fractures. The study choses one relatively intact wall surface to measure the tilt parameters. The tilt azimuth and angle are 312° and 6°, respectively. The distance between the building and the fault is approximately 20.2 m (Table 2, Figures 7(a) and 7(d)). Correspondingly, lidar data show that the tilt azimuth and angle of the northwest sidewall surface are 318° and 5°, respectively, whereas the distance from the fault is approximately 19.2 m (Table 3, Figure 7(g)).

Building S12, which is constructed of brick, severely collapses, and only part of the wall is preserved with shear cracks. The maximum tilt of the building is located on the northwest sidewall, with a tilt azimuth and an angle of 325° and 1°, respectively. The distance between the building and the fault is approximately 28.2 m (Table 2, Figures 8(a) and 8(b)). Correspondingly, lidar data show that the tilted azimuth and angle of the northwest sidewall are 319° and 1.0°, respectively, whereas the distance from the fault is approximately 27.5 m (Table 3, Figure 8(d)).

Therefore, the study suggests that the tilt parameters and the distance from the fault obtained by the field measurements and lidar cloud-point data for buildings S5–S12 are basically consistent. Moreover, all the buildings on the hanging wall appear to show a northwestward-tilted deformation with a greater degree of tilting than that on the footwall of the fault.

4.2. Modelled Tilted Deformation of Buildings

Considering that geological conditions are complex in the Shengxigou section, the study simplified the simulation parameters to schematically reveal how thrust fault deformation affects the tilted deformation of near-fault buildings. Specifically, the building is set on the hanging wall and is 0.03 m from the fault with a dip angle of 60°. The thickness of the loose sediment particles is 0.045 m, and the normal and shear strengths of the loose sediment particles are 5.16e3 N/m and 1.72e3 N/m, respectively (Figure 9(a)). When the vertical displacement is set to 0.035 m, the block representing the building shows tilted deformation with an angle of 2.4°, while the tilted azimuth is consistent with the slip direction of the thrust fault (Figures 9(b) and 9(c)). The aforementioned results suggest that fault displacement might directly influence the tilted deformation of buildings (Figure 9(c)). Specifically, the model base representing the bedrock is directly offset by the fault, whereas the particles representing the overlying loose sediments are not directly offset. Instead, the particles form a scarp that shows a bending deformation zone where the affected range related to the deformation is much wider than the surface point corresponding to the fault tip. Moreover, the closer it is to the surface fault point, the more dominant the bending of the surface morphology. Additionally, the degree of bending and the affected range on the hanging wall are significantly greater than those on the footwall, which is consistent with the asymmetric tilted deformation of buildings observed in the field on both sides of the fault.

5.1. Comparison of Tilted Buildings on the Hanging Wall and Footwall

Based on the comparison of the tilted deformation of buildings measured from the compass and lidar methods (Tables 2 and 3), the study suggests that the tilted azimuths of the buildings on the hanging wall are primarily between 300° and 350°, with a concentrated distribution at approximately 320° (Figure 10(a)). Comparatively, the tilt azimuth of the buildings on the footwall is mainly between 300° and 330°, with a concentrated distribution at approximately 310° (Figure 10(b)). This finding shows that the buildings on both the hanging wall and footwall tilted with a consistent azimuth. Moreover, the tilted azimuths of all the buildings are basically perpendicular to the strike of the coseismic surface rupture, which indicates that the tilted deformation of all the buildings might be directly influenced by the thrust deformation of the fault.

Additionally, by analyzing the tilting angles of the buildings on the hanging wall and footwall, the study found that the further the building is from the fault, the smaller the tilt angle of the buildings. Considering that the relationship between the tilt angles and distance from the fault might obey the power law decay model [53-55], the study fitted the tilted data on the hanging wall and footwall to the distance from the fault as Angle(h) = 19.6 L−1.0 and Angle(f) = 2.7 L−1.2 (Figure 11). Comparatively, the tilt angle of the buildings on the hanging wall is significantly greater than that on the footwall. The convergence trend for the tilting angles of the buildings on the hanging wall is steep, whereas the convergence trend on the footwall is gentle. This finding indicates that the decay trend for the tilt angles of the buildings on both sides of the fault is asymmetric. In addition, field investigations have shown that many collapsed buildings are close to faults. Most of them are located on the hanging wall, while only a few collapsed buildings are on the footwall. These lines of evidence suggest that the intensity of the tilted deformation of buildings on the hanging wall is significantly stronger than that on the footwall (Figure 11), which is also consistent with the deformational characteristics of thrust faults. Specifically, the coseismic displacement amount on the hanging wall of thrust faults is significantly greater than that on the footwall, and the displacement amount shows a power law decay trend as the distance from the fault increases [55]. Therefore, the distribution pattern of the tilt angles of the buildings on both sides of the fault further suggests that the tilting buildings are directly influenced by the deformation of thrust faults.

5.2. Factors Influencing the Tilted Deformation of Buildings

Considering that the coseismic deformation of thrust faults might be the direct cause of the tilted damage of near-fault buildings, the study further clarifies its influence mechanisms by using numerical simulation. Since field observations show that the degree of tilting of buildings on the hanging wall is significantly greater than that on the footwall, the study focuses on the tilted deformation of buildings on the hanging wall during the simulation.

First, the study compared the influence on the variations in the distance of buildings from the fault. If the other parameters (e.g. fault dip angle, particle thickness, and strength) are the same, the tilt angles of the buildings on the hanging wall are 13.7°, 2.4°, and 0° when the distances from the fault are 2 cm, 3 cm, and 4 cm, respectively (Figures 12(a), 12(b), and 12(c)). The study suggests that the degree of tilted deformation of the buildings on the hanging wall decreases as the distance from the fault increases. Correspondingly, the degree of tilted deformation of the buildings on the footwall is less influenced than that on the hanging wall (Figure 12(d)). Therefore, the study suggests that the tilted deformation of buildings might be strongly related to their distance from the fault and that there is asymmetry in the tilted deformation of the hanging wall and footwall, which is also consistent with the findings of field investigations.

Additionally, considering that parameters such as the fault dip angle, thickness, and strength of the loose deposits in the Shenxigou section are unknown, the study further establishes a model to reveal whether these parameters have an impact on the degree of tilted deformation of buildings. If the other parameters (e.g. distance from the fault, particle thickness, and particle strength) are the same, when the study set the fault dip angles at 45°, 60°, and 75°, the corresponding tilt angles of the buildings are 18.8°, 2.4°, and 0.5°, respectively (Figures 12(a), 12(e), and 12(f)). Therefore, the study suggests that increasing the angle of the thrust fault might weaken the degree of tilted deformation of the near-fault buildings. In other words, compared with a high-angle thrust fault, a low-angle thrust fault is likely to further enhance the tilted deformation of buildings.

Similarly, the study varied the sediment thickness at 4.5 cm, 6.8 cm, and 9.0 cm when the other parameters (distance from the fault, fault dip angle, and particle strength) were the same, and the corresponding tilt angles of the buildings were 2.4°, 25°, and 18.6°, respectively (Figures 12(a), 12(g), and 12(h)). Therefore, the study suggests that the thickness of loose deposits can also affect the degree of tilted deformation of buildings and that this relationship appears to be more complex. Initially, an increase in the thickness of loose deposits might lead to an increase in the tilt angle of buildings. When the thickness approaches a certain value, the tilt angle reaches its maximum value and then gradually decreases as the thickness increases. This evolutionary process could be explained by the deformation of thrust faults. When the deposits are thin, thrust faults can offset the surface, forming a faulted scarp, while the deposits near the fault do not bend much. However, when the deposits become thicker, the direct offset at the surface near the fault is likely to gradually change to bending deformation, forming a fold scarp, which leads to an increase in the tilted deformation of near-fault buildings. When the thickness of deposits exceeds a certain threshold, the thrust faulting effect cannot easily extend to the surface and is accommodated by a wider folding deformational zone; therefore, the corresponding tilt angle of buildings begins to gradually decrease. These stages are consistent with the effect of the tri-shear zone related to thrust fault-related fold theory [56].

The study varied the normal (Kn) and shear (Ks) strengths of the sediment at 5.16e3 N/m and 1.72e3 N/m, 5.16e4 N/m and 1.72e4 N/m, and 1.03e5 N/m and 3.44e4 N/m when the other parameters (distance from the fault, fault dip angle, and particle thickness) were the same, and the corresponding tilt angles of the buildings were 2.4°, 7.8°, and 7.1°, respectively (Figures 12(a), 12(i), and 12(j)). Therefore, the study suggests that the strength of loose deposits can also affect the degree of tilted deformation of buildings and that this relationship also appears to be complex. This is because particles with greater strength have more solid and dense structures, which leads to a more focused deformation zone during thrust deformation. In other words, surface deformation near a fault is likely to change from diffuse to localized, which could increase the tilt angles of buildings. However, when the particle strength increases to a certain threshold, the solidity and compactness of the particles gradually exhibit characteristics similar to those of bedrock. The thrust faults might tend to offset the surface, forming a faulted scarp rather than a folded scarp, which means that the tri-shear deformation might weaken and decrease the tilt angle of buildings.

Overall, the distance of the building from the fault, the dip angle of the fault, and the thickness and strength of the loose sediments has significant impacts on the tilted deformation of near-fault buildings. The movement of thrust faults directly controls the deformational range and intensity of loose sediments, which exhibit different deformational styles and degrees that further influence the tilted deformation of buildings.

5.3. Tilted Deformational Effect on Active Thrust Faults and Implications for Seismic Mitigation

The results from measurements and numerical simulations of the buildings along the Shenxigou coseismic surface rupture zone during the 2008 Wenchuan Mw7.9 earthquake show that (1) the closer the buildings are to the thrust fault, the greater the displacement of the buildings, and the more concentrated the tilt angles. Far from the fault, the degree of tilted deformation of the buildings rapidly decreases, and the tilt angles are relatively scattered. If the study defines 2° as the significant tilt of the building, then the deformation range on the hanging wall of the fault that can cause significant tilting of the buildings reaches approximately 27 m, whereas the corresponding footwall is approximately 14 m. This finding indicates that the degree of tilted deformation of the buildings on the hanging wall is significantly greater than that on the footwall. (2) The azimuth of tilted deformation for all buildings is toward the northwest, concentrated at approximately 320°, which is nearly perpendicular to the strike (approximately 40°–50°) of the coseismic surface rupture at the Shenxigou section. This finding indicates that the tilted deformation of buildings might be directly influenced by the compressional movement of the thrusting fault.

For example, the 1999 Chi-Chi Mw7.6 earthquake in Taiwan involved a thrust fault similar to the seismogenic structure of the 2008 Wenchuan Mw7.9 earthquake. The Chi-Chi earthquake also exhibited similar building damage, which shows that (1) the closer a building is to the fault, the more severe the building damage and that (2) the building damage on the hanging wall of the fault was greater than that on the footwall [10-12]. To explain the aforementioned characteristics, some scholars have suggested that the strong horizontal and vertical acceleration effects of earthquake ground motion are important causes of severe damage to near-fault structures [16, 57-59]. Other researchers have suggested that the degree of damage to near-fault buildings varies due to differences in their materials and structures [11, 60, 61]. However, these two factors do not adequately explain the characteristics of the tilted deformation of buildings in the Shenxigou section. The effects of near-fault ground motion may explain the differences in the tilting angles of buildings between the hanging wall and the footwall but cannot explain the uniform tilted azimuth of the buildings. The differences in building materials and structural factors might show more randomness that was constrained by previous construction conditions. Regarding the influence of soil-structure interaction, previous studies indicate that the primary damage pattern can be characterized by soil liquefaction, potentially leading to tilted deformation of buildings [62, 63]. This deformation is closely linked to soil properties and groundwater under dynamic loading associated with earthquakes [64]. It is noteworthy that no surface liquefaction was observed at the site during post-earthquake field investigations. However, these three factors may further amplify the differences in the tilted deformation of near-fault buildings. Considering that the modelling results are based on simplified geological conditions, the study is unable to quantify the aforementioned factors in detail regarding their impact on building damage. Therefore, to minimize the economic losses and casualties caused by large earthquakes, buildings should be located as far from active faults as possible. However, when it is difficult to effectively avoid them, the footwall of thrust faults may suffer less damage than the hanging wall, which could be helpful for earthquake prevention and disaster reduction. In the future, a comprehensive study that incorporates geophysical exploration, geological investigation, on-site geotechnical testing, and building structural testing will be further needed to analyze the damage to buildings located near active faults during seismic mitigation efforts.

The coseismic surface ruptures at the Shenxigou section associated with the 2008 Wenchuan Mw7.9 earthquake caused tilted deformation of buildings on the two sides of the thrust fault. Based on the measurements of twelve typical buildings, the study found that the primary tilted azimuth of the buildings was 320° and that the tilt angle gradually decreased with increasing distance from the fault. The fitting functions for the tilt angles of the buildings on the hanging wall and the footwall were Angle(h) = 19.6 L−1.0 and Angle(f) = 2.7 L−1.2. The study suggests that the degree of tilted deformation of the buildings on the hanging wall was significantly greater than that on the footwall, which shows an asymmetric tilted deformational characteristic. Numerical simulation results further revealed that parameters such as the distance from the fault, the fault angle, and the thickness and strength of loose deposits could affect the degree of tilted deformation of near-fault buildings. The results could be helpful for better understanding the mechanisms affecting the tilted deformation of buildings near active faults and provide a scientific basis for the avoidance of active thrust faults and the assessment of seismic hazards.

The data used to support the findings of this study are included within the article.

The authors declare no conflict of interest.

This work was financially supported by the 2nd Tibetan Plateau Scientific Expedition and Research (2019QZKK0901) and the National Science Foundation of China (Grant no. 42372241, 42072244).

The study thanks Peisheng Luo, Dongming Li, and Changwei Shen for their field help.