The Seismic Surface Rupture Zone in the Western Segment of the Northern Margin Fault of the Hami Basin and Its Causal Interpretation, Eastern Tianshan

Abstract

The Eastern Tianshan region, influenced by the far-field effect of northward compression and expansion of the Qinghai-Xizang block, features highly developed Late Quaternary active faults that exhibit significant neotectonic activity. Historically, the Barkol-Yiwu Basin, located to the north of the Eastern Tianshan, experienced two major earthquakes in 1842 and 1914, each with a magnitude of M7¹/₂. In contrast, the Hami Basin on the southern margin of the Eastern Tianshan has no historical records of any major earthquakes, and its seismic potential, mechanisms, and future earthquake hazards remain unclear. Based on satellite image interpretation and field surveys, this study identified a relatively recent and well-preserved seismic surface rupture zone with good continuity in the Liushugou area of the western segment of the Northern Margin Fault of the Hami Basin (HMNF), which is the seismogenic structure responsible for the rupture. The surface rupture zone originates at Kekejin in the east, extends intermittently westward through Daipuseke Bulake and Liushugou, and terminates at Wuzun Bulake, with a total length of approximately 21 km. The rupture zone traverses the youngest geomorphic surface units, such as river beds or floodplains and first-order terraces (platforms), and is characterized by a series of single or multiple reverse fault scarps. The morphology of fault scarps is clear, presenting a light soil color with heights ranging from 0.15 m to 2.13 m and an average displacement of 0.56 m, suggesting that this surface rupture zone likely represents the most recent seismic event. Comparison with historical earthquake records in the Eastern Tianshan region suggests that the rupture zone may have been formed simultaneously with the Xiongkuer rupture zone by the 1842 M7¹/₂ earthquake along the boundary faults on both sides of the Barkol Mountains, exhibiting a flower-like structural pattern. Alternatively, it might represent a separate, unrecorded seismic event occurring shortly after the 1842 earthquake. The estimated magnitude of the associated earthquake is about 6.6~6.9. Given that surface-rupturing earthquakes have already occurred in the western segment, the study indicates that the Erdaogou–Nanshankou section of the HMNF has surpassed the average recurrence interval for major earthquakes, indicating a potential future earthquake hazard.

1. Introduction

Seismic surface rupture zones are direct geomorphic characterizations of recent fault activities. In-depth studies of characteristics such as surface rupture patterns, spatial geometry, and coseismic displacement distribution can provide essential insights into neotectonic activity, rupture features, mechanisms, and future earthquake hazards of faults, as well as regional tectonic deformation and dynamics. Traditional research methods for studying seismic surface rupture zones primarily rely on field surveys and manual measurements along the fault. However, the rapid development of Unmanned Aerial Vehicle (UAV) photogrammetry in recent years, characterized by high efficiency, ease of operation, and relatively high precision, has enabled its widespread application in active tectonic research and detailed measurements of historical and recent earthquake rupture zones [1,2,3,4,5].

Due to the far-field effect of the northward compression and expansion of the Qinghai-Xizang block, the late Quaternary active faults in the Eastern Tianshan region (Shan means mountain in Chinese) are highly developed [6], exhibiting intense neotectonic activity and possessing the tectonic conditions for generating strong earthquakes. Based on seismic geological surveys and historical documentation, previous studies suggest that two M7¹/₂ (specifically used for magnitudes of historical earthquakes) earthquakes occurred in the northern part of the Barkol-Yiwu Basin in the Eastern Tianshan in 1842 and 1914 [7,8]. In contrast, there are no historical records of significant earthquakes at the southern edge of the Eastern Tianshan within the Hami Basin, leaving its seismic potential, rupture characteristics, earthquake mechanisms, and future earthquake hazards unclear. During our field study of the tectonic activity characteristics on the Northern Margin Fault of the Hami Basin during the Late Quaternary, we discovered an undocumented seismic surface rupture zone (hereafter referred to as the Liushugou Rupture Zone) in the Kekejin–Liushugou–Wuzun Bulake area of the western segment of the fault. Through satellite image interpretation, field investigation, and low-altitude UAV imagery, we obtained quantitative parameters, including the spatial geometry, coseismic displacement distribution, and the fault activity characteristics revealed by this seismic surface rupture zone.

Based on the historical earthquake catalog for the Eastern Tianshan region, only two M7¹/₂ earthquakes in 1842 and 1914 are recorded [7,8]. However, previous research has identified three relatively recent seismic surface rupture zones: the Tazi Bulake Rupture Zone [8], the Xiongkuer Rupture Zone, and the Yiwu Salt Pond Rupture Zone [9,10]. These studies infer that the associated faults are the Jianquanzi–Luobaoquan Fault, the Southern Margin Fault of the Barkol Basin, and the Southern Margin Fault of the Kuisu-Yiwu Basin, respectively. Because of historical limitations and the lack of timely post-earthquake field investigations, there is currently some disagreement regarding the relationship between these two M7¹/₂ earthquakes in the Eastern Tianshan and the three identified seismic rupture zones, as well as their causative structures and earthquake mechanisms [11]. It remains unclear whether the newly discovered Liushugou surface rupture zone along the HMNF, which is located relatively close to the epicentral areas of two historical major earthquakes, is associated with these earthquakes. Based on the geometric distribution, deformation characteristics, and coseismic displacement distribution of the rupture zone, and combined with a comparative analysis of the active tectonic features of the Eastern Tianshan region and other surface rupture zones, this study aims to comprehensively discuss the genesis and mechanisms of the Liushugou seismic surface rupture zone in the western segment of the northern edge of the Hami Basin.

2. Regional Tectonic Setting

2.1. Overview of Active Faults in the Eastern Tianshan

The Tianshan are situated on the southern margin of the North Asia Orogenic Belt within the Eurasian Plate. They extend north to the Junggar Basin and the Kazakh Platform and south to the Tarim Basin, spanning 250 km–350 km in width and 2500 km in length [12]. Throughout geological history, the Tianshan orogenic belt has undergone multiple episodes of intense crustal deformation. Notably, during the Himalayan tectonic events, the enormous northward push of the Indian Plate reactivated the Tianshan orogenic belt, leading to significant compression and rapid uplift of the region [6,13,14,15]. The extent and rate of compression in the Tianshan are comparable to those in the Qinghai-Xizang Plateau [16]. The current geomorphology and frequent seismic activity of the Tianshan indicate that tectonic deformation in this region remains intense [12].

The Tianshan can be broadly divided into four parts. Among these, the western, southern, and northern Tianshan regions primarily absorb the energy from the India–Eurasia plate collision and exhibit compressive shortening. In contrast, the Altai region to the north and the Gobi-Altai region to the east predominantly experience strike-slip and block rotation [9]. The Eastern Tianshan, situated between these regions, serves as a significant tectonic transition zone. Studying the tectonic deformation and strong seismic activity of the Eastern Tianshan region during the Late Quaternary is crucial for understanding the far-field effect of the northward push of the Indian Plate on the Tianshan orogenic uplift, the transition in tectonic deformation styles and the latest tectonic activity features, holding significant geological importance.

The Eastern Tianshan mainly includes the Bogda Mountains, the Barkol Mountains, and the Karlik Mountains. The major faults in the region, from west to east, include the Northern Margin Fault of the Turpan Basin (TNF), the Central Fault Zone (Fold Belt) of the Turpan Basin, the Jianquanzi–Luobaoquan Fault (J-LF), the Southern Margin Fault of the Barkol Basin (BSF), the Northern Margin Fault of the Hami Basin (HMNF), the Central Fault of the Karlik Mountains (KCF), the Northern Margin Fault of the Yiwu Basin (YWNF), the Weizixia Fault (WZXF), and the Xiamaya Fault (XMYF) (Figure 1). Among these, the J-LF and the western segment of the BSF are sinistral strike-slip faults with thrust components, while the KCF is a sinistral strike-slip fault. The remaining faults are all compressional thrust faults.

The TNF is located at the southern foothills of the Bogda Mountains and in the north of the Turpan Basin, with its strike changing from NEE to NWW as it extends eastward. The Late Quaternary activity of the main fault has migrated toward the Central Fold Belt of the Basin [16]. The Central Fold Belt of the Turpan Basin extends in an EW direction around the area of the Yila Lake in the northern part of Tuokexun and the Southern Lake Coal Mine of Qiketai Town. There are reverse faults developed on the southern frontal side of the fold, with a length of over 200 km. The recurrence interval of large earthquakes in this region since the Holocene is approximately 3 ka [17]. The J-LF is a continuation of the TNF, extending eastward. The Tazi Bulake rupture zone in the western segment of the J-LF indicates that it is a Holocene-active fault. Upon entering the Barkol Mountains, the fault splits into two branches along the northern and southern sides of the mountains. The northern branch is the BSF, while the southern one is the HMNF. The BSF is a Holocene-active fault. The average sinistral strike-slip rate of the western segment of this fault during the Holocene is 2.4–2.8 mm/a. The Xiongkuer seismic surface rupture zone on this segment may have been formed by the 1842 M7¹/₂ earthquake. The eastern segment of the BSF is a typical reverse fault, with an uplift rate of 0.15–0.17 mm/a and a shortening rate of 0.13–0.22 mm/a [9]. The western segment of the YWSF is mainly a reverse fault with a vertical activity rate of 0.06–0.08 mm/a. The eastern segment, the Xiamaya section, exhibits significant strike-slip characteristics since the Late Quaternary, with a sinistral strike-slip rate of 1.1–1.4 mm/a [18]. The Yiwu Salt Pond surface rupture zone might be formed by the 1914 M7¹/₂ earthquake [9].

Figure 1. Seismotectonic map of Eastern Tianshan: (a) the location of Eastern Tianshan; (b) primary active faults and earthquakes in Eastern Tianshan; (c) the distribution features of HMNF. TNF: The Northern Margin Fault of Turpan Basin; J-LF: Jianquanzi–Luobaoquan Fault; ZFF: Zhifang Fault; BSF: The Southern Margin Fault of Barkol Basin; BNF: The Northern Margin Fault of Barkol Basin; HMNF: The Northern Margin Fault of Hami Basin; K-YSF: The Southern Margin Fault of Kuisu-Yiwu Basin; KCF: The Central Fault of Karlik Mountains; WZXF: Weizixia Fault; XMYF: Xiamaya Fault; YWSF: The Southern Margin Fault of Yiwu Basin; GTSFS: Gobi–Tianshan Sinistral Strike-Slip Faults. The base map is based on 30 m DEM of USGS [19]. The fault data are modified from studies [9,11,13,20]. The earthquake dates are from the China Earthquake Catalogue (1831BC-1969AD) [21] and the National Earthquake Date Centre [22].

Figure 1. Seismotectonic map of Eastern Tianshan: (a) the location of Eastern Tianshan; (b) primary active faults and earthquakes in Eastern Tianshan; (c) the distribution features of HMNF. TNF: The Northern Margin Fault of Turpan Basin; J-LF: Jianquanzi–Luobaoquan Fault; ZFF: Zhifang Fault; BSF: The Southern Margin Fault of Barkol Basin; BNF: The Northern Margin Fault of Barkol Basin; HMNF: The Northern Margin Fault of Hami Basin; K-YSF: The Southern Margin Fault of Kuisu-Yiwu Basin; KCF: The Central Fault of Karlik Mountains; WZXF: Weizixia Fault; XMYF: Xiamaya Fault; YWSF: The Southern Margin Fault of Yiwu Basin; GTSFS: Gobi–Tianshan Sinistral Strike-Slip Faults. The base map is based on 30 m DEM of USGS [19]. The fault data are modified from studies [9,11,13,20]. The earthquake dates are from the China Earthquake Catalogue (1831BC-1969AD) [21] and the National Earthquake Date Centre [22].

2.2. Late Quaternary Tectonic Deformation Characteristics of the Northern Margin Fault of the Hami Basin

The HMNF, as the eastern extension of the J-LF into the southern flank of the Barkol Mountains, extends along the basin–mountain boundary on the southern side of the Barkol Mountains. It extends from the Yiwanquan area in the west to the Tianshengquan and Luotuojingzi areas in the east, with an overall NWW orientation and a total length of approximately 230 km. It is a major deep thrust fault that cuts through the regional Moho discontinuity [23]. Previous studies on this fault [7,9,18] have primarily focused on the deformation characteristics of the thrust fault scarp. Bai et al. [7] reported vertical slip rates of 0.65 ± 0.08 mm/a and 0.32 ± 0.03 mm/a for the Nanshankou and Zhongyingtai segments of the fault, respectively. Wu F. [9] determined a vertical slip rate of 0.10–0.18 mm/a and a horizontal shortening rate of 0.17–0.31 mm/a for the Dewaili segment over the past 50 ka. Ren [18] found a vertical slip rate of 0.06–0.08 mm/a for the Xishanxiang segment and a vertical slip rate of 0.12 mm/a along with a horizontal shortening rate of 0.08 mm/a for the Zhongyingtai segment. In addition to determining the fault activity rates since the Late Quaternary, Bai et al. [24] further subdivided the HMNF into five segments based on its geometric distribution: the West Liushugou–Sidaogou, Aketasi–Xishan, Hulugou–Nanshankou, Shichengzi–Yushugou, and Shangmiaoergou–Eastern Tianshengquan, exhibiting a right-stepping en echelon pattern. More broadly, the HMNF can generally be divided into two major sections, with Yushugou serving as the boundary, of which the western one is a Holocene fault, while the eastern one is a Late Pleistocene fault (Figure 1c). The newly discovered rupture zone in this study is located on the West Liushugou–Sidaogou segment of the fault.

Previous studies on the HMNF primarily focused on the deformation characteristics of the thrust fault scarp, with no detailed reports on tectonic deformation patterns, segmentation activity characteristics, or seismic surface rupture zones along the fault. In fact, the latest tectonic deformation characteristics of the HMNF are dominated by thrust faulting and thrust fault-fold zones. Along the fault, areas such as Qincheng, Hongquan, Nanshankou, Zhongyingtai, and Liushugou all exhibit thrust fault-fold zones (Figure 1c), with the Nanshankou–Liushugou area simultaneously exhibiting well-developed thrust faulting, folding, and fault scarps.

Based on the different elevations of the geomorphic surfaces along the fault (Figure 2) and their depositional characteristics, we roughly divided the geomorphic surfaces into four stages, from youngest to oldest, labeled as Fan1 to Fan4, using the Liushugou segment as a representative example. Fan1 represents the most recent deposition in the area and is found alongside relatively larger channels. Fan2 is a slightly older fluvial–alluvial terrace located between channels or gullies, and it is the most widely distributed in the region, with its surface covered by desert varnish-coated black gravels. Portions of the Fan2 terraces have undergone folding deformation (Figure 2). The newly discovered seismic surface rupture zone cuts across modern gully floodplains and the Fan1–Fan3 fluvial–alluvial terraces.

3. Data Acquisition and Processing

3.1. Data Acquisition

In recent years, advancements in UAV technology, computer vision theory, and automated feature matching algorithms have led to the integration of Structure from Motion (SfM) 3D reconstruction technology, originating from computer vision, into photogrammetry. This integration has significantly increased the level of automation in the photogrammetric process [2,25,26]. SfM refers to the process of efficiently extracting corresponding features from three or more images using advanced feature-matching algorithms and tracking these features from one image to another (Figure 3). This process initially recovers the relative positions of the camera during photography, which are then optimized using a nonlinear least squares method to ultimately determine the camera’s position and orientation, as well as the 3D spatial coordinates of the photographed object [2,27,28,29,30,31]. Compared to traditional photogrammetry methods, the SfM technique can quickly acquire high-precision topographic and geomorphic data from photographs taken from different angles of the target, significantly reducing measurement costs [2,26,32].

In this study, we employed a DJI Phantom 4 Pro V 2.0 UAV for low-altitude photogrammetry. This UAV has a flight weight of 1375 g, a maximum flight altitude of 6000 m, and can withstand wind speeds up to 10 m/s. It operates in temperatures ranging from 0 to 40 °C, with a maximum flight duration of approximately 30 min and an effective working time of 15–20 min per flight. Meanwhile, it is equipped with an integrated dual-mode positioning module that includes the Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS). It is also fitted with an FC6310S camera, which features a 1-inch CMOS sensor with a 20-megapixel resolution and a lens with an equivalent 35 mm focal length, meeting the requirements for rapidly acquiring high-resolution images of seismic surface rupture zones.

The imaging quality of UAV photogrammetry is influenced by environmental conditions, such as flight path, elevation, forward overlap, and side overlap [33]. The study area for UAV aerial photography, focusing on the surface rupture zone along the western section of the HMNF near Liushugou, is situated in the basin–mountain boundary region. The survey area contains unfavorable conditions such as frontal uplift, power poles, and strong winds, which could, to some extent, affect the fieldwork and imaging quality. Considering these practical conditions, the flight altitude of the drone was set between 80 and 150 m, with a side overlap rate of 75–85%, to capture the highest possible resolution images while ensuring safe flight. Generally, it is sufficient to edit and execute a single aerial photography mission for a survey area. However, upon the occurrence of accelerated battery drain due to weather conditions and the need for obstacle avoidance, multiple flight missions may be required to complete aerial photography for a given area. In such cases, it is essential to ensure that the different flight missions maintain consistent or similar overlaps in the surveyed areas, flight altitude, flight path overlap, and side overlap to facilitate later image synthesis. On average, 2–4 batteries were needed to complete the aerial photography for each work area in this study.

3.2. Data Processing

In this study, the data collected by the UAV was processed using Agisoft Metashape Professional (×64) software (https://www.agisoft.com/features/professional-edition/, accessed on 7 November 2024). The processing workflow was as follows: adding photos (photo folder) → aligning photos → generating a dense point cloud→ generating a mesh → generating textures (both this process and the preceding ones are chosen for high quality) → generating a Digital Elevation Model (DEM) → generating a Digital Orthophoto Map (DOM) → exporting DEM and DOM images. The resolutions of both the DOM and DEM can reach approximately 4 cm/pix. Based on these outputs and combined with information obtained from field surveys, the DEM and DOM images were visually interpreted using ArcGIS 10.8 software to accurately determine geomorphic surface stages and the geometric distribution of the rupture zones.

4. Basic Characteristics of the Seismic Surface Rupture Zone

The newly discovered Liushugou Rupture Zone extends along the western section of the HMNF on the southern side of the Barkol Mountains. It starts in the east from the alluvial fan terraces west of Erdaogou and east of Kekejin, extending westward discontinuously through DaipuSeke Bulake, Buletuotieke, Liushugou, and ultimately terminating in the floodplain of Wuzun Bulake. The entire rupture zone stretches approximately 21 km with an overall trend of NWW. The rupture zone offsets the most recent landforms, such as alluvial flats and first-level terraces, and is primarily characterized by typical normal and reverse thrust scarps, whose free surfaces are fresh and clearly visible, displaying a light-yellow soil color, in contrast to the black gravel covered with desert varnish on the original alluvial landforms on both sides of the rupture zone. Based on the overall distribution characteristics of the rupture zone, it can be divided into five segments: Kekejin, Toudaogou, Daipuseke Bulake, Liushugou, and Wuzun Bulake. Among these, the rupture zone located between Kekejin and Liushugou displays the most pronounced scarps, appearing as clearly visible gray-white stripes on satellite images (Figure 4). We conducted UAV aerial photography and analysis of four segments: Kekejin, DaipuSeke Bulake, Liushugou, and Wuzun Bulake, and obtained the following insights.

4.1. Characteristics of the Surface Rupture Zone

4.1.1. Kekejin Segment

The eastern endpoint of the Liushugou Rupture Zone is located at the alluvial fan terraces east of Kekejin and west of Erdaogou. On the eastern side of Erdaogou, there are no significantly small reverse fault scarps, only older, large reverse fault scarps. The geomorphic surfaces in the Kekejin section can be divided into three stages: T0-alluvial flat, Fan1, and Fan2. The rupture zone offsets the Fan2 surfaces, creating two rows of reverse scarps with an NWW orientation, approximately 400 m apart and extending for 2.6 km (Figure 5a). The free surfaces of the reverse scarps in the rupture zone are well-preserved, displaying a light-yellow color, while the original land surfaces on either side are black due to gravel covered with desert varnish, providing a clear contrast.

The rupture zone has altered the distribution of surface water flow on the original slope. Surface water flow accumulates at the base of the reverse fault scarps and flows along scarps, causing some erosion and leading to substantial vegetation growth along the scarps (Figure 5e,f). The southern rupture zone offsets the Fan2 surfaces with minimal height of the scarp. The northern rupture zone offsets the T0, Fan1, and the western part of the Fan2 surfaces, forming a reverse scarp with a height of around 0.13 m within the T0 floodplain (possibly modified by water erosion) and reverse scarps with height ranging from 0.25 m to 0.78 m on the Fan2 surfaces (Figure 5c,e,f). The height of the reverse scarps increases towards the eastern side of the Fan2 surface, where the rupture zone might terminate (Figure 5a,c).

4.1.2. Daipuseke Bulake Segment

The surface rupture zone of the DaipuSeke Bulake segment is relatively continuous, beginning in the east at DaipuSeke Bulake and extending westward, where it disappears after crossing the alluvial fan terraces south of Buletuotieke. The overall trend shifts from NW to nearly WE, with the rupture zone mainly distributed around Daipuseke Bulake and the southern area of Buletuotieke. The geomorphic surfaces of the Daipuseke Bulake section can be divided into four stages: T0-alluvial flat, Fan1, Fan2, and Fan3 (Figure 6b).

The surface rupture zone in the Daipuseke Bulake area primarily trends NW, with a length of approximately 400 m. Image interpretation reveals two rows of rupture zones in this area, with the southern one being slightly longer. The rupture zone mainly offsets the Fan2 alluvial fan, forming a reverse fault scarp with a height of around 0.45 m. In addition, there is a fault spring on the eastern side of the scarp (Figure 6b,c).

West of Daipuseke Bulake, the rupture zone disappears within the wide river channel. Then, the rupture zone reappears again on the Fan2 alluvial flat to the west of this river channel and the south of Buletuotieke, extending approximately 2.3 km with a nearly W-E trend. This section of the surface rupture zone exhibits the most typical features with well-developed reverse fault scarps. The rupture zone offsets the T0 alluvial flat, forming a reverse fault scarp with a height of about 0.7 m. To the east of this point, the Fan2 alluvial flat is offset, creating a reverse fault scarp approximately with a height of 0.6 m. The free face of the scarp is light yellowish-white, contrasting with the black alluvial fan surface covered with gravel coated with desert varnish. The overall heights of the scarps in this section range from 0.45 m to 1.3 m (Figure 6b,d–f).

4.1.3. Liushugou Segment

The geomorphic surfaces in the Liushugou area can be divided into four stages: Fan1, Fan2, Fan3, and Fan4 (Figure 7a). This section of the rupture zone generally trends W-E and extends approximately 2.5 km. The eastern end of the rupture zone begins at the eastern side of Liushugou, near Zhagadun, where it offsets the Fan2 alluvial flat. At a location 280 m east of the County Road X090, the rupture forms a reverse fault scarp with a height of about 2 m. The maximum vertical offset of the Liushugou rupture zone measured here is 2.13 m (Figure 7c and Figure 8a).

West of the maximum displacement point, the rupture zone splits into two branches. The southern branch extends about 150 m westward before terminating and offsets the alluvial fan, forming a reverse fault scarp approximately with a height of 0.6 m. The northern branch continues west, offsetting the first-order river terrace west of County Road X090, forming small scarps with heights ranging from 0.4 m to 0.6 m. The scarp profile indicates a slight uplift of the T1 terrace surface, showing weak folding deformation (Figure 7). Adjacent to this, on the west side of the first-order terrace, the rupture zone passes through the folded, deformed Fan2 alluvial fan, creating a reverse fault scarp with a height of 0.8 m on the forelimb of the fold. On the core and rear limb of the fold, the rupture zone forms reverse fault scarps with heights of 0.4 m and 0.8 m, respectively. These scarp features are similar to those observed in the Daipuseke Bulake section, with a total coseismic vertical offset of 2 m (Figure 7).

The scarp on the forelimb and back limb of the fold on the Fan2 alluvial fan extends about 200 m westward before disappearing. The reverse fault scarp on the core of the fold extends approximately 700 m westward before terminating, with an additional reverse fault scarp about 100 m long and 0.6 m high developing on its southern side (Figure 7).

In a gully about 3 km west of the Fan2 fold, the surface rupture zone offsets a recent channel, forming a reverse fault scarp approximately with a height of 0.4 m (Figure 8f). Given the multi-segment distribution of the rupture zone, the coseismic offsets should be assessed through cumulative analysis of the small scarps, resulting in coseismic offsets of 1.2 m, 2 m, and 2.13 m.

4.1.4. Wuzun Bulake Segment

When extending into the Wuzun Bulake area from west of Liushugou, no significant fault scarp distribution was observed. In the Wuzun Bulake area, the rupture zone reappears, trending SWW, with a length of approximately 700 m and displaying typical reverse fault scarps. The landforms in the Wuzun Bulake area can be roughly divided into T0 alluvial flats and Fan1 alluvial fan terraces. The Fan1 terraces were previously offset by the HMNF, forming a major scarp. The rupture zone continues to offset the Fan1 terrace, creating minor scarps superimposed on the substantial scarp and also offsets the T0 alluvial flat, forming low scarps with heights ranging from approximately 0.15 m to 0.38 m (Figure 9b). Additionally, a small scarp with a height of about 0.2 m was observed on the east bank of a small gully within the alluvial flat. Continuing west of Wuzun Bulake, no significant minor fault scarps were observed. Given the reduced height of the fault scarps compared to those in the Liushugou–Daipuseke Bulake area, it is suggested that Wuzun Bulake represents the western terminus of the Liushugou seismic surface rupture zone.

4.2. Coseismic Offset Distribution

In this study, aside from using a tape measure or distance meter in the field to measure some typical offset scarps for indoor calibration, the primary approach involved processing DEM images obtained from UAV aerial surveys using ArcMap 10.8 software to generate shaded relief maps. By combining these with DOM orthophoto images, visually identifiable typical scarp locations along the fault zone were selected for coseismic scarp vertical displacement measurements. A total of 51 displacement values were identified, with one additional coseismic displacement measured manually. The smallest coseismic displacement recorded is 0.13 m, and the largest is 2.13 m. Most coseismic displacements range between 0.4 m and 1.3 m, with an average coseismic displacement of 0.56 m.

In the Kekejin section, 10 coseismic displacements were identified, with values ranging from 0.13 m to 0.78 m. In the Daipuseke Bulake section, 15 coseismic displacements were identified, with a minimum of 0.5 m and a maximum of 1.3 m. In the Liushugou section, nine displacements were identified, with a minimum displacement of 0.4 m. Considering that this section of the fault zone is divided into multiple rows, the real coseismic displacement should be analyzed by summing the heights of the minor scarps, resulting in a minimum coseismic displacement of 1.2 m and a maximum of 2.13 m. For the Liushugou–Wuzun Bulake area, a manually measured coseismic displacement of 0.4 m was obtained (Figure 8f). In the Wuzun Bulake section, 17 coseismic displacements were identified, with values ranging from 0.15 to 0.6 m. Overall, the vertical displacements along the Liushugou fault zone approximately follow a regular distribution pattern, consistent with typical displacement distribution characteristics of reverse fault surface rupture zones (Figure 10).

5. Mechanism of Seismic Fault Zone Formation and Fault Hazard Assessment

5.1. Discussion on the Formation Age of the Fault Zone

During the field investigation of the Liushugou Rupture Zone, it was observed that the fault has displaced the youngest alluvial flats and other geomorphic surfaces, indicating that the Liushugou rupture zone seems to be formed in a very young period. Currently, there is no absolute dating method for the most recent surface rupture zones. The Liushugou Rupture Zone is situated primarily in the Piedmont region along the southern margin of the Eastern Tianshan. As it is relatively remote from human settlements and experiencing minimal anthropogenic activity, this area has preserved both the original geomorphic surfaces and the free faces of the fault scarps in a relatively intact state. Due to long-term exposure to intense sunlight and other cosmic rays, the gravel surfaces on the original geomorphic surfaces are covered with a layer of black desert varnish, giving these surfaces a characteristic dark appearance. Once altered, however, the exposed surfaces starkly contrast with the surrounding black-colored original surfaces.

We noticed that a dirt road near the Liushugou Rupture Zone, along with an adjacent abandoned dirt road and a mid-20th-century canal, contrasted noticeably with the surrounding original geomorphic surfaces (Figure 11a,b). The original surfaces exhibit their typical dark coloration, whereas the currently used dirt road appears lighter, with tire tracks showing a white color (Figure 11b-I). The abandoned dirt road has turned grayish-white (Figure 11b-II), and the abandoned canal, lined with gravel, shows a pale yellowish-white color (Figure 11b-III). The formation ages of the currently used and abandoned dirt roads are definitely relatively young. We can compare the light grayish-white or pale yellowish-white appearance of the Liushugou Rupture Zone with that of the roads (Figure 11c,d). This method of relative comparison suggests that the formation age of the rupture zone is relatively recent. If it had formed much earlier, the free face of the scarp would have developed a darker gray color, more similar to the original geomorphic surfaces (Figure 11e).

5.2. Causal Mechanism of the Rupture Zone

In general, earthquakes with magnitudes of 6³/₄ and higher in the inland regions of China produce surface rupture zones of varying scales [34]. The lower magnitude threshold for earthquakes capable of generating surface rupture zones is approximately 6.6. Historical seismic records indicate that only two earthquakes with magnitudes above 6.6 have been recorded in the Eastern Tianshan region: the M7¹/₂ earthquakes in 1842 and 1914. Historical records show that the areas most affected by these earthquakes were, in order of the severity of earthquake damage to the area: Barkol > Hami > Shanshan. The isoseismal maps of these two historical earthquakes indicate that the long axes of the isoseismal ellipses are oriented NW, consistent with the strike of the Barkol Mountains-Karlik Mountains (Figure 12). This suggests that these two earthquakes may be closely related to the faults along the northern and southern boundaries of the Barkol Mountains-Karli Mountains range [9]. Confirmed rupture zones in the region include the Tazi Bulake Rupture Zone, the Xiongkuer Rupture Zone, and the Yiwu Salt Pond Rupture Zone. According to research by Wu F. [9], the Xiongkuer Rupture Zone may have been formed by the 1842 M7¹/₂ earthquake, while the Yiwu Salt Pond Rupture Zone is likely associated with the 1914 M7¹/₂ earthquake. The Tazi Bulake Rupture Zone may have been formed due to an unrecorded seismic event. The rupture zone discovered in the Liushugou area, located on the western section of the HMNF, extends for a total length of 21 km, trending NWW. This orientation is nearly parallel to the long axes of the isoseismal ellipses of the 1842 and 1914 earthquakes. The distance between this rupture zone and the epicentral region of the two major earthquakes near Barkol County is only about 30 km. The rupture zone is located within the VIII intensity zone of the 1842 earthquake and the VII as well as VIII intensity zones of the 1914 earthquake (Figure 12). Taken together, these factors suggest a close relationship between the Liushugou rupture zone and the two M7¹/₂ earthquakes in the Eastern Tianshan region.

The Yiwu Salt Pond Rupture Zone is located in the Salt Pond–Kuisu area to the west of Yiwu County, with a thrust coseismic displacement of 1.5 m and a length of at least 40 km [9]. Later, it was re-evaluated by Xu [11], who extended the rupture zone to 90 km. The Tazi Bulake Rupture Zone, situated approximately 70 km NE of Shanshan, crosses the S241 road and extends for about 20 km. The latest fault activity in this zone has resulted in approximately 4 m of left-lateral displacement, along with a vertical displacement of 1 m [9]. The Yiwu Salt Pond and Tazi Bulake rupture zones are approximately 120 km and 150 km away from the Liushugou Rupture Zone studied in this research, respectively. Given the significant separation between these rupture zones (or causative faults) and the magnitudes of the two major earthquakes in the Eastern Tianshan region, it seems unlikely that the Liushugou Rupture Zone formed simultaneously with one of these rupture zones during a single seismic event.

The Xiongkuer Rupture Zone is located in the Xiongkuer area, southwest of Barkol County, adjacent to the northern side of the Barkol Mountains. This rupture zone extends approximately 45 km in length, with a left-lateral coseismic displacement of 4 ± 1 m and no thrust component [9]. The Liushugou Rupture Zone lies about 25 km southeast of the Xiongkuer Rupture Zone and approximately 30 km south of the epicentral area near Barkol County. Additionally, the Liushugou Rupture Zone displaces the youngest geomorphic units along the HMNF, such as alluvial flats and the first-order terrace, with the morphology of the rupture scarp being relatively new. The morphological features of the scarp surface are similar to that of the Xiongkuer Rupture Zone (Figure 13), suggesting that the Liushugou Rupture Zone may have been formed on the southern side of Barkol Mountains during the 1842 M7¹/₂ earthquake in Barkol. The surface rupture likely resulted from the concurrent activity of a strike-slip fault and a thrust fault during a major seismic event, creating two distinct rupture zones on the northern and southern flanks of the Barkol Mountains. This pattern, typical of a positive flower structure (Figure 14), resembles the deformation observed on the northern and southern flanks of Longshoushan. The boundary faults of Longshoushan exhibit both thrust and strike-slip components, with the northern margin fault of Longshoushan being identified as the causative fault of the 1954 Shandan M_S7¹/₄ earthquake [35,36,37]. Zhao et al. [38] discovered a rupture zone over 20 km long along the southern margin fault of Longshoushan. Based on previous studies, it is believed that the 1954 Shandan M_S7¹/₄ earthquake caused simultaneous surface ruptures along both the northern and southern margin faults of Longshoushan, exhibiting a positive flower structure deformation pattern.

According to the empirical relationships established by Deng [34] for the Xinjiang region, the magnitude (M) of thrust faults is related to surface rupture length (L) and displacement (D) by the equation M = 5.28 + lgL + lgD, and for strike-slip faults by the equation M = 5.42 + lgL + lgD. Additionally, the relationship between magnitude (M) and energy release (E) is given by the equation lgE = 4.8 + 1.5 M [39]. According to the research conducted by Wu F. [9], which determined a length of 45 km and a coseismic displacement of 4 ± 1 m for the Xiongkuer rupture zone, and the insights that the Liushugou Rupture Zone is characterized by a length of 21 km and a maximum offset of 2.13 m in this study, the estimated magnitude of the 1842 earthquake in the Eastern Tianshan region is approximately M7.6.

However, there is also a possibility that the Liushugou seismic surface rupture zone could be formed by an unrecorded clustered seismic event that occurred close in time to the 1842 M7¹/₂ earthquake. Using the same empirical formula M = 5.28 + lgL + lgD, the magnitude of this potential event is estimated to be around M6.9.

Additionally, considering the empirical formulas derived by Wells and Coppersmith [40], M = 6.52 (±0.11) + 0.44 (±0.26) log(MD) and M = 6.64 (±0.16) + 0.13 (±0.36) log(AD) (where MD is the maximum coseismic displacement and AD is the average coseismic displacement), the magnitude is calculated to be between M6.5 and M6.6. Given that the widely accepted minimum magnitude for generating surface ruptures in the interior of mainland China is 6.6, it is believed that the magnitude of this event is likely to be 6.6~6.9.

5.3. Seismic Hazard of the Northern Margin Fault of the Hami Basin

The HMNF can be broadly divided into eastern and western segments, with the Yushugou area serving as the boundary. The Liushugou surface rupture zone and the low Holocene scarps around Nanshankou indicate that the latest activity of the western segment of the fault occurred during the Holocene, whereas the latest activity of the eastern segment dates back to the Late Pleistocene, with no evidence of Holocene activity. Wu F. [9] conducted a paleo-seismic study by excavating a trench across a typical thrust scarp in Dewaili (Figure 1c and Figure 14), identifying two paleo-seismic events (U1, U2) and constraining the ages of the stratigraphic layers. This study estimated the average recurrence interval of the Northern Margin Fault of the Hami Basin, ranging from 5.6 ± 1.3 ka to 9.5 ± 1.1 ka.

The Liushugou rupture zone discovered in this study is likely a result of the 1842 M7½ earthquake, indicating that the western segment of the fault, specifically the Erdaogou to Wuzun Bulake section, has a relatively short elapsed time since the last major earthquake. This suggests a lower probability of a significant earthquake occurring within the average recurrence interval of the fault in the near future. However, no seismic rupture zones or earthquake records have been identified in the Erdaogou to Nanshankou section on the west segment of the HMNF, suggesting a longer elapsed time, possibly nearing the average recurrence interval of the fault. This suggests a potential for future seismic activity, warranting increased attention and further study to more accurately assess the seismic hazard in this area.

As for the eastern segment of the fault, there is no evidence of Holocene activity based on current research, suggesting a lower likelihood of future seismic events.

6. Conclusions

Based on the detailed field survey and precise interpretation of UAV imagery of the western segment of the HMNF, the essential characteristics of the seismic surface rupture zone on this segment have been obtained. The main findings are as follows:

(1)

The total length of the Liushugou rupture zone is approximately 21 km, with a maximum coseismic displacement of 2.13 m and an average displacement of 0.56 m. The seismogenic structure is associated with the western segment of the HMNF.

(2)

The Liushugou rupture zone is likely related to the 1842 M7¹/₂ earthquake in the Eastern Tianshan region, during which both the northern and southern boundary faults of the Barkol Mountains simultaneously ruptured the ground surface, characterized by a flower-like pattern, and the estimated magnitude of the earthquake is 7.6.

It is also possible that the Liushugou surface rupture zone resulted from a separate, clustered seismic event close in time to the 1842 M7¹/₂ earthquake. The estimated magnitude of such an earthquake would be about 6.6~6.9.

(3)

The paleo-seismicity of the western segment of the HMNF, particularly from Erdaogou to Nanshankou, suggests that the elapsed time since the last earthquake is close to the average activity cycle of the fault. Therefore, there is a higher likelihood of future seismic activity in this segment.