Holocene Activity Characteristics and Seismic Risk of Major Earthquakes in the Middle Segment of the Jinshajiang Fault Zone, East of the Qinghai–Tibetan Plateau

1. Introduction

The Jinshajiang fault zone is an important fault located in the eastern part of the Qinghai–Tibetan Plateau. It is considered to be the western boundary fault zone between the Sichuan–Yunnan block and the eastern Xizang block and is characterized by reverse dip-slip motion [1]. Since the Cenozoic, the collision of the Indian Plate with the Eurasian Plate has caused strike-slip motion along major faults and the significant uplift of the Tibetan Plateau, leading to intense fault activity and folding deformation in the eastern boundary region of the Xizang block [2,3,4]. The Jinshajiang fault is located at the forefront of the eastern Xizang block, and its activity since the Late Cenozoic has primarily manifested as intense east–west (EW) compression, which absorbed part of the movement of the eastern Tibet block towards the Sichuan–Yunnan block via EW shortening [5,6,7].

The geometric structure of the Jinshajiang fault zone is complex, intersected by the northeast (NE)-trending Batang fault and the northwest (NW)-trending Deqin–Zhongdian–Daju fault, dividing the fault zone into northern, middle and southern sections. Of these, the northern segment is NW-trending with a relatively simple structure. The latest research indicates that the northern section reflects Late Pleistocene activity [8]; the central section of the fault is more complex, consisting of multiple fault branches oriented south–north (SN), NE or north-northwest (NNW), and displays Holocene–Late Pleistocene activity. Meanwhile, the activity in the southern section of the fault is weak [6,9]. Views about the motion characteristics and slip rate of the Jinshajiang fault zone differ. Wang et al. (2008) believe that the current activity of the Jinshajiang fault zone is weak, according to GPS data used to constrain and invert the current activity rates of major faults in the Sichuan–Yunnan region [10]. Conversely, the GPS inversion results of Xu et al. (2020) suggest that the activity of the northern section of the Jinshajiang fault zone is not significant, while the middle and southern segments exhibit obvious right-lateral strike-slip movement, with slip rates of 4.9 mm/a and 5.5 mm/a, respectively [11]. On the basis of seismic geological surveys, Zhou et al. (2005) believe that the Hetaoping fault and the Hongjunshan fault in the middle segment of the fault zone exhibit left-lateral strike-slip movement, with the Yarigong fault in the middle segment exhibiting right-lateral strike-slip movement with a slip rate of 3.3–4.1 mm/a [6]. The current seismic activity along the Jinshajiang fault zone is unevenly distributed. Only the Batang fault within the fault zone has experienced an M7.3 earthquake (in 1870) since the beginning of historical records of strong earthquakes in the area [12,13]. There are no historical records of earthquakes stronger than M6 along the northern and southern sections of the main fault; however, the middle section has experienced multiple earthquakes ranging from 6.0 to 6.9 in magnitude [6,12]. Yi et al. (2017) analysed the focal mechanism solutions of the 2016 Litang M5.1 earthquake swarm and the 1989 Batang M6.7 earthquake swarm, both of which occurred in the middle segment of the Jinshajiang fault zone [12]. They found that these earthquakes exhibited normal faulting characteristics, which do not match the motion characteristics of the main faults in the fault zone. They suggest that the seismogenic structures are nearly EW-trending secondary faults within the fault zone. On the basis of GPS and minor seismic activity studies, Xu et al. (2020) believe that the central segment of the Jinshajiang fault zone has a higher risk of strong earthquakes [11]. However, the structure of the middle section of the fault zone is complex, consisting of multiple Holocene-active faults, and the assessment of its seismic hazard still lacks confirmation via direct data from strong seismic activity.

The main fault in the central segment of the Jinshajiang fault zone exhibits Holocene activity; however, there are no major earthquakes in the historical records. Therefore, the potential for future strong seismic hazards deserves attention. Via interpretations of high-resolution satellite images and trench excavations, this paper examines the major earthquake rupture behaviour of the middle segment of the Jinshajiang fault zone and assesses its potential risk for major earthquakes.

2. Regional Seismic and Tectonic Setting

The Jinshajiang fault zone is located on the easternmost side of the Sanjiang tectonic belt, belonging to the back-arc basin formed by the expansion of the eastern margin of the ancient Tethys Ocean, and has undergone multiple and complex tectonic evolutionary episodes [14,15]. Since the Cenozoic, with the collision of the Indian and Eurasian plates, the strong uplift of the Tibetan Plateau has led to lateral slip along the main strike-slip fault system, including the Jinshajiang fault zone [16,17,18,19]. There has been more than 300 km of strike-slip movement along the Jinshajiang fault zone since the Paleogene [20,21].

In addition to the Jinshajiang fault zone, the main active faults in the study area include the Nujiang, Lancangjiang and Deqin–Zhongdian–Daju fault zones on the southwest side of the Sichuan–Yunnan block, the Ganzi-Yushu and Xianshuihe faults on the northern boundary of the block and, within the block, the Maisu, Darlangsunggou and Litang faults (Figure 1). Of these, the Nujiang fault is located at the westernmost boundary of the Sanjiang tectonic zone. The Nujiang fault has multiple branches with Holocene activity and a horizontal slip rate of 0.6–1.7 mm/a [22,23]. The Lancangjiang fault is a large regional fault. Its western branch is a Holocene-active fault, while its eastern branch exhibits different characteristics of segmented activity [24,25,26]. The Garzê–Yushu and Xianshuihe faults are boundary faults of the Sichuan–Yunnan block, not only exhibiting strong Holocene activity and high horizontal slip rates but also having experienced multiple strong and major earthquakes historically [27,28,29,30,31,32,33,34]. In the northern part of the study area, the Maisu fault is an EW-trending Holocene active fault in the southern part of the Garzê–Yushu fault. It preserves a seismic surface rupture zone with a length of 45 km and may accommodate a portion of the horizontal deformation of the Garzê–Yushu fault [35]. The Litang fault is an important NW-trending strike-slip fault within the Sichuan–Yunnan block, controlling the formation and development of some Tertiary–Quaternary basins. Its Late Quaternary slip rate was 2.3–5.0 mm/a [36,37]. The southern section experienced an M7.3 earthquake in 1948 and still preserves a well-defined seismic surface rupture. The Deqin–Zhongdian–Daju fault exhibits significant Late Quaternary activity with a horizontal slip rate of 1.7–2.0 mm/a; it is also considered part of the northwestern boundary of the Sichuan–Yunnan block [38]. Currently, moderate seismic activity is relatively frequent along the fault. The NW-trending, NE-trending and SN-trending major faults form the basic tectonic framework of the study area and control the seismic activity within the region.

The distribution of seismic activity in the study area is uneven. The boundary faults of the block are still the main seismogenic structures for major earthquakes, particularly the Garzê–Yushu and Xianshuihe faults. In addition, the step-over of the two faults is an active site for strong earthquakes. Furthermore, the Batang, Litang and Deqin–Zhongdian–Daju faults have all historically experienced major earthquakes and currently exhibit relatively frequent seismic activity [6,36,38]. Currently, seismic activity along the Jinshajiang fault zone is primarily concentrated on the eastern side of the fault zone, exhibiting a clustered distribution. However, the main earthquakes in these sequences are generally normal faulting earthquakes [12]. Meanwhile, the current seismic activities of the other active faults are relatively weak and sporadically distributed.

3. Data and Method

The active fault data and earthquake catalogue used in this article were provided primarily by the Sichuan Earthquake Agency. For the fault data, we used GF-7 satellite data to enable a detailed interpretation of the fault tracks and conducted field seismogeological work to identify the tracks of the newest activity along the Jinshajiang fault zone and to revise the fault data. On the basis of remote-sensing interpretation and field surveys, typical geomorphic displacement points were selected; then, high-resolution geomorphic images and digital elevation data of landforms were obtained via unmanned aerial vehicle (UAV) and structure from motion (SfM) technology [39,40,41]. Next, the evolution process of the geomorphic displacement was analysed to determine the amount of displacement and the displacement values were obtained using ESRI ArcMap. Finally, samples for carbon dating were collected from the corresponding geomorphic surfaces to estimate the slip rate of the Jinshajiang fault zone.

We also selected two fault troughs and excavated two trenches across them to conduct palaeoseismological studies. We used a scraper to smooth the trench walls and identified palaeoseismic events via the deformation and faulting characteristics of the strata exposed in the trenches. Then, samples for radiocarbon dating were collected layer by layer from the strata and sent to the Beta Analytic Inc. laboratory (USA) for testing. The radiocarbon ages were determined using accelerator mass spectrometry referenced to 1950 AD. Finally, we estimated the palaeoseismic events and obtained the palaeoearthquake sequence to assess the potential seismic hazard.

3. Result

3.1. Holocene Activity Characteristics of the Main Fault

3.1.1. The Main Fault Geometry

The main faults of the middle segment of the Jinshajiang fault zone include the NNW-trending Hongjunshan and Hetaoping faults, the north–northeast (NNE)-trending Yarigong fault, the NE-trending Ciwu fault and the nearly SN-trending western branch fault and Bendu fault (Figure 2a). Of these, the Hongjunshan, Hetaoping, Yarigong and Ciwu faults exhibit significant Holocene activity and are the main focus of this study (Figure 2b).

3.1.2. The fault Holocene Activity Characteristics

(1) Hongjunshan and Hetaoping faults

The northern branch of the middle segment of the Jinshajiang fault zone is composed of the Hongjunshan and Hetaoping faults, which are nearly parallel and generally NNW-trending. Zhou et al. (2005) suggested that the two faults exhibit left-lateral strike-slip characteristics [6]. On the basis of the dating result of a sag pond along the fault, the occurrence time of the latest earthquake is approximately 770 ± 30 a BP [6]. In Hongdong Village of Yarigong Town, Batang County, Sichuan Province, we also observed a fault profile in which the fault cut through the top of the Late Quaternary alluvial sand–gravel layer (Figure 3a), causing the Quaternary strata near the fault to gradually thin and thicken in the hanging wall (Figure 3b). Near the top of the profile, the fault splits into two branches, both cutting through all of the layers beneath the surface soil layer (Figure 3c).

(2) Yarigong fault

In Yarigong Town, the Holocene activity of the Yarigong fault, which is a NNE-trending branch of the Jinshajiang fault zone, is very prominent. The Yarigong fault has caused a series of gullies to exhibit right-lateral offsets, forming linear gully landforms on alluvial fans (Figure 4a). South of Yarigong Town, the Yarigong fault cuts across a gully, displacing it with a right-lateral offset of approximately 144 ± 16 m, forming a reverse steep slope of approximately 13 m (Figure 4b). The cutting depths of the gullies on both sides of the fault vary, with the upstream erosion being shallower on the northwestern side of the fault and deeper on the southeastern side. This erosion difference may be attributable to the fault activity. To ensure the reliability of estimating the fault slip rate, we collected optically stimulated luminescence (OSL) dating samples from the T4 and T3 terraces of the Muqu River (Figure 4c,d). The dating results were approximately 55.4 ± 3.4 and 31.9 ± 1.8 ka B.P. (Table 1), respectively. On the basis of these results, the upper limit of the fault’s right-lateral slip rate is estimated to be 4.5 ± 0.5 mm/a, the lower limit is 3.4 ± 0.2 mm/a, and the average is 4.0 ± 1.8 mm/a.

Several geological profiles are exposed along the Yarigong fault, all of which show that the fault has displaced the Late Quaternary strata. The profile on the northern side of Ran Village shows that the fault displaced the Holocene gravel layer (Figure 5a). Another profile in Dalong Village reveals that the fault has displaced the Late Miocene sand layers, which exhibit bedding with many poorly rounded gravel particles (Figure 5b). An exposed fault profile (5d) was observed in an open sand quarry north of Lide Village in Zhongzan Town, located on the T2 terrace of the Liqu River. The upper layers of the clear-bedded strata in the profile consist primarily of calcareous clay and gravel layers. Using a grey–green fine sand layer as a marker bed, the fault displacement is approximately 2.5 m. The fault dip direction is 123°, and the dip angle is 73° (Figure 5c). The sample (LC-05-C08) carbon dating result of the grey–green fine sand layer at the top of the profile is 15,190 ± 90 a B.P., indicating fault activity during the Late Pleistocene.

(3) Ciwu fault

The Ciwu fault is a NE-trending branch of the middle segment of the Jinshajiang fault zone and is a Holocene-active fault. Near Nidou Village, in Zhengdou Town, Xiangcheng County, a geological profile of the Ciwu fault is exposed (Figure 6) on the roadside, in which the fault has displaced the Late Quaternary sand–gravel layers. The strata on the footwall of the fault is a greyish-blue gravel layer (Stratum ① in Figure 6) with poorly rounded gravel. The strata on the handing wall of the fault is composed of a gravel-bearing sand layer (Stratum ② in Figure 6) with grey fine gravel lenses (Stratum ③ in Figure 6). The fault is covered by a brown soil layer (Stratum ④ in Figure 6) on top.

The Jinshajiang fault cuts across an alluvial fan southwest of Nidou Village, forming linear fault trough gullies and reverse fault scarps on the alluvial fan (Figure 7a). The fault scarp is approximately 0.5–2-m high (Figure 7b,c).

In Kagong Village, Ciwu Town, Derong County, the Ciwu River mainly develops the T1 and T2 terraces. The Ciwu fault right-laterally displaces the T2 terrace by approximately 60 ± 5 m (Figure 8a). An OSL dating sample (LX-G-910) was taken from the sand layer at the top of the T2 terrace (Figure 8b), yielding a result of 22.1 ± 1.0 ka B.P. Accordingly, the horizontal slip rate of the Ciwu fault since the Late Pleistocene is estimated at 2.71 ± 0.3 mm/a.

3.2. Palaeoseismic Studies

3.2.1. Trench TC1 at Bugge

The Yarigong fault cuts across the alluvial fans near Bugge Village, forming a well-defined and linear fault trough. We used UAV and SfM technology to obtain the topographic data along this fault segment. Trench TC1 at Bugge was excavated across the fault trough (Figure 9).

The TC1 trench reveals seven stratigraphic units, listed from old to new as follows (Figure 10).

U1: Grey–white and yellow–brown gravel layer, containing many highly weathered granite gravel particles, with angular to sub-angular shapes.

U2: Yellow–brown clay layer, containing many small gravel particles, with sizes generally less than 5 cm.

U3: Deep brown silt layer, containing many gravel particles, with gravel often deposited in clusters.

U4: Brown, dark grey sand and gravel mixed with yellow–brown clay clumps, with significant folding and deformation.

U5: Brown sandy soil layer, containing many gravel particles. This unit gradually thins towards the west.

U6: Yellow sand–gravel layer, with an especially higher accumulation of gravel at the bottom of the layer.

U7: Grey soil layer.

On the basis of an analysis of the faulted strata, sedimentation and deformation characteristics, three palaeoseismic events were identified in the TC1 trench. In Event I, faults F1 and F2 displaced units U1 and U2, forming a fault scarp, and subsequently U3 was deposited in the footwall of the fault scarp. Unit U2 is a yellow–brown clay layer, and unit U3 a dark brown silt layer. The units represent different depositional environments, and the compaction of U2 is greater than that of U3. In accordance with the results of the dating samples BGC14 from U2 and BGC27 from U3, Event I is constrained to have occurred between 5970 ± 30 a BP and 3520 ± 30 a BP. (Table 2). Event II mainly involved fault F3 displacing unit U3, forming a wedge fault zone, followed by the deposition of U4 with mixed sedimentary components. Constrained by the results of samples BGC27 from U3 and BGC26 from U4, the occurrence time of Event II should be between 3520 ± 30 a BP and 1940 ± 30 a BP. In Event III, faults F2, F4 and F5 displaced unit U4, resulting in the folding and deformation of this unit, which was subsequently covered by units U5, U6 and U7. The result of the dating sample BGC08 collected in U5 is 1220 ± 30 a BP. Therefore, Event III occurred between 1600 ± 30 a BP and 1220 ± 30 a BP.

3.2.2. TC2 Trench at Nidou

The TC2 trench is located on an alluvial fan south of Nidou Village, Xiangcheng County. The Ciwu fault cuts across the alluvial fan, forming a linear fault trough. We obtained high-resolution geomorphic images and digital elevation data of the area around trench TC2 using UAV and SfM technology. The Ciwu fault has right-laterally displaced the gully on the southern side of the trench by 18 ± 2 m (Figure 11). Descriptions of the strata revealed in trench TC2 are given from old to new as follows (Figure 12).

U1: Grey–yellow clay layer.

U2: Grey–brown clay layer.

U3: Yellow–brown powdery clay layer, containing a small amount of gravel.

U4: Dark brown, dark grey silty sand layer, containing a small amount of small gravel particles, rich in organic material.

U5: Grey–white, grey–brown silty sand layer, containing many small gravel particles, with grain sizes generally less than 3 cm.

U6: Yellow–brown sandy soil layer, rich in organic material at the bottom, dark brown and dark grey in colour.

U7: Sandy yellow soil layer, rich in plant roots, with a small amount of gravel.

On the basis of the faulted stratigraphy and deformation characteristics of the units in trench TC2, three palaeoseismic events were identified. In Event I, the fault primarily displaced unit U1, resulting in deformation of U1. Units U1 and U2 are in unconformable contact. The radiocarbon dates for samples NDC29 from U1 and NDC53 from U2 are 28,290 ± 150 cal BP and 32,250 ± 220 cal BP, respectively. However, the results for two other dating samples, NDC55 and NDC59, from the overlying unit U3 are 25,190 ± 110 cal BP and 20,030 ± 60 cal BP, respectively. Therefore, it is speculated that the test result of NDC53 is older than its actual age. Accordingly, the occurrence time of Event I is constrained to between 28,290 ± 150 and 25,190 ± 110 cal BP. The evidence for Event II is fault displacement of units U2 and U3, followed by the deposition of U4, which is in unconformable contact with U3. Unit U3 fractured, and subsequently unit U4 locally formed some wedge deposition. According to the dating result of NDC24 in unit U4, Event II likely occurred between 19,980 ± 60 cal BP and 19,080 ± 60 cal BP. The obvious evidence for Event III is that the F1 and F2 faults displaced the U4, U5 and U6 units and that the activity of F1 caused significant folding and deformation in U4, U5 and U6. The age of the dating carbon dating sample NDC44 collected from the top of unit U6 is 2750 ± 30 cal BP, suggesting that the occurrence of this event was close to this date. On the southern side of this trench site, the Ciwu fault has displaced a gully by approximately 18 ± 2 m (Figure 11a). The exposed units U1–U5 in trench TC2 are clay or silty layers, representing the original sedimentation prior to the uplift of this terrace. Conversely, U6 and U7 are brown sandy soil layers with coarser sediment grain sizes, likely deposited after the uplift of the terrace and influenced by the fault activity. The fault formed the linear fault trough on the terrace, and subsequently U6 and U7 were deposited in the fault trough. Therefore, according to the age of NDC02 from the top of U5 (8000 ± 30 BP), we estimate that the right-lateral slip rate of the fault at this site is 2.3 ± 0.3 mm/a.

4. Discussion

In this study, we examined the Holocene activity characteristics of the middle segment of the Jinshajiang fault zone using methods such as high-resolution image interpretation, seismic geological surveys and trenches. In Hongdong Village, the latest activity of the Huotaping fault resulted in deformation of the synchronous deposition and faulted the top sandy soil layer in the geological profile. This coincides with the latest earthquake event found by Zhou et al. [6]; this event occurred on the fault at 770 ± 30 a BP. In Yarigong Town, using the ages of the upper and lower terraces of the displaced river, the Late Quaternary slip rate of the Yarigong fault was constrained to between 4.5 ± 0.5 mm/a and 2.6 ± 0.3 mm/a, with an average slip rate of 3.6 ± 0.6 mm/a. This is consistent with the result of Zhou et al. [6]. Trenches TC1 at Bugge and TC2 at Nidou both recorded three palaeoseismic events. The stratigraphy–age model and palaeoseismic sequences were calibrated using OxCal v4.4.4 [42,43]. The occurrence ages of the three palaeoseismic events revealed in trench TC1 at Bugge were constrained to 6745–3848, 3742–1899 and 1494–1112 cal BP (Figure 13a). In addition, three palaeoseismic events were identified in trench TC2 at Nidou dated to 32,566–29,430, 24,056–22,990 and 2875–2723 cal BP (Figure 13b). The recurrence interval of the two latest palaeoseismic events revealed in trench TC2 is as long as 20,000 years. The absence of major earthquake records during such a long period may indicate missing palaeoseismic events. The unconformable contacts between units U4 and U3 and between units U5 and U6 revealed in trench TC2 indicate prolonged erosion periods without records of palaeoseismic events. Even during periods of erosion, it is possible to be a relatively weak or calm seismic activity period. This is because, if major earthquakes continued to occur, they would continue to form negative landforms such as fault troughs and deposit new sediments, thereby recording palaeoseismic events. Therefore, it is believed that the behaviours of the major earthquakes on the Yarigong and Ciwu faults are not consistent. The Yarigong fault indicates active large earthquake events during the Holocene, while the large earthquake activity on Ciwu fault has been relatively stable.

The Jinshajiang fault zone is a complex tectonic suture belt with multi-phase activity and is characterized by multiple sets of parallel or intersecting faults. This fault zone is thought to have distinct thrust characteristics [1,5,44]. However, our study found that the Jinshajiang fault zone’s main active fault with Late Quaternary activity exhibits predominantly strike-slip motion. Both the geological profiles and the trenches reveal that the main faults mostly exhibit a component of normal faulting and do not show significant thrusting motion characteristics. The occurrence of many normal-fault earthquakes in the Jinshajiang fault zone confirms the existence of these normal faults. GPS observations also show significant extensional movement of the block in the Batang-Litang region [45]. In the Jinshajiang fault zone, the Late Quaternary horizontal slip rate of the NE-trending Batang fault is 2–3 mm/a [1,6,46], and the horizontal slip rates of the NNE-trending Yarigong fault and the NE-trending Ciwu fault are 2.6–4.5 mm/a and 2.5 ± 0.5 mm/a, respectively. Although we did not obtain the horizontal slip rates of the NNW-trending Hongjunshan and Hetaoping faults, one can speculate that the single slip rates of the two faults should be less than those of the above-mentioned main faults. The variation in the strikes and horizontal slip rates of the main faults may be the main reason for the development of secondary EW-trending normal faults and relevant earthquakes in the Jinshajiang fault zone. The current moderate earthquakes in the study area are mostly normal-fault types, and the seismogenic structure does not correspond to those of the main faults of the Jinshajiang fault zone [12]. Furthermore, palaeoseismic studies indicate that the last major earthquake on the Ciwu fault occurred approximately 2800 years ago, hence its future seismic hazard deserves attention.

5. Conclusions

This paper examined the characteristics of Holocene activity in the middle segment of the Jinshajiang fault zone using methods such as high-resolution image interpretation, seismic geological surveys and trench studies and found that the predominant movement of the fault zone is strike-slip motion. The Late Quaternary horizontal slip rate of the NNE-trending Yarigong fault and the NE-trending Ciwu fault are 3.6 ± 0.6 mm/a and 2.5 ± 0.5 mm/a, respectively. We identified three palaeoseismic events on the Yarigong fault dated to 6745–3848, 3742–1899 and 1494–1112 cal BP, as well as another three palaeoseismic events on the Ciwu fault constrained to 32,566–29,430, 24,056–22,990 and 2875–2723 cal BP.