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GeoScienceWorld Lithosphere Volume 2021, Article ID 7866379, 17 pages https://doi.org/10.2113/2021/7866379

Research Article Latest Quaternary Active Faulting and Paleoearthquakes on the Southern Segment of the Xiaojiang Fault Zone, SE Tibetan Plateau

1,2 1,2 1,2 3 4 1,2 Peng Guo , Zhujun Han , Shaopeng Dong , Zebin Mao, Nan Hu, Fan Gao, 1,2 and Jiani Li

1Institute of Geology, China Earthquake Administration, Beijing 100029, China 2Key Laboratory of Seismic and Volcanic Hazards, China Earthquake Administration, Beijing 100029, China 3Yunnan Earthquake Bureau, China Earthquake Administration, Kunming 650041, China 4Shanxi Earthquake Bureau, China Earthquake Administration, Xi’an 710068, China

Correspondence should be addressed to Zhujun Han; [email protected] and Shaopeng Dong; [email protected]

Received 24 June 2020; Accepted 7 January 2021; Published 10 February 2021

Academic Editor: Andrea Billi

The Xiaojiang fault zone (XJFZ) is an important part of the Xianshuihe-Xiaojiang fault system, acting as the eastern boundary of the Chuan-Dian block on the southeastern margin of the Tibetan Plateau and accommodating the lateral extrusion of the block. The faulting activity and paleoseismic history on the southern segment of the XJFZ remain poorly understood. Here, trench excavations and radiocarbon dating revealed that four recent surface-rupturing paleoearthquakes have occurred on the Jianshui fault (JSF) in the southern segment of the XJFZ since ~15370 yr BP. The ages of these events, labeled E4-E1 from oldest to youngest, are limited to the following time ranges: 15360-12755, 10845-6900, 1455-670, and 635-145 yr BP, respectively. The most recent event E1 was most likely the 1606 Jianshui earthquake. These events appear to occur unregularly in time. The time interval between the last two events is 726 ± 235 yr, and the average recurrence interval for all four events is 4589 ± 3132 yr. The deformed strata show that the JSF is characterized kinematically by transtension, which likely respond to the apparent change in the direction of clockwise rotation of the Chuan-Dian block around the eastern Himalayan syntaxis. Combined with the analysis of the neighboring NW-striking faults, our study suggests that the south-southeastward motion of the Chuan-Dian block is likely to be ﬁrstly accommodated in part by the right-lateral shear and dip-slip motions of the Qujiang and Shiping faults and continues across the Red River fault zone, then is transmitted southward along the Dien Bien Phu fault. Therefore, the southern segment of the XJFZ plays a dominant role in the tectonic deformation of the southeastern Chuan-Dian block, with a high seismic hazard.

1. Introduction mately 400 km long and can be generally divided into three segments (Figure 1(a), [9–11]). The northern segment is The Xiaojiang fault zone (XJFZ) constitutes the continuous located on the northern side of Dongchuan County and is Xianshuihe-Xiaojiang left-lateral strike-slip fault system, composed of a single fault (Figure 1(a)). The central segment together with the Xianshuihe fault zone, Anninghe-Zemuhe is divided into eastern and western branches that extend fault zone, and Daliangshan fault zone (Figure 1(a)). The from Dongchuan County to Fuxian Lake. The southern seg- fault system acts as the eastern boundary of the Chuan- ment is located on the southern side of Fuxian Lake and is Dian block on the southeastern margin of the Tibetan composed of multiple fault branches to the north of the Plateau and plays a key role in accommodating the lateral Jianshui Basin. There is only one fault branch near and to extrusion of the block and clockwise rotation around the the south of the Jianshui Basin, which is called the Jianshui eastern Himalayan syntaxis (e.g., [1–5]). The tectonic defor- fault (JSF; Figure 1(a); [12]). mation and seismicity of the XJFZ have been intense since The southward motion of the western wall of the XJFZ or the late Quaternary, and the fault zone forms a vital part of Chuan-Dian block was generally considered to be absorbed the north-south seismic belt [6–8]. The XJFZ is approxi- and accommodated by the NW-striking Qujiang and Shiping

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a b c N 1896 M7 GZF

Ganzi Baiyun 1923 M7.3 Baiyun 1816 M7.5 Q LMSFLMSFLMSF Tibetan Plateau 1973 M7.6 XSHF 2008 Mw7.9 1904 M7 TC3 Fig.8 LTFLTFLTF 1893 M7 Bagenjiao Litang1955 M7.5 Sichuan 1870 M7.3 1725 M7 Basin Jianshui 1786 M7.8 Basin JSJF Shimian N Q

1948 M7.3 Lujiang RiverQ

ANHF ANHF ANHF Dongshanzhai N N Chuan-Dian block DLSF 1536 M7.5 Q Jianshui Jianshui 814 M7 Fig.5 TC2 TC1 1515 M7 1850 M7.5 Fig.2 ZMHF Fangmaping LJ-XJHF 1606 M ? Qiaojia Xinzhai Jianshui EQ Xinzhai N-XJF Q 1515 M7.8 1933 M7.8

Dongchuan Q

C-XJF C-XJF 1925 M7 C-XJF Q 1652 M7 1833 M8 ChuxiongChuxiongChuxiong South Kunming China Liangchahe Liangchahe 1970 M7.7 1500 M7 Fuixan Lake 1976 M7.4 RRF 1913 M7

1976 M7.3 QJF Q S-XJF S-XJF 1789S-XJF M7 SPF 1588 M7

1799 M7

JSF JSF

1887 M7 JSF Jianshui Basin NTHF 1941 M7 1988 M7.2 1606 M6.75 Niuguntang 1988 M7.6 0 100 200 Puer Fig.b km 1941 M7 Red River Red River N 1995 M7.3 RedQ River DBFF 1950 M7 04 km Mamuzhai 0 4km

10 mm/yr Strike-slip fault Magnitude Trust fault Q Q.deposits 7.0-7.4 N N.deposits 7.5-8.0 Normal fault Strike-slip fault Well-kept surf. rupture 1606 M? Jianhui EQ

Figure 1: Seismotectonic setting and distribution map of the Jianshui fault in the southern segment of the Xiaojiang fault zone. (a) Main active faults and large earthquakes with magnitudes of ≥7.0 in the study area. Fault locations and slip senses are simpliﬁed from Xu et al. [21]. Red lines represent the Xianshuihe-Xiaojiang left-lateral strike-slip fault system. The rectangular box represents the location of the Jianshui fault. Earthquake locations and magnitudes are from the China Earthquake Information Network. GPS velocity ﬁeld relative to stable Eurasia is from Liang et al. [22]. Abbreviations: GZF = Ganzi fault; XSHF = Xianshuihe fault; ANHF = Anninghe fault; ZMHF = Zemuhe fault; DLSF = Daliangshan fault; N-XJF = northern segment of the Xiaojiang fault; C-XJF = central segment of the Xiaojiang fault; S-XJF = southern segment of the Xiaojiang fault; JSF = Jianshui fault; QJF = Qujiang fault; SPF=Shiping fault; RRF = Red River fault; DBFF = Dien Bien Phu fault; NTHF = Nantinghe fault; LJ-XJHF = Lijiang-Xiaojinhe fault; JSJF = Jinshajiang fault; LTF = Litang fault; LMSF = Longmenshan fault. (b) Google Earth optical image showing the clear linear features of the Jianshui fault. Red arrows represent the location of the fault. (c) Distribution map of the Jianshui fault. The hillshade map is generated from the ASTGTM DEM (30 m resolution).

faults in the form of dextral shearing and transverse shorten- Basin. Accordingly, several large earthquakes with magni- ing [5, 9, 13, 14]. The left-lateral shearing of the Xianshuihe- tudes of ≥7.0 have occurred on the northern and central seg- Xiaojiang fault system might end to the north of the Jianshui ments throughout history (Figure 1(a)), including the 1733

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M7.75 Dongchuan earthquake, 1833 M8.0 Songming earth- Compared with the central and northern segments, the quake, 1500 M7 Yiliang earthquake, and 1789 M7 Huaning southern segment of the XJFZ, including the JSF, is still earthquake [5, 15, 16]. There were minimal activity and no poorly studied. He et al. [9] and Song et al. [11] believed that past surface-rupturing earthquakes in the southern segment the fault did not extend to the Red River valley and termi- of the XJFZ abutting the Red River fault zone [9, 11, 17, 18]. nated near Shanhua village, Jianshui County, 10-15 km from The only earthquake to occur on the western side of the JSF the Red River valley (Figure 1(b)). However, previous pro- is the 1606 M6.75 Jianshui earthquake [15]. However, the posed kinematic models of the SE Tibetan Plateau have GPS velocity field presented by Shen et al. [1] and the kine- shown that the Xiaojiang fault zone passes through the Red matic models of the SE Tibetan Plateau proposed by Schoen- River fault zone southward and connects with NE-striking bohm et al. [19] and Wu et al. [20] show that the crustal faults, including the Dien Bien Phu fault on the south side material of the Chuan-Dian block does not decelerate signifi- (Figure 1(a)), constituting a continuous eastern boundary cantly in the vicinity of the southern segment of the XJFZ of the southeastern margin of the Tibetan Plateau [4, 19, and that clockwise rotation continues across the Red River 20]. Han et al. [12] suggested that the JSF extends from the fault zone. Therefore, studying the late Quaternary faulting north side of the Jianshui Basin to the Red River valley and activity of the southern segment of the XJFZ will contribute estimated the Holocene left-lateral slip rate to be 7:02 ± to understanding the tectonic deformation pattern of the 0:20 mm/yr based on the offset measurements of the faulted Chuan-Dian block and reasonably assessing the seismic risks river terraces and radiocarbon dating of its formation age, of the SE Tibetan Plateau. which is in good agreement with the slip rate constrained To better understand the faulting activity of the southern by GPS data [1]. The paleoseismology of the fault has not segment of the XJFZ and its role in accommodating the tec- been reported. tonic deformation of the SE Tibetan Plateau, the JSF, abutting The Qujiang, Shiping, and Red River fault zones (QJF, the Red River fault zone, was mapped in detail through high- SPF, and RRF) are three important NW-WNW-striking resolution satellite image interpretation, field investigation, faults in the area (Figure 1(a)). They are distributed at nearly and fine-scale measurement of offset landforms. The defor- regular intervals from north to south, and these arcuate faults mation characteristics and paleoseismic history of the fault are convex to the southwest. Three earthquakes with magni- were constrained by the trench excavations and radiocarbon tudes of M ≥ 7 have occurred on the Qujiang fault in dating. Then, we discussed the latest Quaternary faulting recorded history, namely, the 1588 M7.0 Qujiang earthquake, behavior and kinematic property of the JSF and compared 1913 M7.0 Eshan earthquake, and 1970 M7.7 Tonghai earth- the tectonic activity of the fault with those of the neighboring quake [15, 27]. Among them, the 1970 M7.7 Tonghai earth- faults. Finally, we found that four paleoearthquakes occurred quake produced a right-lateral surface rupture zone with a on the JSF during the latest Quaternary and the fault plays a length of 60 km, a maximum right-lateral offset of 2.7 m, dominant role in the tectonic deformation of the southeast- and a maximum vertical offset of 0.47 m [28]. The Holocene ern Chuan-Dian block. right-lateral strike-slip rate of the Qujiang fault is 2.8- 3.5 mm/yr [29, 30]. Two earthquakes with magnitudes of 2. Regional Seismotectonic Setting M7 occurred on the Shiping fault in 1799 and 1887. The Red River fault zone experienced a motion reversal from The Chuan-Dian block is located on the southeastern margin left-lateral to right-lateral in the Neogene [31]. The right- of the Tibetan Plateau, bounding with Sichuan Basin and lateral slip rate of the fault has been estimated to be 2- South China blocks to the east (Figure 1(a)). The block moves 3 mm/yr [32], 2.0-2.6 mm/yr since the middle Pleistocene south-southeastwards due to the convergence and extrusion [33], and 1.2-1.4 mm/yr since the Pliocene [20]. Shi et al. from the India plate to the Eurasian plate [4, 19]. Two groups [2] revealed that the seismic recurrence interval of the fault of active faults with different strikes of nearly N-S and NW- zone has been 6000 ± 1000 yr over the last 30000 yr and that WNW are located in the southeastern Chuan-Dian block the late Quaternary slip rate is 1:1±0:4 mm/yr. (Figure 1(a)). The nearly N-S-striking faults are composed mainly of the XJFZ. Many large earthquakes with magnitudes 3. Methods of M ≥ 7 have occurred on the central and northern segments of the XJFZ throughout history. Among them, the 1833 M8.0 The surface traces of the JSF were mapped in detail based on Songming earthquake occurred on the western fault branch high-resolution (~0.6 m) Google Earth optical image inter- of the central segment and generated a surface rupture zone pretation and field investigations of offset landforms, includ- with a length of approximately 150 km and a maximum ing fault scarps, fault troughs, and left-laterally offset streams, coseismic offset of ~8.4 m [16]. The geologically measured terraces, and ridges. Three trenches were excavated in sag late Quaternary left-lateral slip rates of the central and ponds or small depressions where sediments were relatively northern segments are 10-16 mm/yr [10, 11, 23], and geo- continuous and undisturbed, and radiocarbon samples could detic measurements limit the slip rate of the fault seg- be collected. We used airborne light detection and ranging ments to be 7-10 mm/yr [1, 5, 24]. Previous trench (LiDAR) technique to finely measure faulted landforms near excavations revealed that several paleoearthquakes have the trench excavation sites to obtain high-resolution (0.5 m) occurred since the late Quaternary, and the time intervals digital elevation models (DEMs) after the removal of vegeta- between the events are 2000-4000 yr [11, 25], 2000-2500 yr tion [34–36]. Trenches were excavated across the fault [10], and 340-480 yr [26]. subvertically (Figure 1(c)), and trench sections were

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a N

0 50 100 m

N b Fig.d S5 S1 S2 S4 TC1

S3a S3b Fig.c 0 10050 m

c d

T2 Fault scarp

TC1 T1 S5

View to N View to E

Figure 2: Faulted landforms at the Fangmaping site and the location of trench TC1. (a) Topographic hillshade map from a LiDAR-derived DEM (0.5 m resolution). (b) Interpreted surface traces of the fault (red) and stream channels (blue). The white rectangular box represents the location of trench TC1. (c) Field photo showing a fault scarp. (d) Field photo showing displaced stream S5 and two terraces on both sides of the stream.

systematically cleaned to expose clear evidence of paleoseis- the Beta Analytics in the United States and State Key Labora- mic events. Horizontal level rulers were placed on the walls tory of Earthquake Dynamics in China for dating. The ages of of trenches TC1 and TC2. The structure from motion the samples are corrected as σ calendar ages (68.2% confi- (SFM) method was used to photograph the trench walls with dence interval) with the OxCal 4.3 procedure. The timings an overlap rate between photos of >70%. The SFM mosaic of the paleoseismic events from each trench were also was constructed utilizing Agisoft Photoscan software, and modeled and limited using the OxCal 4.3 software and detailed field logging was carried on the printed map; this the ages of the samples [47, 48]. If the radiocarbon ages map was then digitized using vector software [37–39]. For with unknown stratigraphic sequence were from the same the previously excavated trench TC3, a 1:0m×1:0m grid layer, they were put in a phase in the above modeling pro- was built on the walls to take photos, and square grid paper gram. Then, the progressive constraining method was used was used as a base map for logging in detail; these logs were to constrain the paleoseismic events and their ages from then digitized using vector software [40]. all the trenches [49–51]. Finally, the average recurrence Upward fault termination is generally the most effective interval and time intervals between the events were also for identifying the latest event, and later ruptures are likely estimated. to occur along preexisting fault planes (e.g., [38, 40, 41]). The most recent event was identified based on the offset strata at the uppermost termination of the fault. The older 4. Paleoseismic Investigation fi paleoearthquakes were identi ed using multiple identifying 4.1. Fangmaping Trench (TC1) features, including cross-cutting relationship, colluvial wedges, filled fissures, differential deformation, and angular 4.1.1. Site Location and Offset Landforms. Several streams unconformities between the layers in the trench sections along the slope flow from west to east at the Fangmaping site, (e.g., [38, 40, 42–46]). and flat alluvial terraces are developed on both banks of the We collected 19 radiocarbon samples from the faulted steams (Figures 2(a) and 2(b)). The linear features of the fault ° and unfaulted strata of the three trenches and sent them to striking N22 E can be identified on the DEM-derived

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a Fig d.

Fig c.

b F1-1

U2-4 U2-3 U3-1 F2-1 U2-2 U2-4 U2-3 U2-4 U2-2 U2-2 U2-3 F1-2 U2-1 F2-2 U2-1 U1-3 U-① U1-3 U1-2 U1-2 U1-1 U-① F2-3

F1 U-② U-② 0 50cm F2

c U2-2 d

U2-1 U3-1 U3-1 U2-4 U-① U1-3 U2-3 U2-4 U2-3 U2-2 U2-2 U1-1 U2-1 U1-2 U2-1 F1 U1-3 U-① U1-3

Figure 3: Photomosaic (a), interpretation map (b), and zoomed-in photos (c, d) of the trench section of TC1 at the Fangmaping site. Locations of (c) and (d) are shown in (a). (c) Photo of the oﬀset strata along fault F1. (d) Photo of the oﬀset strata along fault F2.

hillshade map without vegetation (Figure 2(b)), and fault 5:6±0:4 m. Although streams S2 and S5 have been artificially scarps in local areas are well preserved (Figure 2(c)). Both modified, offsets can still be identified (Figure 2(d)). There is streams and terraces are left-laterally displaced by the fault. no left-lateral offset in stream S3a. However, beheaded The offset of stream S1 is well preserved and measured to stream S3b is developed downstream of the fault on the

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eastern side of stream S3a (Figure 2(b)) and is interpreted as a Table 1: Unit description of TC1 at the Fangmaping site. result of the left-lateral offset of stream S3a. Two alluvial terraces can be identified on both banks of stream S5 and are Unit no. Description also offset left-laterally along the fault. U3-1 Gray-black modern soil layer. U2-4 Brown clay layer. 4.1.2. Stratigraphy. Trench T1 represents excavation of the U2-3 Gray-black silt layer. outcrop section of the terrace and has a length of 6.0 m and U2-2 Gray-brown clay layer. a depth of 2.0-2.2 m (Figure 3). The strata consist mainly of Gray-black silt layer mixed with clay. There is a U2-1 a weathered bedrock crust, clay layers on both sides, and clay trend of thinning eastward. layers in a small graben bounded by two groups of faults in U1-3 Gray-brown sandy clay layer. the middle. We divided these strata into ten layers that were numbered in ascending order from bottom to top. The U1-2 Gray-black silt layer. Yellow-brown wedge-shaped clay layer mixed description of the strata is shown in Table 1. U1-1 with pebbles. 4.1.3. Evidence for Events. Based on the oriented gravel clasts U1-① Limestone and weathered crust. in the gravelly clay layers and the deformation and lithologi- Greenish-yellow clay layer mixed with U1-② cal differences in the strata, two groups of faults can be iden- multicolored bands. tified, which dip opposite each other to form a small graben. Based on the analysis of deformed strata and paleoseismic markers, three paleoearthqakes were identified, named EF1, than that of the upper stratum. Samples of the same age in EF2, and EF3, from the youngest to the oldest. the same stratum were placed in a phase, and the ages of the paleoearthquake events were simulated using the OxCal (1) Event EF3. Faults F1 and F2-3 offset the weathered crust 4.3 program. The modeling results show that events EF3, of limestone U-① and greenish-yellow clay layer U-② EF2, and EF1 occurred before 1640, 1475-675, and after (Figures 3(a)–3(c)). A wedge-shaped deposit, gray-yellow 625-495 yr BP, respectively (Figure 4). gravelly clay layer U1-1, formed on the eastern side of fault F1 and is interpreted as a scarp-derived colluvial wedge deposited following an earthquake. The top contact of the 4.2. Dongcun Trench (TC2) gray-black silt layer U1-2 on the top of fault F2-3 is relatively 4.2.1. Site Location and Offset Landforms. Trench TC2 is stable and exhibits no obvious deformation. Additionally, located at the Dongcun site to the southeast of Jianshui layer U1-2 forms a clear unconformity with the underlying County. The geomorphology of the area is dominated by ① ② fl ff strata U1-1, U- , and U- ,re ecting di erential deforma- low hills and valleys. The fault cuts through the hillside tion. Therefore, event EF3 occurred before the sedimentation (Figure 5(a)). Field photos and the DEM-derived hillshade of U1-1. map without vegetation show that linear features are obvious along the fault and that offset landforms are clear (Figure 5). (2) Event EF2. The gray-brown sandy clay layer U1-3 is The fault shows seismic surface rupture zones, including a faulted by faults F1-2 and F2-2. The gray-black clayey silt fault scarp and a sag pond. Three small streams along the layer U2-1 on top of F1-2 and F2-2 is continuous and undis- slope were synchronously left-laterally offset (Figure 5(c)). fi turbed (Figures 3(b) and 3(c)). A lled wedge or wedge- The back-slipped measurements of the high-resolution hill- shaped deformation zone formed between faults F2-1 and shade map show that streams S1, S2, and S3 have similar F2-2. The thickness of layer U2-1 thins eastward, and this amounts of left-lateral displacement of 6.8 m (Figure 5(d)). unit is interpreted as a scarp-derived wedge-shaped sediment The sag pond is developed on the eastern side of stream S2, following an earthquake. Layer U1-3 along fault F2-1 has a where the terrain is relatively gentle (Figure 5(d)). The sag larger displacement than the overlying layer U2-1, indicating pond was chosen to be excavated for TC2. A landslide was ff di erent amounts of deformation between layers U1-3 and observed along the fault at the western end of the study area U2-1. Therefore, the above evidence suggests that event (Figure 5(a)), but it destroyed the continuity of the surface EF2 occurred after deposition of U1-3 and before deposition ruptures; this landslide may have been caused by the most of U2-1. recent event. (3) Event EF1. Faults F1-1 and F2-1 offset layers U2-2, U2-3, U2-4, and U3-1 and almost extend to the ground surface 4.2.2. Stratigraphy. Trench TC2 is approximately 14.0 m (Figures 3(b) and 3(d)). The most recent event occurred after long, 2.0 m wide, and 2.8-3.5 m deep. The strata consist the sedimentation of layer U3-1. The well-preserved fault mainly of weathered limestone crust and fault rocks on both scarps are interpreted to have been caused by event EF1. sides of the south wall and several sets of gravelly clay layers in the middle part (Figures 6 and 7). The distribution of the 4.1.4. Radiocarbon Dating of Events. Five radiocarbon clay layers is controlled by the fault zones, and they form samples were collected and tested from the trench section an extensional graben-shaped structure. The fault rocks can (Table 2). The ages of these samples are in a normal sequence be roughly divided into 3 sets of rocks with different degrees (Figure 3(b)); that is, the age of the lower stratum is older of faulting. Five sets of layers, named in ascending order from

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Table 2: Radiocarbon samples and dating results from the three trenches along the Jianshui fault.

Lab code Sample no. Radiocarbon age (a BP)1 Calibration years (cal BP)2μ ± σ Analyzed material Lab3 Trench no. Beta-482197 FMP-C-2 630 ± 30 606 ± 34 Organic sediment Beta-482199 FMP-C-6 720 ± 30 668 ± 26 Organic sediment Beta-482200 FMP-C-9 680 ± 30 629 ± 41 Organic sediment TC1 Beta-482201 FMP-C-12 1750 ± 30 1658 ± 43 Charcoal Beta-482202 FMP-C-14 1610 ± 30 1487 ± 47 Organic sediment Beta-493806 DC-C-1 9620 ± 30 10964 ± 110 Organic sediment Beta-493807 DC-C-2 10810 ± 30 12715 ± 17 Organic sediment Beta-493805 DC-C-5 12940 ± 40 15463 ± 106 Organic sediment BETA Beta-493804 DC-C-6 9920 ± 30 11311 ± 52 Organic sediment Beta-493808 DC-C-7 10320 ± 30 12139 ± 104 Organic sediment TC2 Beta-493814 DC-C-25 6010 ± 30 6849 ± 44 Organic sediment Beta-493813 DC-C-28 210 ± 30 182 ± 94 Organic sediment Beta-493813 DC-C-27 1780 ± 30 1696 ± 54 Organic sediment Beta-493813 DC-C-23 15780 ± 50 19029 ± 79 Organic sediment Beta-493803 DC-C-22 13490 ± 40 16239 ± 91 Organic sediment CG-2013-126 XJ-C4 425 ± 20 495 ± 20 Charcoal CG-2013-127 XJ-C5 3600 ± 25 3908 ± 38 Charcoal SKLED TC3 CG-2013-128 XJ-C6 4900 ± 25 5627 ± 21 Organic sediment CG-2013-129 XJ-C7 10530 ± 40 12491 ± 60 Organic sediment Note. 1The age is in radiocarbon years using the Libby half-life of 5568 years, and the uncertainties are reported as 1σ. 2The calendar dates were calculated using the OxCal v4.3 program ([47]; [48]). 3BETA: Beta Analytic Inc., Miami, Florida 33155, USA; SKLED: State Key Laboratory of Earthquake Dynamics (SKLED), A-1, Huayanli, Beijing 100029, China.

EF1

R_Date FMP-C-2

R_Date FMP-C-9

R_Date FMP-C-6

Phase U2-3

EF2

R_Date FMP-C-14

R_Date FMP-C-12

Phase U1-2

EF3

Sequence TC1

2200 2000 1800 1600 1400 1200 1000 800 600 400 Cal yr BP

Figure 4: Timings of paleoseismic events recorded in TC1 constrained with OxCal modeling.

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a View to E

TC2

Landslide

b N c

S1u S2u S3u TC2 Fig.c

S1d S2d S3d

20m 10m

d View to E e

Ofset: 6.8 m

TC2

Figure 5: Faulted landforms in the Dongcun site and the location of trench TC2. (a) Field photo showing oﬀset landforms at the Dongcun site. Red arrows point to the surface rupture, and blue lines represent major stream channels. (b) Hillshade map generated from a terrestrial LiDAR-derived DEM (0.25 m grid size). (c) Contour map (interval: 0.2 m). Separated channels with the smallest local oﬀset are traced (blue lines along channel thalwegs) and labeled numerically upstream “u” and downstream “d” of the fault. These channels are projected onto an idealized planar fault (red line). The white rectangular box shows the trench location. (d) Field photo showing the location of TC2. (e) Stream channels are approximately realigned on the hillshade map with ~6.8 m of back-slip.

bottom to top, can be recognized from the trench section. an unconformity with the underlying layers U1, U2, and The description of strata is shown in Table 3. fault rocks, reflecting differential deformation. The above deposition and deformation features indicate that event 4.2.3. Evidence for Events. The trench sections show that the ED3 occurred before deposition of U3 and after deposition fault zone comprises a series of faults and fault rocks between of U2. the faults (Figures 6 and 7). The faults form a negative flower structure, controlling the distribution of deposits. Based on (2) Event ED2. Fault F3 offsets the gray-brown clay layer U3 the analysis of deformed strata and paleoseismic markers, (Figure 6). No deformation is found in the overlying brown three paleoseismic events were recognized, named ED1, silty clay layer U4 at the top of fault F3, showing that F3 offset ED2, and ED3, from youngest to oldest. layer U3 but did not offset layer U4. Additionally, layer U3 contains a large amount of peat, which is interpreted to have (1) Event ED3. Faults F6, F7, F8, and F9 offset the black been the paleo ground surface. Therefore, these observations organic-containing silt layer U1 and the brownish sandy show that event ED2 occurred between the sedimentation of and gravelly clay layer U2 (Figures 6 and 7). The overlying layers U3 and U4. brownish clay layer U3 mixed with peat at the top of these four faults is relatively stable and exhibits no visible defor- (3) Event ED1. Faults F2, F5, and F10 offset brown silty clay mation. Layers U1 and U2 have more deformation and a layer U4, and F11 ruptures upward to the ground surface more limited distribution than layer U3. Layer U3 features (Figure 6). No faulting deformation is observed in the

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b U5-1 100cm U4 U5-2 U5-1 U4 F10 U3 U3 F13 F5 U5-2 F1 Z2 F4 U3 F11 F3 F9 F2 U2 F6 F8 Z2 U2 F12 F14 Z2 F7 Ub Z3 Z1 U1 Z2

Fault zone

Figure 6: Photomosaic (a) and interpretation map (b) of the south wall of trench TC2 at the Dongcun site.

a b

U5-1

F5 F10 U4

F9 F7 F6 F8 U2 U2 Z1 Z3 Z2 Z1 Ub Z2 U1 50cm

Figure 7: Photomosaic (a) and interpretation map (b) of the north wall of trench TC2 at the Dongcun site.

overlying layer U5-1 (Figure 7). Additionally, a well- 4.2.4. Radiocarbon Dating of Events. Ten radiocarbon sam- preserved surface rupture zone is observed at this site ples were collected and tested from the trench section (Figure 5). The deposition and geomorphology features indi- (Table 2). The ages of most samples are in a normal sequence cate that the most recent event was likely to occur after the (Figures 6(b) and 7(b)). However, the age of sample DC-C-23 deposition of layer U4 and might almost have ruptured (19029 ± 79 yr BP) in layer U3 in the north wall is much older upward to the ground surface. than that of other samples (~11000-13000 yr BP) in U3 in the

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Table 3: Unit description of trench TC2 at the Dongcun site. (1) Event EL2. Fault F1 offset dark brown subangular clayey gravel layer U2, resulting in faulted contact between U1 Unit no. Description and U2 (Figures 10(a) and 10(b)). Above F1, no deformation U5-1 Gray-black organic-rich, gravelly clay layer. is found in the overlying brown gravel layer U3. Layer U3 U5-2 Brown-reddish clay layer. features an unconformity with the underlying layer U2 and fl ff U4 Brownish silty clay layer. the limestone tectonic zone, re ecting di erential deforma- U3 Brownish clay layer mixed with peats. tion. The above features indicate that a seismic event occurred after sedimentation of U2 but before that of U3. U2 Brownish clay layer mixed with gravels and sands. U1 Black organic-rich silt layer. (2) Event EL1. Faults F2 and F3 faulted layers U3 and U4, and Ub Highly weathered limestone. the overlying layer U6 is relatively stable without visible Z1 Fine-grained cataclastic zone. deformation (Figure 10(b)), indicating that the contact Z2 Coarse-grained cataclastic zone. between the U6 and underlying strata is a sedimentary con- Z3 Clayified fault zone. tact and that only a single event happened between the sedimentation of U3 and U6. Layer U5 is a wedge-shaped mixed deposit and is interpreted as a scarp-derived colluvial wedge south wall and older than that of samples DC-C-22 deposited following an earthquake. The fault zones of F2 and (16239 ± 91 yr BP) and DC-C-5 (15463 ± 106) from underly- F3 are wider than those of the other faults (Figures 10(b)– ing layer U1 (Figures 6(b) and 7(b)). The sample was likely to 10(e)), reflecting differential deformation. Thus, the above be reworked from an older deposit. Therefore, we interpreted evidence shows that event EL1 occurred between the sedi- that the age of sample DC-C-23 might be an outlier, and mentation of layers U4 and U6. the sample was excluded when limiting the timings of paleoseismic events. Samples of the same age in the same 4.3.4. Radiocarbon Dating of Events. Four radiocarbon sam- stratum were placed in a phase, and the age of paleoearth- ples were collected and tested from the trench section quake events was simulated using the OxCal 4.3 program. (Table 2). The ages of the samples are in a normal sequence The modeling results show that events ED3, ED2, and (Figure 10). The modeling results show that events EL2 and ED1 occurred at 15365-12725, 10840-6890, and 1685- EL1 occurred at 12490-5655 and 3845-505 yr BP, respectively 185 yr BP, respectively (Figure 8). (Figure 11).

4.3. Luoshuidong Trench Site (TC3) 5. Discussion 4.3.1. Site Location and Offset Landforms. The Luoshuidong 5.1. Paleoseismic Sequence of the JSF. The progressive con- site is located in the northern part of the fault. Streams and straining method was used to constrain the paleoseismic alluvial fans are developed along the slope (Figures 9(a) and events and ages in the three trenches (Figure 12). The 9(b)). Satellite images and field investigations reveal that the trenches of TC1 and TC2 showed that faults have cut the fault shows a linear fault scarp and fault trough (Figures 9(b) layer with a formation age of ~625 yr BP but have not cut and 9(c)). Three streams are left-laterally offset when crossing the layer with a formation age of ~185 yr BP (Figures 3 and the fault (Figure 9(b)). The alluvial fans and small ridges on 7). Although trench TC3 did not reveal evidence of faulting both banks of the streams also show left-lateral offsets. The of the modern soil layer (Figure 10), fresh earthquake surface back-slip measurements of the satellite image show that these rupture zones were observed at all three trench sites. Thus, three-stream channels have similar amounts of left-lateral off- the most recent events EF1 and ED1 identified in the two set of ~4.2 m (Figure 9(d)). A small depression is developed at trenches were interpreted to be the same event (E1 of the the front of the fault scarp and which is conducive to deposi- fault). Based on the analysis of the geomorphic, geologic, tion. Thus, trench T3 was excavated in the small depression and historical data, the most recent event E1 of the fault is across the fault scarp with a height of 2.5-3.7 m (Figure 9(c)). most likely the 1606 Jianshui earthquake. ff 4.3.2. Stratigraphy. Trench TC3 is approximately 15.0 m Layer U1-3 in TC1 is o set along faults F1-2 and F2-2, long, 2.0 m wide, and 2.0-2.7 m deep. The strata of the trench and overlying layer U2-1 is not deformed (Figure 3). Layer exhibit a graben-like structure, with fault zones on both sides U2-1 is a scarp-derived wedge-shaped deposit deposited fi of the section and clay and gravel layers in the middle. Six sets following an earthquake. A lled wedge is developed of layers, named in ascending order from bottom to top, can between faults F2-1 and F2-2. The displacement of layer be recognized from the trench section (Figures 10(a)–10(c)). U1-3 along fault F2-1 is larger than that of the overlying The description of strata is shown in Table 4. layer U2-1. Therefore, the evidence of the penultimate event EF2 is strong in TC1 (Figure 3). In addition, the 4.3.3. Evidence for Events. According to the aligned gravel age of paleoseismic event EL1 in TC3 overlaps with event clasts in the gravel layer and the deformation and lithological EF2 in TC1 (Figure 12). Therefore, EF2 and EL1 in differences among the strata, two groups of faults with oppo- trenches TC1 and TC3 were interpreted as seismic event site dips were identified. Based on the deformed strata and E2 of the fault. paleoseismic markers, two paleoseismic events were recog- Evidence for event EL2 in trench TC3 at the Luoshuidong nized, named EL1 and EL2, from the youngest to the oldest. site is weak (Figure 10), but the age range of the event

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R_Date DC-C-28

ED1

R_Date DC-C-27

R_Date DC-C-25

Phase U4

ED2

R_Date DC-C-1

R_Date DC-C-6

R_Date DC-C-7

R_Date DC-C-2

Phase U3

ED3

R_Date DC-C-5

R_Date DC-C-22

Phase U1

Sequence TC2

20000 15000 10000 5000 0 Cal yr BP

Figure 8: Timings of paleoseismic events recorded in TC2 constrained with OxCal modeling.

overlaps with the age range of event ED2 in trench TC2 5.2. Seismic Recurrence Characteristics of the JSF. We used the (Figure 12). Layers U3 and U4 in TC2 have differential defor- OxCal model to calculate intervals with σ variability of mation (Figures 6 and 7), and layer U3 is interpreted to have 5199 ± 1429, 7840 ± 1214, and 726 ± 235 yr between events been the paleo ground surface. Considering the evidence of E4, E3, E2, and E1, respectively. This indicates an average the event from these two trenches, this event was interpreted recurrence interval of 4589 ± 3132 yr. The time intervals as paleoseismic event E3 of the fault. between the three old events E4, E3, and E2 are much longer Event ED3 was the oldest event in trench TC2 and was than the time interval between the two young events E2 and not observed in the other two trenches (Figure 12). Layers E1. There are two possible explanations for this trend: (1) the U1 and U2 in TC2 have more deformation and a more lim- fault features irregular earthquake recurrence; (2) some ited distribution than layer U3 (Figures 6 and 7). Layer U3 events may be missing. features a distinct unconformity with the underlying layers Trenches TC1 and TC2 are only ~1.5 km apart U1, U2, and fault rocks, reflecting differential deformation. (Figure 1(c)), and the surface trace of the fault is continuous The strong evidence permits us to interpret this event as between these sites. The two locations should have suffered paleoseismic event E4 of the fault. from the same number of seismic events. Trench TC1 is Therefore, four paleoearthquakes occurring since located on a gentle alluvial terrace. Multiple sets of thin clay 15370 yr BP were constrained from these three trenches layers have been deposited since ~1658 ± 43 yr BP and are (Figure 12). The ages of events E4, E3, E2, and E1, from distributed in a small graben. Therefore, the strata in trench the oldest to the youngest, are 15360-12755, 10845-6900, TC1 are most likely continuous, and the paleoseismic events 1455-670, and 635-145 yr BP, respectively, and the most recorded should be complete. Although trench TC2 is located recent event might correspond to the 1606 Jianshui on a flat area on a mountain slope, few layers of sediment earthquake. have been deposited since ~6849 ± 44 yr BP, and events E1

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a N b Down N stream S2 S1 S3

TC3

100m 100m

c d S2 S1 S3

TC3 Fault scarp Depression Ofset:4.2 m

View to NW

Figure 9: Faulted landforms at the Luoshuidong site and the location of trench TC3. (a) Optical imagery from Google Earth collected in 2014 (~0.6 m resolution). Red arrows show the surface traces of the fault. (b) Interpreted fault (red) and numbered channels (blue). Trench TC3 is shown with the white bar. (c) Field photo shows the fault scarp and location of the trench. (d) The restoration with the smallest oﬀset for best retroﬁtting the stream beds into an initial alignment is 4.2 m.

and E2 were not separated in TC2. Trench TC3 was exca- movement of the central and northern segments of the vated in a small depression, and the morphology of the Xiaojiang fault, Anninghe fault, and Zemuhe fault. depression was well preserved. However, the age ranges of An irregular earthquake recurrence of a fault may be the oldest two events constrained from TC2 and TC3 are rel- related to the interaction among faults in a fault system atively large. Additionally, the erosion from precipitation is [26, 29, 53, 54]. The geometric structure of the southern strong in the study area, and the sedimentary strata might segment of the XJFZ is more complex than that of the be discontinuous. We cannot rule out the possibility of miss- central and northern segments. Multiple fault branches ing events in the obtained paleoseismic sequence. However, are developed in the southern segment, and the interaction the time interval between events E2 and E1 is 726 ± 235 yr, among the faults may be obvious. In addition, the eastern which is quite different from the time intervals between terminations of the NW-striking Qujiang and Shiping events E4, E3, and E2. We are more inclined to believe that faults are located near the southern segment of the XJFZ the fault is featured by irregular earthquake recurrence. (Figure 1(a)). Both faults show right-lateral strike-slip Therefore, the time interval of 726 ± 235 yr between the last motion during the Holocene (Han et al., 1993; [29]). two events is more representative of the recent activity of Therefore, the fault systems around the JSF are complex, the fault than the earthquake recurrence interval of 4589 ± and the earthquake recurrence of the JSF may be affected 3132 yr averaging from four events. by the interaction both among the internal fault branches The central segment of the XJFZ and the Anninghe- of the nearly NS-striking XJFZ and with the NW- Zemuhe fault zone also show irregular seismic recurrence trending strike-slip faults. intervals. Li et al. [26] revealed that six paleoearthquakes occurred at irregular intervals on the west branch of the 5.3. Kinematic Property of the JSF. The XJFZ exhibits an arcu- central segment of the XJFZ over 40000 yr through trench ate shape that is convex eastward (Figure 1(a)). The strike of excavations and concluded that the three youngest events the fault zone changes from NNW in the northern segment were continuous and gave an average earthquake recur- to nearly N-S in the central segment to NNE in the southern rence interval of 370-480 yr. Wang et al. [29] found that segment. Li et al. [26] suggested that the northern part of the five paleoseismic events that occurred on the Zemuhe fault central segment has experienced long-term compression and since 8000 yr appeared to be unevenly spaced in time and shearing based on the analysis of the thrusting and folding of estimated the average earthquake recurrence interval to be strata in a trench. The three trenches in this study reveal that ~2300 yr. Similarly, Wang et al. [52] revealed that the time the fault branches of the JSF present a normal dip-slip com- intervals between five paleoearthquakes on the southern ponent and create graben or negative flower structures. In segment of the Anninghe fault since 3400 yr vary widely terms of large landform expression, it can be observed from and range from ~130 to 2200 yr. These events are the DEM, satellite image, and field investigations that some unevenly spaced in time, with an average seismic recur- late Quaternary basins are distributed along the JSF rence interval of approximately 600-800 yr. The similar (Figures 1(b) and 1(c)), such as the Baiyun Basin, Xinzhai seismic recurrence behaviors of these faults indicate that Basin, and Goujie Basin. In addition, the calculated focal the JSF appears to positively respond to uneven tectonic mechanism of small earthquakes located near the southern

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a South wall

b U3 U7 U1 U6 U5 U4 U2 U3 U1 F1 F2 0 1m F3 F6 F5 F4

Fault scarp View to N d e Shearing plane Fig.d North wall c Oriented linear gravels

U7 U7 U3 U4 F2 U2 Fig.e U4 0 1m F2 U2

Figure 10: Photomosaic and interpretation map of trench T3 at the Luoshuidong site. (a) Photomosaic image of the south wall. (b) Interpretation map of the south wall. (c) Interpretation map of the north wall. (d) Expanded photo of the north wall. The location is shown in (c). (e) Expanded photo showing oriented gravels along the fault. The location is shown in (d).

Table 4: Unit description of trench TC3 at the Luoshuidong site. and shows as a left-lateral strike-slip motion with a normal component [55, 56]. Therefore, the above observations and Unit Description analysis permit us to believe the JSF is characterized kinemat- no. ically by extension and shearing as a whole. U7 Gray-brown soil layer. The change in the strike of the XJFZ is accompanied by a Gray-brown wedge-shaped mixed deposits composed of change in the kinematic properties, indicating that the U6 gravel and sandy soil. interaction between the Chuan-Dian and South China U5 Yellow-brown wedge-shaped clayey gravel layer. blocks transforms from transpression to transtension. U4 Reddish-brown sandy clay layer. These changes are likely caused by the apparent change U3 Brown subangular clayey gravel layer. in the direction of clockwise rotation of the Chuan-Dian block around the eastern Himalayan syntaxis. It also sug- U2 Dark brown subangular clayey gravel layer. gests that the SE Tibetan Plateau turns to move south- U1 Gray-white limestone tectonic zone. southwestwards at the south of the central segment of the XJFZ and might have not expanded eastwards further. So the lateral extrusion of the SE Tibetan Plateau does not segment of the XJFZ shows that the normal strike-slip make an obvious impact on the South China block. motion dominated along the fault zone (Hu et al., 2013). The transtension is also observed along the Dien Bien Phu 5.4. Role of the JSF in Accommodating Regional Tectonic fault on the southern side of the JSF, which strikes NNE Deformation. The nearly N-S-striking XJFZ and NW-

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R_Date XJ-C4

EL1

R_Date XJ-C5

R_Date XJ-C6

EL2

R_Date XJ-C7

Sequence TC3

16000 14000 12000 10000 8000 6000 4000 2000 0 Cal yr BP

Figure 11: Timings of paleoseismic events recorded in TC3 constrained with OxCal modeling.

E4 E3 E2 E1 15360-12755 10845-6900 1455-670 1606

TC1 EF2 EF1

TC2 ED3 ED2 ED1

TC3 EL2 EL1

16000 14000 12000 10000 8000 6000 4000 2000 0 Age (yr BP)

Figure 12: Constraints of the paleoseismic events and ages from the three trenches.

striking Qujiang, Shiping, and Red River faults are the main and northern segments of the XJFZ [26]. Furthermore, faults in the southeastern Chuan-Dian block. These three the geological left-lateral slip rate of the JSF are ~7 mm/yr NW-striking faults are mainly manifested as right-lateral [12], which is also slightly lower than 10-16 mm/yr of the strike-slip motion. The Qujiang and Shiping faults are central and northern segments of the XJFZ, but much located on the western side of the southern segment of higher than 2.8-3.5 mm/yr, 3-3.6 mm/yr, and 1-3 mm/yr the XJFZ and on the northern side of the JSF of the Qujiang, Shiping, and Red River fault zones. GPS (Figure 1(a)). The western segments of the two faults strike slip rates also show a similar variation trend. The slip rate NW, and the eastern segments strike from WNW to nearly constrained by the GPS velocity proﬁle across Qujiang, E-W, forming a curved shape that is convex to the south- Shiping, and Red River fault zones with a width of southwest. The Red River fault zone is located on the 70 km is ~4.5 mm/yr [5]. GPS slip rate of the JSF was southern side of the JSF. estimated to be ~7 mm/yr [1], which is slightly lower than The XJFZ mainly shows a left-lateral strike-slip 7-10 mm/yr of the central and northern segments of the motion. Our results suggest that the average earthquake XJFZ and higher than that of the three NW-striking faults. recurrence interval of the JSF since ~15000 yr is 4589 ± Therefore, the above faulting activity parameters indicate 3132 yr, which is longer than 2000-2500 yr of the central that the late Quaternary activity of the southern segment and northern segments of the XJZ [10], but shorter than of the Xiaojiang fault zone may be slightly weaker than 6000 ± 1000 yr of the Red River fault zone [2]. The interval that of the central and northern segments but stronger of 726 ± 235 yr between the most two recent earthquakes than that of the NW-striking Qujiang, Shiping, and Red of the JSF is also longer than 370-480 yr of the central River fault zones.

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Considering the fault scale, tectonic location, and faulting Therefore, the southern segment of the XJFZ plays a dom- activity, the XJFZ is an important part of the eastern bound- inant role in the tectonic deformation of the southeastern ary of the Chuan-Dian block and accommodates the south- Chuan-Dian block on the SE Tibetan Plateau, with a high ward movement of the block. The Qujiang and Shiping seismic hazard. faults are interpreted as second-order tectonic features accommodating deformation inside the block, which may Data Availability be caused by bookshelf faulting generated by regional differential crustal shear. The Dien Bien Phu fault on the southern All chronological data are shown in the tables, and satellite side of the XJFZ has been an active left-lateral strike-slip fault imageries are accessible in Google Earth. zone in the Quaternary with a probable average slip rate of 2.5 mm/yr [55], sharing the spatial alignment of the Conflicts of Interest Xianshuihe-Xiaojiang fault system. So in the regional kinematic mode, the SSE-directed motion of the southeastern The authors declare that they have no conflicts of interest. Chuan-Dian block is likely to be firstly accommodated in part by right-lateral shear and dip-slip motion on the Acknowledgments Qujiang and Shiping faults and continues across the Red River fault, then is transmitted southward along the Dien This study is cosupported by the National Natural Science Bien Phu fault. The Red River fault zone has already been Foundation of China (Grant Nos. 41772218 and 41372219) not the main boundary faults of the Chuan-Dian block. and the China Active Fault Survey Project (Grant No. Compared with the NW-striking fault zones, the southern 201108001). segment of the XJFZ may play a dominant role in the tectonic deformation of the southeastern Chuan-Dian block on the SE References Tibetan Plateau, with a high seismic hazard. [1] Z. K. Shen, J. Lü, M. Wang, and R. Bürgmann, “Contemporary crustal deformation around the southeast borderland of the 6. Conclusion Tibetan Plateau,” Journal of Geophysical Research: Solid Earth, vol. 110, article B11409, 2005. Trench excavations and radiocarbon dating revealed that the “ four most recent surface-rupturing paleoearthquakes on the [2] X. Shi, Y. Wang, K. Sieh et al., Fault slip and GPS velocities across the Shan plateau define a curved southwestward crustal JSF have occurred since approximately 15370 yr BP. 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