Abstract

The Tancheng–Lujiang (Tanlu) fault zone is the most active fault zone in eastern China. In this zone, the Anqiu–Juxian fault represents the most recently active fault and has the clearest surface traces and the highest seismic risk. This study comprehensively analyzes the kinematic characteristics of the Jiangsu segment of the Anqiu–Juxian fault using field geological surveys, trenches, shallow seismic reflection surveys, combined borehole section exploration, and middepth seismic reflection surveys. The results show that the Jiangsu segment of the Anqiu–Juxian fault features a single branch in the bedrock outcrop area, with reverse strike-slip motion near North Maling Mountain and Chonggang Mountain and normal strike-slip motion near South Maling Mountain. The sedimentary zone features two normal strike-slip faults (east and western branches), which represent the synsedimentary boundaries of a half-graben rift basin. The kinematic process is represented by rotational movement along the strike-slip fault with a curved path. The resulting tensile and compressive stresses are accommodated by dip-slip movement at both ends of the strike-slip fault. The activity of the Jiangsu segment of the Anqiu-Juxian fault can be divided into two periods. The first period of activity occurred before the later part of the Late Pleistocene, when movement along this curved segment occurred, forming the western branch of the Xinyi segment and the eastern branch of the Suqian segment. The second period of activity started in the later part of the Late Pleistocene and continues today. It is characterized by activity on the western branch of the Xinyi segment and the western branch of the Suqian segment of the Jiangsu segment, while the eastern branch of the Xinyi segment and the eastern branch of the Suqian segment became inactive and can be considered Late Pleistocene faults. The maximum vertical slip rate of the Jiangsu segment of the Anqiu–Juxian fault since the Pleistocene has been 0.28 mm/a. The Jiangsu segment of the Anqiu–Juxian fault formed via dextral strike-slip faulting, mainly due to the southward movement of the region to the east of the fault.

1. Introduction

The kinematic characteristics of a fault zone are important parameters for studying the activity and behavior of the fault zone and evaluating the earthquake risk. The Tancheng–Lujiang (Tanlu) fault zone is an important active fault zone and boundary tectonic zone in eastern China, also a world-famous intraplate strike slip fault within a plate, where several large earthquakes of magnitude 7 or higher have occurred since records have been kept. The Anqiu–Juxian fault was the seismogenic fault of the M 8.5 Tancheng earthquake in 1668 [15], and the kinematic characteristics of this fault have long been a focus of attention in the geosciences. Regarding the kinematic characteristics of the Anqiu–Juxian fault since the Late Pleistocene, some scholars believe that they have involved a combination of thrusting and dextral strike-slip faulting [69], but others believe that they have involved mainly dextral strike-slip faulting with minor thrusting [3, 10] or only dextral strike-slip faulting [11].

Due to the destruction of the tectonic landscape by human activities, there are few studies on the transcurrent activity rate of this fault. Jiang et al. [12] used aerial photographs from 1960 and measured the characteristic meter-scale horizontal displacement of a gully since the start of the Holocene and inferred a dextral strike-slip rate since the start of the Holocene of 2.2-2.6 mm/a. Li et al. [13] calculated a modern strike-slip rate of 0.9-1.2 mm/a using GPS measurement data.

Investigations of numerous strike-slip faults have found that large-scale strike-slip faults do not extend along a straight line. Generally, they have complex structures, often composed of multiple discontinuous secondary faults arranged pinnately or en echelon, with discontinuous stepovers, extensional zones, or uplift zones between them [1420]. Previous researchers have proposed different models of fault slippage by characterizing the kinematics of the various secondary strike-slip faults and their connections. For example, Schwartz and Coppersmith [21] proposed three types of slip models for strike-slip faults, namely, the characteristic seismic model, the uniform slip model, and the variable slip model, after studying the San Andreas fault in the United States. Deng [22] proposed a rotational movement model for strike-slip faults after field observations of the seismic surface rupture zone of the Fuyun fault. Gao et al. [23] conducted a finite element numerical simulation of rotational movement in the Fuyun fault zone. According to their simulation, the rotational movement causes vertical movement on the two sides of the fault, resulting in a four-quadrant distribution of uplift and subsidence; the tensile stress area subsides, forming a basin, and the compressive stress area uplifts. Wang and Geng [24] further summarized the characteristics of rotational motion and characteristic earthquakes and concluded that the rotational motion of strike-slip faults is a mechanism that leads to the occurrence of characteristic earthquakes and that the rotational axis is the location of these earthquakes. Chao et al. [25] proposed a model of characteristic earthquakes for the middle section of the Tanlu fault zone by studying the kinematic characteristics and seismic activity of the Juxian–Tancheng segment.

The kinematic features of the Anqiu–Juxian fault have been revealed by many previous studies on uplifted regions, such as Maling Mountain and Chonggang Mountain, through natural outcrops and trenches [3, 4, 2528]. However, since the fault is mostly hidden, few studies have been conducted on the kinematic features of the fault in the hidden region. Previous models of strike-slip faults focus on the tectonic characteristics of stepovers between the secondary faults. In these models, the stepovers were either considered normal faults due to tensile stress or thrust faults due to compression, and the changes in the sense of motion (normal or reverse) of the secondary faults were ignored. In this paper, by integrating multiple research methods, such as field geological surveys in the area of outcropping bedrock, trenching, shallow seismic reflection surveys (SRSs) in subsidence areas, composite drilling in certain sections, and a middepth SRS, the kinematic characteristics of the Jiangsu segment of the Anqiu–Juxian fault are comprehensively analyzed, and the kinematic pattern of the fault is discussed based on the changes in the activity of each secondary fault. The kinematic model might have implications for the study of other intraplate strike-slip faults in the world. The activity characteristics of the Tanlu fault zone are closely related to the westward subduction of the Pacific plate and the Philippine plate, and the kinematic characteristics of the Jiangsu segment of the Anqiu-Juxian fault can reflect the coupling relationship between the Tanlu fault zone and the subduction of the Pacific plate since the late Quaternary.

2. Geological Background

The Tanlu fault zone is a large-scale, north–northeast-striking active fault zone in eastern China that extends from Heilongjiang in the north to the Yangtze River in the south, running through the eastern part of mainland China over a distance of 2400 km [29, 30]. It is also a major zone of strong seismic activity in eastern China [3133]. According to its geometric structure and activity, the Tanlu fault zone can be divided into four segments from north to south (Figure 1(a)), namely, the Hegang–Tieling segment (I), Xialiaohe–Laizhouwan segment (II), Weifang–Jiashan segment (III), and Jiashan–Guangji segment (IV) [26]. Among them, the Weifang–Jiashan segment is the most active part of the Tanlu fault zone [4, 5, 34]. The Weifang–Jiashan segment is composed of five parallel faults (Figure 1(b)). From east to west, they are the Changyi–Dadian fault (hereinafter referred to as F1), Anqiu–Juxian fault (F5), Baifenzi–Fulaishan fault (F2), Yishui–Tangtou fault (F3), and Tangwu–Gegou fault (F4). Among them, faults F1, F2, F3, and F4 constitute a structural pattern comprising a horst with two grabens, one on each side.

Fault F5 is a recently generated fault between faults F1 and F2 and is the latest fault to have formed in the active era, and it has the most obvious surface traces. The maximum horizontal slip rate of fault F5 reached 2.86 mm/a [35], but no destructive earthquakes have occurred over the past 3000 years on F5, which is very close to the destructive earthquake recurrence period. Fault F5 is classified as having the highest seismic risk [3, 4, 25]. Fault F5 is divided from north to south into the Anqiu segment, Juxian–Tancheng segment, and Jiangsu segment (Figure 1(b), [4, 5, 26]). The total length of the Jiangsu segment of fault F5 is 170 km. The overall strike is 5-15°. This fault is mostly hidden and is only exposed at the surface in bedrock outcrops near Maling Mountain, Zhangshan Mountain, and Chonggang Mountain, where it manifests as the overthrusting of the Wangshi Formation’s purplish-red sandy shale (K2W) onto Late Pleistocene loess. The crushed zone ranges in width from a few meters to more than 50 m and contains colorful fault gouge [27].

The study area is located on the southern edge of the Yishu hilly and plain region. The topography from northeast to southwest shows a high–low–high–low pattern. The sedimentary strata can be divided from top to bottom into beds of grayish-yellow silty fine sand and grayish-black clay deposited in the Holocene (Qh); Upper Pleistocene brownish-yellow clay with densely distributed calcareous concretions (Q3); Middle Pleistocene grayish-yellow clay with grayish-green reticulated silty clay (Q2); and Lower Pleistocene grayish-yellow medium-coarse sand and medium-fine sand (Q1).

3. Materials and Methods

According to the geological conditions, such as the topography, geomorphology, and thickness of the Quaternary strata, the methods of field geological survey and trenching were adopted in outcropping areas of the fault, while SRSs, combined borehole section exploration, and a middepth SRS were adopted in areas where the fault is buried. Different dating methods were performed on samples from the trenches and boreholes according to the stratigraphic characteristics and approximate age.

3.1. Middepth Seismic Reflection Survey

The middepth SRS was performed across the Tanlu fault zone. According to the site condition requirements for the SRS, a seismic reflection line approximately 6 km north of South Maling Mountain was selected (Figure 1(b)). The geometric structure of the middepth part of fault F5 and its contact relationships with other branch faults of the Tanlu fault zone were determined according to the results of the middepth SRS.

3.1.1. Data Acquisition

A French 428XL 24-bit digital seismograph was used. The data recording format was SEG-Y, the sampling interval was 2 ms, and the record length was 8.0 s. The detector was a DSU1 single-component digital oscilloscope. The settings of the observation system were as follows: numberofreceivinggroups=1200, excitationmode=midpointexcitation, groupinterval=8m, offset=120m, fold>30, and commondepthpointdistance=4m.

3.1.2. Data Processing

The raw earthquake data were comprehensively processed using multiple sets of processing software. The main processing modules, e.g., refraction static correction, frequency division, speed scanning, residual static correction, and poststack denoising, were used in the processing, and satisfactory processing results were achieved. The overall resolution of the time profile was high, the effective wave band was wide, the interference wave was well suppressed, and the reflected waves were distinct, indicating that the processing flow of two-dimensional seismic data was rational, the parameters were appropriately selected, and the quality of the processing results was reliable.

3.2. Seismic Reflection Survey

The SRS was performed mainly to detect the nature of fault activity in the hidden area of the fault. Based on the results of the above Tanlu fault zone mapping and the site condition requirements for the SRS, a seismic reflection line was laid across fault F5. From north to south with the Xinyi River as the boundary, the seismic reflection line was divided into lines X5-1 to X5-4 for the Xinyi segment and S5-5 to S5-8 for the Suqian segment (Figure 1(c)) to reveal the geometric structure and fault properties of the hidden area of fault F5 in the study area.

3.2.1. Data Acquisition

Based on our specifications for the acquisition and receiver instrument, excitation methods, expansion arrangements, field data collection and processing, and interpretation of field tests, the S-Land seismograph (SI, USA) was selected. The natural frequency of the detector was 40 Hz. The seismic source was a 3-ton KZ-03 controllable seismic source for both longitudinal and transverse waves (Beiao, China), and the frequency of the seismic source was 20-160 Hz. The parameters of the shallow SRS observation system were as follows: group interval of 3 m, 96 receiving groups, fold of 15, sampling interval of 0.5 ms, record length of 2 s, and minimum offset of 9-36 m.

3.2.2. Data Processing

Various digital processing steps of the obtained reflection data were completed by the dedicated reflection data processing software GRISYS [36]. The data analysis and processing mainly included basic processes such as preprocessing, filtering, statics correction, amplitude compensation, constant-speed scanning, vespagrams, dynamic correction and superposition, distortion removal, modification processing, and profile output. The processing sequence and content were rationally combined and selected based on the task demand.

3.3. Combined Borehole Section

According to the nature of the activity and geometric characteristics of fault F5, the obvious mutation point of the reflection lineups on the shallow SRS profile was selected, and combined borehole section profiling on both sides of the breakpoint was performed. The borehole arrangement followed a doubling exploration pattern [37, 38], with at least three boreholes in the hanging wall and three boreholes in the footwall of the fault, and the spacing between boreholes was no more than 10 m.

The combined borehole sections were located along seismic reflection lines X5-1 and X5-2 (Figure 1(c)), which are the Zhangcang combined borehole section and the Huangshulu combined borehole section, respectively. The detection results served mainly to verify the SRS results and reveal the nature and active periods of fault F5. The nature of the fault activity was mainly analyzed based on the comparison of the marker bed. The marker bed had to meet the following conditions: (1) the rock strata have special lithology and a special sedimentary structure or special interlayer interfaces; (2) no or very few similar strata or interfaces appear longitudinally; (3) strata or interfaces are distributed stably in the lateral direction, and the lithology and thickness change little; and (4) the characteristics are distinct and easy to identify [39]. The active periods of the fault were restricted based on the location of the breakpoint on the fault, the age of the fault, and the overburdened strata exposed by the combined borehole section. According to the vertical displacement of the marker bed and the dating results of the marker bed, the rate of vertical motion of the fault was calculated.

3.4. Dating Methods

To determine the active periods of the fault, dating samples were taken from the trench near Maling Mountain and the two combined borehole sections. According to the regional stratigraphic sedimentary characteristics and the dating range of the dating methods, the following dating methods were selected: carbon-14 (14C) dating, optically stimulated luminescence (OSL) dating, and electron spin resonance (ESR) dating. A total of 13 samples were tested, including three samples by 14C dating, 10 samples by OSL dating, and three samples by ESR dating.

The samples for 14C dating were sent to Beta Analytic (USA) for processing. The samples were subjected to pickling pretreatment before dating, and then, the pretreated samples were analyzed using a National Electrostatics Corporation accelerator mass spectrometer and a Thermo isotope ratio mass spectrometer to complete the experimental tests. The conventional radiocarbon age was calculated using the Libby half-life (5,568 years) and was corrected by isotope fractionation. We then used the high probability density method to calibrate the dating results to the Gregorian calendar [40].

3.4.1. OSL Dating Method

We analyzed samples via OSL at the Key Laboratory of Quaternary Chronology and Hydro-Environmental Evolution, China Geological Survey. The middle section of each sample, which was not disturbed, was used for measurement. We dried and ground 20 g of each sample to measure the U, Th, and K contents. We added hydrogen peroxide and hydrochloric acid to the remaining sample to remove carbonates and organic matter. We then isolated the particle groups from 4 to 11 μm by using the hydrostatic settlement method. We treated the particle groups with fluorosilicate to obtain fine-grained quartz for further measurement. The dose equivalent (DE) of the sample was determined by using a Risoe DA-20-C/D OSL automeasuring system. The natural OSL dose was measured via the middle-grain single-aliquot regenerative-dose (SAR) method. The U, Th, and K contents contributing to the environmental dose rate (D) were measured with an ELEMENT inductively coupled plasma mass spectrometer. Finally, the sample ages were determined according to the equation AgeA=DE/D [4144]. Using the saturation index method to fit the DE value ensured that the dating results of the samples with unsaturated growth curves were accurate, while the dating results of the samples with saturated growth curves were omitted. The final sample dating results were compared with the regional stratigraphic sedimentary ages to determine the correct dating results.

3.4.2. ESR Dating Method

The sampling principle of ESR dating samples was strictly followed during sampling, and all samples were prepared and tested in the Metrology and Testing Department of the China Institute of Atomic Energy. For sediment, the ESR dating can be expressed as follows: ageT=totaldoseofnaturalradiationTD/annualradiationdoseD. Approximately 100 g of the sample was taken from the center of the raw sample, weighed on a balance, and dried in an oven at low temperature (below 45°C) until it reached a constant weight. From this weight, the moisture content of the sample was calculated. The samples were physically ground, sieved, and chemically processed to obtain samples with higher quartz contents. The ESR measurement was performed with a Bruker EMX ESR spectrometer (BRUKER, Germany). According to the sample type and estimated age, the corresponding ESR dating signal was used. The Ge center and E’ center signal were used at room temperature, and the E’, Al center, or Ti center signal was used at low temperature. Finally, the TD was calculated by fitting and extrapolating the intensity of the ESR signals of different irradiated samples. The annual dose determination was performed with a thick-source alpha counter (Daybreak 583, UAS) that was used to analyze the U and Th contents in the sample, and we ensured that the count reached a certain level. The K content in the samples was analyzed with an FR640 flame photometer (Shanghai, China). According to the levels of trace elements, such as U, Th, and K, in the sample, along with the water content in the sample and the sampling depth, the D value of the sample was calculated. Finally, from the above data, the age of the sample was calculated.

4. Results

4.1. Natural Outcrops and Trenching

The Jiangsu segment of fault F5 passes northward through North Maling Mountain, South Maling Mountain, and the Xinyi River and extends southward to Chonggang Mountain (Figure 1(c)). Due to the heavy outcrop coverage, most of the segment is located in the border zone between the uplifted low hills and alluvial plains.

Several natural outcrops can be seen on the eastern side of the sand pit approximately 200 m northeast of Hezhuang on North Maling Mountain (Figures 2(a)–2(d)). The fault is dominated by eastward shortening and thrusting. The sandstone of the Wangshi Formation on the eastern side of the fault is thrust onto the clayey silt stratum on the western side. Two sets of fault gouges can be seen in the fault zone, with thicknesses of approximately 30 cm. The directional distribution of the sand and gravel of the Wang Formation forming a westward arc east of the fault and the fault gouge show that the fault is dominated by thrust activity (Figures 2(a) and 2(b)), and the fault at the surface has a dip of approximately 45°. Before its destruction by human activities, the western Q3 loess atop the Wangshi Formation sandstone was nearly horizontal (Figure 2(d)), and the strata in the natural sand pit on the western side of the fault were deposited horizontally. The dip direction of the fault is approximately 96° (Figure 2(c)). No branch faults were developed on the western side of the fault. The trench results near North Maling Mountain [26] therefore indicate that F5 is mainly dominated by thrust activity. According to the OSL results, this segment of fault F5 was active in the Holocene.

The South Maling Mountain trench exposes a fault zone (Figure 3(a)) that is approximately 1.5 m wide, with three developed faults, all of which are normal faults. The western side of the fault is purplish-red sandy conglomerate, and the eastern side is grayish-yellow clay. The dip angle is approximately 56°. The South Maling Mountain uplift area is dominated by eastward extension and normal faulting. According to the results of 14C dating (Table 1), this segment of fault F5 was active in the Holocene.

The segment on the southern bank of the Xinyi River (Figures 2(e) and 2(f)) reveals that the bluish-gray clay stratum rich in large calcareous concretions on the eastern side of the fault has been thrust onto the blackish-brown clay on the western side. The top of the calcareous concretion-rich stratum on the eastern side is uneven, while the top of the calcareous concretion-rich stratum on the western side is relatively flat. The offset of the calcareous concretion-rich clay stratum is nearly 1 m, and the offset of the brownish-gray clay is approximately 30 cm. Zhang et al. [27] determined the age of the top of the brownish-gray clay stratum to be 3.8±0.3ka BP by OSL. Therefore, this segment of fault F5 was active in the Holocene.

Cao et al. [28] excavated a trench on the western side of Chonggang Mountain. The Wangshi Formation sandstone on the eastern side of the fault is thrust over yellowish-brown clay on the western side (Figure 3(b)). Faulting on the eastern side of the fault is densely developed, whereas the western side contains no branch faults. The dip angle of the fault is approximately 50°, and it steepens downward. According to the dating results, this segment of fault F5 was active in the Holocene.

The analysis of the above fault activities in the North Maling Mountain, Xinyi River, and Chonggang Mountain areas shows that the old strata on the eastern side of fault F5 have been thrust over flat-lying Pleistocene or Holocene strata on the other side. On the eastern side of the fault, branch faults are densely developed, and the formation is largely deformed. The formation on the western side of the fault is relatively flat, and no branch faults have developed. The dip angles of the faults are all relatively gentle (approximately 50°) and are close to 0° in some local areas, indicating that the fault is dominated by thrust activity. Fault F5, exposed in the South Maling Mountain trench, is an east-dipping normal fault with a dip angle of approximately 56°, and branch faults are developed on the eastern side of the main section. Therefore, the faults at North Maling Mountain and Chonggang Mountain in the bedrock outcrop area have different activity patterns than those at South Maling Mountain.

4.2. Shallow Seismic Profiles

Through data processing and single shot record comparison, a total of 16 faults were revealed by eight shallow seismic profiles. According to the relative positions of the faults, the faults were divided into two branches: F5E and F5W. On the shallow seismic profiles, fault F5 is mainly manifested in three activity modes: (1) the reverse fault (Figure 4(a)). With the F5-1 fault as the boundary, there are obvious differences in the in-phase axes between the eastern and western sides. Two sets of reflected wave groups P1 and Pg were developed on the western side of F5-1. The P1 reflection energy is weak, and the phase axis is relatively flat in the cover layers. The Tg wave group (bedrock surface reflected wave) was more stable, with fewer ups and downs. Within the scope of Nos. 750 to 810, the Pg wave group indicates rapid uplifts, the phase axes are obviously faulted, and a diffracted wave pattern is developed at the fault site. On the eastern side of the fault breakpoint of F5-1, only a Pg wave group is developed, and this wave group is obviously uplifted and shows a trend of continuing uplift to the east. According to the detection results of the combined borehole profile (Figure 5), the depth of each wave group and the fault location are reasonable. The shallow seismic profile in Figure 4(d) shows the characteristics of reverse fault activity. (2) The fault depression characteristics of the superimposed profile (Figure 4(b)) show that the F5 fault is bounded by the F5W-3 fault and the F5E-2 fault, showing fault depression characteristics. The wave sets on the eastern and western sides of the F5W-3 fault are abundantly developed. At this fracture point, the in-phase axes of the Pg, P2, and P1 wave sets are obviously displaced and segmented and curved. On the eastern side of the F5W-3 fault, only the top surface reflection wave group (Pg) of bedrock is developed, and the top surface reflection wave group (Pg) of bedrock has been uplifted considerably. The fall of the Pg wave group on the eastern and western sides of the fault is approximately 230 ms. The reflection wave sets between the two faults are abundant, showing obvious depression and bending characteristics, and diffraction waves are developed, which reflects the strong bending deformation of the strata under the influence of tectonic activities. According to the combined borehole profile across the F5W-3 fault (Figure 6), the depth of each wave group and the fault location are relatively reasonable. The shallow seismic profiles in Figures 4(c), 4(f), and 4(h) show similar fault depression characteristics. (3) Both normal faults and reverse faults are developed (Figure 4(e)). The bedrock on the western side of fault FW-9 is shallow, and the cover layer is very thin, while the bedrock surface on the eastern side of FW-9 suddenly deepens to 50-120 m, and the fault corresponds to the contact between the Wangshi Formation sandstone and the Quaternary strata on the eastern side. The F5E-8 fault faulted the P1, P2, Pg, and P4 wave groups from top to bottom, and the section is roughly parallel to the FW-9 fault, showing upward thrusting of the hanging wall (eastern side). The apparent fault displacement is approximately 3-10 m, the fault plane is inclined to the east, and the apparent dip angle is steep, indicating a reverse fault. The properties of each fault can be seen in Table 2, in which the age of activity is determined according to the combined borehole section. The bedrock top dislocation revealed by each fault varies greatly, which generally shows that the bedrock dislocation distance between the subsidence areas of Xinyi city and Suqian city is greatest and gradually decreases on both sides. According to the bedrock dislocation of the eastern and western branches of the fault, the bedrock dislocation distance of F5E in the Xinyi segment is longer, approximately 160-180 m, and the bedrock dislocation distance of F5W is shorter, approximately 20-30 m. In the Suqian section, the bedrock dislocation distance of F5E is large, with a maximum of approximately 280 m, and the dislocation distance of the western branch is small, approximately 30-50 m. Xu et al. [45] and Cao et al. [28] conducted a large number of seismic profiles and combined borehole profiles for the Suqian section, and they concluded that the Suqian section of fault F5 is mainly composed of two main faults that tilt toward each other and developed nearly vertically, with F5W being the Holocene active fault and F5E being the Late Pleistocene active fault.

According to the seismic profile (Figure 4), seismic reflection line X5-1 on the south side of North Maling Mountain shows thrusting and extrusion, extending southward to the sedimentary basin, where the normal faulting and extension have a double-branching-to-triple-branching transition, shifting back to thrusting and extrusion at seismic reflection line X5-4 and the Xinyi River area. To the south in the Suqian segment, the activity changes to normal faulting, the double branching shifts to triple branching, and it shifts back to the extrusion and thrusting along a single branch near Chonggang Mountain. The Jiangsu section of fault F5 is mainly composed of eastern and western faults in the buried area, which gradually merge into a single fault in the North Maling Mountain, South Maling Mountain, and Chonggang Mountain bedrock outcropping areas, and secondary faults are developed in the center of the basin.

A comparative analysis of the activity characteristics of the eastern and western faults in the Xinyi and Suqian segments shows that the Jiangsu segment of fault F5 can be divided into two branches according to the amount of bedrock top displacement. One branch is the west-dipping eastern branch of the Xinyi segment and the east-dipping western branch of the Suqian segment, with bedrock displacement greater than 160 m. The other branch is the east-dipping western branch of the Xinyi segment and the west-dipping eastern branch of the Suqian segment, with bedrock displacement less than 50 m. In the synsedimentary boundary of the basins controlled by the eastern and western branches of the fault F5 Jiangsu segment in the subsidence area, the branch with a larger bedrock displacement controls the formation of the rift basin. The controlling fault in the rift basin has gradually changed from the eastern branch of the Xinyi segment to the western branch of the Suqian segment, and the transition area is from South Maling Mountain to seismic reflection lines S5-5. The latest activity in the Jiangsu segment of fault F5 (after the Holocene) moved along the western branch, forming a half-graben rift basin. The eastern branch of the Jiangsu segment of fault F5 was inactive, and secondary faults were developed in the rift basin, forming the three faults in Tangdian and Sankeshuxiang.

4.3. Results of Combined Borehole Section Exploration

4.3.1. Combined Borehole Section of Zhangcangcun

The row of boreholes in Zhangcangcun crosses fault F5 and is approximately 2.5 km from North Maling Mountain, where natural outcrops are observed (Figure 7). The boreholes were deployed on both sides of the seismic reflection line X5-1 breakpoint (stake number 804).

According to regional stratigraphic comparison and sample dating results (Tables 1, 3, and 4), the combined borehole section at this site revealed that the strata are mainly Quaternary (Q), the Neogene Suqian Formation (N2s), and the Upper Cretaceous Wangshi Formation (K2w). The lithologies in the boreholes are shown in Table 5. The depth of the Holocene bottom boundary ranges from 2.1 m to 3.2 m, and the main lithology is grayish-black and grayish-brown silty clay. The depth range of the upper Pleistocene bottom boundary is 7.6-8.9 m, and the main lithology is grayish-yellow, bluish-gray, and brownish-gray clay, in which calcareous concretions are uniformly distributed in masses, and black iron–manganese concretions are densely distributed. The depth range of the bottom boundary of the middle Pleistocene is 17.8-19.9 m, and the main lithology is grayish-yellow and yellowish-gray clay. Grayish-green kaolin is distributed in porphyritic and lamellar layers, and black iron–manganese concretions are densely distributed. The depth range of the bottom boundary of the lower Pleistocene is 24.2-69.6 m, and the main lithology is grayish-white, grayish-black, or yellow medium-coarse sand intercalated with gravel. The depth of the bottom boundary of the Neogene Suqian Formation ranges from 91.5 to 92.8 m, with mainly grayish-white or grayish-green medium-coarse sand intercalated with brownish-yellow clay and grayish-yellow or brownish-yellow silty fine sand beds. Data from the four boreholes in the eastern Suqian Group are missing.

According to the stratigraphic comparison (Figure 5), the bedrock has an obvious displacement of 69.3 m. The ZC-5 borehole crosses the fault, and there are two obvious fault planes with dip angles of 80° and 71°. On both sides of the fault are purplish-red gravelly clay and grayish-yellow gravelly sandy clay. The breccias are mostly purplish-red, appearing as east-dipping reverse faults. This row of boreholes exposes two faults, corresponding to the two fault planes exposed by the boreholes. The two faults form a fault zone, which extends upward between the ZC-5 and ZC-6 boreholes. According to the stratigraphic comparison, the bottom of the middle Pleistocene and the bottom of the upper Pleistocene in boreholes ZC-5 and ZC-6 hardly change in depth. Fault F5 appears as an early Pleistocene fault at this point.

4.3.2. Combined Borehole Section near Huangshu Road

The row of boreholes near Huangshu Road crosses fault F5 and features a total of seven boreholes and a total length of 638 m. The holes were arranged on both sides of the western breakpoint (stake number 1612) of seismic reflection line X5-2, with a minimum borehole spacing of 8 m (Figure 8).

According to the regional strata, the Zhangcang combined borehole section strata comparison, and the sample dating results (Table 1), the strata exposed in this segment are Quaternary strata. The lithologies in the boreholes are shown in Table 6. The depth of the Holocene bottom boundary ranges from 2.3 m to 5.9 m, and the main lithology is grayish-black and grayish-brown silty clay. The depth of the bottom boundary of the upper Pleistocene is 23.5-40.4 m, and the main lithology is grayish-yellow, bluish-gray, and brownish-gray clay. The calcareous concretions are uniformly distributed in masses, and black iron–manganese concretions are densely distributed. The depth range of the bottom boundary of the middle Pleistocene is 28.2-54.7 m, and the main lithology is grayish-yellow and yellowish-gray clay. The grayish-green kaolin is distributed in patchy and lamellar shapes. The bottom of the lower Pleistocene system was not observed, but the main lithology of this stratum is grayish-white and yellow medium-coarse sand, with gravel and calcareous concretions.

According to the analysis of the combined borehole sections (Figure 6), the brownish-red clay layer (6) was selected as marker bed ①, the middle Pleistocene clay layer (8) was selected as marker bed ②, and the lower Pleistocene brown clay bed (10) was selected as marker bed ③. According to the comparison of the marker beds, the row of boreholes reveals an east-dipping normal fault. The fault extends downward between boreholes HS6 and HS7. The depth of the upper fault point is approximately 4 m. The thickness difference between the Holocene gray and black clay beds on both sides is obvious, and the displacement of the marker beds increases downward. The displacement of each marker bed is shown in Table 7. The western branch of fault F5 was active in the Holocene.

4.4. Middepth Exploration Results

Through the comparison of data processing and single shot records, in the stacking time profile (Figure 9), a total of four major groups of reflected waves were tracked, namely, the TQ wave, TK wave, TG1 wave, and TG2 wave. Among them, the TQ wave features are clear, the reflected wave energy is strong, the continuity is good, the frequency is high, and it is easy to identify and track. The reflected wave group of the TK wave is messy, with lower frequency and poor continuity. In some areas, there is weak reflection or no reflection, which is obviously different from the overlying layer and the underlying layer, so it is only marked uniformly. The reflected frequency of the TK wave is relatively low, generally in the range of 35-40 Hz, and the characteristics of the TG1 wave group are similar to the attitude of the upper strata, but the wave group has strong energy and good continuity, and the resolution is obviously different from that of the overburden strata. The TG2 reflected wave group has good continuity, and the attitude and energy are obviously different from those of the upper strata. Most areas are easily compared and traced, and the characteristics are obvious at the two ends of the seismic profiles. By comparative analysis of regional geological data and borehole data, the stratigraphic units from shallow to deep include Quaternary, Neogene, Cretaceous, Sinian, and Lower Proterozoic. The Cretaceous unit is thick and mainly contains the sandstone and conglomerate of the Wang Group, which has an angular unconformable contact with the upper Neogene and the lower Sinian or Proterozoic Donghai Group gneiss basement, and the Quaternary unit on the eastern side of F1 has an angular unconformable contact with the Proterozoic Donghai Group gneiss. Comparative analysis of the average velocities of different wave groups, as well as the depth of the seismic wave travel time and drilling data, shows that the TQ wave group represents the Cenozoic bottom interface reflection wave, the TK wave group represents the Cretaceous internal reflection wave, the TG1 wave group represents the Sinian system interface reflection wave, and the TG2 wave group represents the upper interface reflection wave of the Proterozoic East China Sea group of metamorphic gneiss.

In the seismic reflection line range, in addition to the Cenozoic stratigraphy, which is relatively thin with small changes, the shallow stratigraphy in general has large changes in the lateral direction. Mesozoic strata are influenced by changes in basement undulations, and thickness changes are evident, with the greatest thickness deposited in the graben-like region of the Tanlu fault zone. The coexistence of folds and faults in the stratum, with obvious changes in occurrence, shows a strong influence of tectonic activity. Below the Mesozoic boundary is the Sinian system, whose overall thickness in the Tanlu fault zone is thick in the west and thin in the east, with obvious changes in stratigraphic occurrence and undeveloped folds, controlled by the deep basement and more developed fault structures. The top of the Proterozoic boundary, as the basement of the Tanlu fault zone, controls the sedimentary and tectonic morphology of the entire Tanlu fault zone.

According to the morphology and characteristics of the reflection lineups, such as the breaks, bifurcation mergers, distortions, and lineup occurrence changes, a total of six faults are interpreted, five of which are relatively stable in scale (F1-F5), while the sixth is a newly discovered fault located between F3 and F4, provisionally named F3w. The tectonic pattern is characterized by one horst with a graben on each side, with the western graben being wide and gentle and the eastern graben being narrow and steep. Fault F5 is located inside the eastern graben and features a single-branch normal fault with an apparent westward dip and an apparent dip angle of 86°. It extends upward below the bottom interface of the Cenozoic boundary and cuts downward through the bottom interface of the Cenozoic boundary (TQ), the Mesozoic boundary, and the top interface of the Proterozoic boundary (TG2).

The deep and shallow structures of the fault zone are closely related in terms of spatial location, geometric structure, and activity properties, presenting causal correlation and depth coordination. Comparing the results of the shallow SRS and the combined borehole section, it can be seen that fault F5 is a near-vertical fault in the middle and deep parts, is dominated by strike-slip activity, and extends into the shallow surface layer, which is the easiest to fracture and features a gradually shallowing dip angle. The shallow surface bedrock in the mountains and the Quaternary soft soil layer are the most easily fractured units. Therefore, fault F5 in bedrock outcrops, such as those in the North and South Maling Mountains and Chonggang Mountain, shows a single branch with many fractures toward the surface. In thick sediment areas such as Xinyi city and Suqian city, fault F5 is divided into two branches in the upper part. The latest activity is along the western branch of the fault. The upper breakpoint gradually becomes shallower as it extends toward the bedrock mountains.

5. Discussion

The natural outcrops, geologic trenches, seismic profiles, and combined borehole sections show that the Jiangsu segment of fault F5 is mainly compressional in the areas of North Maling Mountain, the Xinyi River, and Chonggang Mountain. Along the fault in the uplifted area of North Maling Mountain and Chonggang Mountain, the Wangshi Formation sandstone on the side toward the mountain has been thrust onto the Late Pleistocene loess on the side toward the plain, while the Xinyi River shows that the Late Pleistocene calcareous clay layer has been thrust onto the Holocene blackish-brown clay on the western side. The thick sediment areas of Xinyi city, Suqian city, and South Maling Mountain are dominated by extensional normal fault activity, with two east–west-branching faults controlling the boundary of the half-graben rift basin and secondary faults developing in the basin.

5.1. Vertical Slip Rate of the Jiangsu Segment of Fault F5

In the uplifted areas, such as North Maling Mountain, South Maling Mountain, and Chonggang Mountain, one side of the fault is composed of bedrock (the Wangshi Formation), and the cover strata are thin or even missing, which makes it impossible to compare with the marker bed, and the vertical uplift rate cannot be calculated. The row of boreholes of Huangshu Road in the subsidence area crosses the western branch of fault F5. The displacement of the bottom boundary of the Holocene black clay layer is 1.1 m (Table 7). The age of the black clay 14C dating sample obtained from the trench near Huangshu Road is 2970±30BP. The depth of the bottom boundary of the black clay layer in the row of boreholes is 4.8-5.9 m. Comparative analysis shows that the age of the black clay layer in seismic reflection line X5-1 is 3920±30BP. The bottom age of the black clay layer in the row of boreholes at Huangshu Road is closer to 3920±30BP (Table 1). The vertical slip rate of the subsidence zone since the start of the Holocene is 0.28 mm/a. According to the SRS, the Jiangsu segment of fault F5 can be divided into eastern and western branches. Since the Late Pleistocene, because both the eastern and western branches have been active, it is impossible to determine whether the two branches faulted together in the same earthquake or separately. Therefore, this paper only gives the accurate vertical slip rate since the start of the Holocene. According to the displacement of the top of the bedrock, the western branch of X5-2 exhibits the largest displacement in the Xinyi segment of fault F5 since the start of the Holocene. The bedrock top displacement decreases to the north and south, so the vertical slip rate represents the largest vertical sedimentation slip rate in the Xinyi segment of fault F5 since the start of the Holocene.

Jiang et al. [12] calculated the horizontal slip displacement to be 9 m by using aerial film from 1960, and Jiao et al. [35] used an unmanned aerial vehicle (UAV) to measure the horizontal slip displacement at 10.1 m. By using the timing of the latest paleo-earthquakes, these authors calculated the maximum horizontal slip rates to be 2.6 mm/a and 2.86 mm/a, respectively. Using the latest fault event time of 3920 yr in this paper, horizontal slip rates of 2.3 mm/a and 2.6 mm/a can be calculated. Considering the influence of human activities, the horizontal slip rate of fault F5 is more reasonable at 2.3 mm/a. Fault F5 is primarily characterized by strike-slip movement, and the ratio of horizontal to vertical slip rates is approximately 9-10. By comparative analysis, Li et al. [13] used the horizontal slip rate of 0.9-1.2 mm/a calculated from GPS data in North China from 2009 to 2014; we can see that the current horizontal slip rate of fault F5 in the Jiangsu section has decreased by half, possibly indicating the accumulation of strain energy.

5.2. Kinematics of Faulting of the Jiangsu Segment of the F5 Fault

The Jiangsu segment of fault F5 is dominated by strike-slip motion, with compression and thrusting in North Maling Mountain, the Xinyi River, and Chonggang Mountain and extension and normal faulting in the sedimentary basin and South Maling Mountain and Roach Mountain. The kinematic process is represented by the rotational movement of the strike-slip fault, and the tensile stress and compressive stress produced are accommodated by the slip tendencies at the ends of the fault, corresponding to normal and reverse faults. Te vertical displacement (evidenced by the displacement of the top of the bedrock) is shown to be small at the pivot location in the middle and gradually increases toward the sides. The Jiangsu segment of fault F5 appears as a curved structure in the superficial layer. From North Maling Mountain in the north, the east-dipping reverse fault gradually changes to a west-dipping normal fault in Xinyi city, with the largest bedrock displacement. The middle segment is transformed from the east-dipping normal fault at South Maling Mountain to the west-dipping eastern branch of fault F5 in Suqian, and the bedrock displacement is small. The east-dipping western branch of fault F5 in the Suqian segment is transformed into an east-dipping reverse fault at Sihong Chonggang Mountain. The branch faults merge into one branch in the deep part. The Jiangsu segment of fault F5 in the study area has a triple curved structure, with two pivot surfaces developed, namely, between X5-1 and X5-2 and between X5-4 and S5-5. According to the activity analysis of different segments of the Jiangsu segment of fault F5, this segment has experienced two active periods since the beginning of the Quaternary. The first period was before the late portion of the Late Pleistocene. Fault F5 moved along the curved structure of the eastern branch of the Xinyi segment and the western branch of the Suqian segment. The bedrock has a large displacement, with a maximum of approximately 180 m, forming the dominant fault of a half-graben rift basin (Figure 10(a)). The second period occurred after the later part of the Late Pleistocene. The Jiangsu segment of fault F5 has continued to move along the western branch of the Xinyi segment and the western branch of the Suqian segment until the present. The bedrock displacement is small (less than 50 m). The eastern branch of the Xinyi segment and the eastern branch of the Suqian segment were inactive and represent Late Pleistocene faults (Figure 10(b)).

5.3. Kinematics of Faulting of the Tanlu Fault Zone since the Latest Paleoseismic Event

Chao et al. [25] studied the slip and earthquake characteristics of fault F5. Based on their findings, fault F5 can be divided into three segments (Figure 1(b)), namely, the Anqiu segment (northern segment), the Juxian–Tancheng segment (middle segment), and the Jiangsu segment (southern segment). The Anqiu segment consists of two parallel secondary faults, with the central segment dominated by extension and normal faulting, controlling the Shibuzi rift basin and shifting to extrusion and thrusting to the north and south. The Juxian–Tancheng segment consists of five secondary faults, with the central segment dominated by extension and normal faulting, controlling the Banquan Basin and shifting to extrusion and thrusting on both sides. The Jiangsu segment is also characterized by alternations in the sense of slip, changing from extrusion and thrusting to extension and normal faulting back to extrusion and thrusting. Based on the comparison of the activity characteristics of the Anqiu segment (northern segment), Juxian–Tancheng segment (middle segment), and Jiangsu segment (southern segment), fault F5 satisfies the characteristics of a curved fault structure. Chao et al. [25, 26] concluded that the motion of the Anqiu–Juxian fault represents the characteristic seismic slip based on the nature of the motion and the displacement distribution of each segment. Wang and Geng [24] further summarized the characteristics of rotational movement and characteristic earthquakes and concluded that the rotational movement of the strike-slip fault is a mechanism that leads to the occurrence of characteristic earthquakes. Therefore, the kinematic characteristics of fault F5 show rotational movement along a curved structure.

The Tanlu fault zone plays an important role in the evolution of the lithospheric mantle beneath East Asia, acting as a deep channel for the ascent of melts and fluids, resulting in higher heat flow, higher seismicity, lower P-wave velocity anomalies, a thinner lithosphere, a higher degree of lithospheric modification, and a greater amount of newly accreted lithosphere near the fault [46]. The Tanlu fault zone originated early in the collisional orogeny of the North China and South China plates, followed by translational movement again during Early Cretaceous tectonic activity associated with the Pacific plate [4749]. Zhang and Dong [50] summarized the process of the Mesozoic kinematic evolution of the Tanlu fault zone as two major movement periods and five stages of development, with the Late Cretaceous transforming from sinistral strike-slip to dextral strike-slip. Some researchers have suggested that the Tanlu fault zone was right biased in the Early Cretaceous [51]. As the Tanlu fault zone has undergone tectonic transitions over multiple periods, this process has resulted in great variability in the activity characteristics of the various segments [52]. The latest paleoseismic studies have shown that the kinematics of the Yilan-Yitong Fault Zone (YYFZ) in the late Quaternary were dominated by reverse dextral faulting and normal strike-slip faulting. Horizontal displacements are the largest along the central part of the YYFZ and smallest at both of its ends. The distribution of vertical displacement varies along the YYFZ, ranging from ~40 cm to ~200 cm along the entire segment [53]. The Hegang–Tieling segment showed local activity in the early Quaternary and has a normal fault nature. The fault activity of the Xialiaohe–Laizhouwan segment is strong, manifesting as a strong extensional fault since the start of the Quaternary. The development and exposure of the Quaternary faulting in the Weifang–Jiashan segment are the best, and it is characterized by extrusion, thrusting, and dextral strike-slip faulting. Additionally, the Jiashan–Guangji segment locally shows early Quaternary normal fault activity [25]. The nature of the activity of the Tanlu fault zone from north to south can be described as an extensional normal faulting–thrusting–extensional normal faulting pattern. The comparative analysis of the fault F5 kinematic features suggests that during the early Quaternary, the shallow surface layer of the entire Tanlu fault zone may have been active, forming a curved structure, and the interiors of different segments are composed of small curved secondary faults (Figure 11). Alternating landscape changes in uplift and subsidence areas are formed, and half-graben-type faulted basins in the stepovers are developed. Since the middle to late Quaternary period, there has been strong activity in the central two sections of the Tanlu fault zone, and this activity decreases to the north and south. The Tanlu fault zone has been dominated by activity in one of the curved structures (fault F5) since the mid- to late-Quaternary period, while the other branch faults (F1-F4) have become inactive. Fault F5 has been dominated by the activity of the western branch since the Late Pleistocene, while the eastern branch has become inactive. The structural pattern of one horst with a graben on either side in the Weifang–Jiashan segment of the Tanlu fault zone may be due to the formation of a half-graben basin by this curved structure of the Tanlu fault zone, influenced by northwest-dipping faults and long-term evolution.

Since the beginning of the Quaternary, the convergence of the Pacific plate has accelerated, with the Pacific plate and the Philippine plate subducting westward and pushing against eastern mainland China. The south–north continent-to-continent collision of the Indian plate and the Eurasian plate has led to eastward tectonic extrusion in western China, which in turn pushes eastward against eastern China. Based on the deep stress environment of the seismic mechanism solution response, Zhu et al. [54] showed that the deep stress environment in the Tancheng–Xinyi segment is dominated by strike-slip and normal faulting, with the main stress oriented in the northeast direction, and the main stress in the Suqian segment is oriented in the nearly east–west direction. Against this background of regional dynamics, fault F5 is dominated by dextral strike-slip faulting, with the eastern side of the fault moving southward and the western side moving relatively northward. We refer to the faster moving plate as the active plate of the fault motion. Based on natural outcrops and exploratory trenches, our findings reveal that fault F5 and branch faults are mostly developed on the eastern side of the main fault. It can be inferred that the Jiangsu segment of fault F5 features southward dextral strike-slip faulting with the region to the east of the fault acting as the active plate (Figure 11).

6. Conclusions

  • (1)

    The natural outcrops near North Maling Mountain, the Xinyi River, and Chonggang Mountain expose fault F5. The old strata on the eastern side of the fault have been thrust over the gently sloping Pleistocene or Holocene strata on the other side, indicating that the fault was highly active in the Late Pleistocene and even in the Holocene, and the latest active period of the fault is shown to be the Holocene. The dip angle of the fault is relatively gentle (approximately 50°) and nearly 0° in some places, which shows that the main part of the fault is dominated by thrusting

  • (2)

    According to the seismic profile of the hidden area across the Jiangsu segment of fault F5, the main body of the Jiangsu segment of fault F5 consists of two branches (eastern and western branches), dominating the synsedimentary boundary of the basin and forming a half-graben rift basin. Secondary faults are developed in the rift basin, forming the three faults in Tangdian and Sankeshuxiang. The fault controlling the rift basin gradually changed from the eastern branch of the Xinyi segment to the western branch of the Suqian segment. Since the start of the Holocene, the Jiangsu segment of fault F5 has moved along the western branch

  • (3)

    The Jiangsu segment of fault F5 is dominated by strike-slip motion, with extrusion and thrusting in North Maling Mountain, the Xinyi River, and Chonggang Mountain and extension and normal faulting in the sedimentary basin, South Maling Mountain, and Zhang Mountain. The kinematic process of this fault is represented by rotational movement along the strike-slip fault with a curved path, and the resulting tensile and compressive stresses are accommodated by slip at both ends of the strike-slip fault. The activity of the Jiangsu segment of fault F5 can be divided into two periods. The first period occurred before the later part of the Late Pleistocene, when the curved structure of the Jiangsu segment of fault F5 was active, forming the western branch of the Xinyi segment and the eastern branch of the Suqian segment. The second activity started in the later part of the Late Pleistocene, when the western branch of the Xinyi segment and the western branch of the Suqian segment of the Jiangsu segment of fault F5 became active and have continued to be active up to the present, and the eastern branch of the Xinyi segment and the eastern branch of the Suqian segment became inactive and are considered Late Pleistocene faults. According to the combined borehole section exploration of the Huangshu Road area, the maximum vertical slip rate of fault F5 since the start of the Holocene is 0.28 mm/a. The Jiangsu segment of fault F5 formed through dextral strike-slip activity, mainly due to the southward movement of the region to the east of the fault

Data Availability

Data will be made available on request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

We thank the anonymous reviewers and the editor for their help improving the manuscript. This work was jointly supported by the Active Fault Detection and Seismic Risk Assessment Project of Xinyi City, Youth Science Foundation of the Earthquake Administration of Jiangsu Province (201802) and a research grant from the National Institute of Natural Hazards, MEMC (No. ZDJ2019-21).

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