The Chishan section of Tan-Lu fault zone is located in Sixian County, northern Anhui Province. Research on the characteristics of Quaternary fault activity of this section began in the 1990s, which includes microgeomorphology survey, trench excavation, dating sample collection and measurement, and so on. Through these studies, many valuable data and results were accumulated, which laid a good foundation for the current research. Based on the field geological survey and previous studies, two geological trenches were excavated, which are named trench XJ1 and XJ2 respectively. Among them, very rich remains of ancient earthquakes were found in trench XJ1 and analyzed as major contents in this paper, and few relics of ancient earthquake were found in trench XJ2, which are not involved in this paper.

In the trench XJ1, ten strata units were revealed, labeled as U₁ to U₁₀ from old to young, respectively. Layer U₁ is the Cretaceous sandstone with a thickness about 0.5~1.0m, lying on the bottom of the west wall of the trench. Layer U₂ is yellowish brown clay with a thickness of 1~2.5m, located at the bottom of the eastern side of trench profile. One OSL sample is collected in the middle of this layer with an age more than 150k {\mathrm{a}}^{}BP, which indicates the layer was deposited before the Mid Pleistocene. Layer U₃ is purple clay-sand, which is wide at the bottom around 6.5m and narrow at the top around 2.5m, and the top extends about 7m continuously from west to east. Layer U₄ is motley gravel with a thickness about 2.0~2.5m, which is below layer U₉ and above layer U₄ on the west side of the trench wall. Layer U₅ is gravel containing a lot of clay and a few of sandstone clumps, wide at the top about 3m and narrow at the bottom about 2m. Layer U₆ is light green gravel containing some sand and clay, thick in the west about 0.8m and thin in the east about 0.2m, extending around 7m discontinuously from west to east. Layer U₇ is grayish white gravel with sand and clay, thick in the west around 1.0m and thin in the east around 0.2m, extending about 5m continuously from west to east. Layer U₈ is yellow clay with a thickness of 0.5~2.0m, located below layer U₉ and above U₇. One peat sample was taken from the top of the layer and the age of this sample is 21.57~21.22k {\mathrm{a}}^{}BP measured by Beta Analytic Inc in the United States, which indicates this layer was deposited in Late Epipleistocene. Layer U₉ is black clay with a thickness of 0.5~1.5m, which is located above Layer U₄, U₅, U₇ and U₈ and is the latest disturbed layer in the trench. One peat sample was taken from the bottom of this layer and the age of this sample is 11.10~10.75k {\mathrm{a}}^{}BP measured by Beta Analytic Inc in the United States, which indicates this layer was deposited in the early Holocene. Layer U₁₀ is the cultivation layer with a thickness of 0.2~0.5m, located on the topmost of the trench wall.

Three faults were revealed in these layers, named as F₁ and F₂ and F₃ respectively from east to west. Three paleoseismic events were identified, which are labeled as E₁ and E₂ and E₃ respectively from old to new. The E₁ represents a thrust activity of fault F₁. After the deposition of layers U₅, U₃ and U₂ finished, the hanging wall U₅ of fault F₁ thrust upward above the footwall U₈, and the soft layer U₃ in between was squeezed and rubbed upward, forming lenticles in the layer, which indicates the movement direction of the hanging wall of F₁ is thrust upward. A compressional overfall scarp was formed by this event, then the layer U₆ was deposited on the east side of the scarp, whose age is not measured. But the dating of layer U₂ beneath the fault F₁ yields an age before Mid Pleistocene, which constrains the lower limit age of E₁ to be after Mid Pleistocene. The E₂ represents a thrust faulting of fault F₂. After the deposition of layer U₆, a new thrust faulting occurred on fault F₂, which cut through layer U₅ and formed a thrust fault scarp. Later, U₇ and U₈ were deposited on the east of the scarp. The layer U₇ is gravel, whose age is not measured, but the layer U₈ is dated as the Late Epipleistocene, which constrains the upper limit age of events E₁ and E₂ to be after Late Epipleistocene. The E₃ represents a strike-slip normal faulting of Fault F₃, which faulted the layer U₃. According to the age of the layer U₃, we can constrain the lower limit age of E₃ to be the Early Holocene, which indicates that the Chishan section of the Tan-Lu fault zone is still active after the Early Holocene.

To sum up, two geological trenches were excavated at the Chishan section of Tan-Lu fault zone, named as trench XJ1 and XJ2 respectively, and three main faults were revealed on the wall of trench XJ1, named as F₁, F₂ and F₃ from east to west, and three paleoseismic events were identified, which are labeled as E₁ and E₂ and E₃ respectively from old to new. The latest ancient seismic event faulted the Early Holocene layer, indicating the Chishan section of the Tan-Lu fault zone is still active after the Late Holocene, and the latest activity is of strike-slip normal faulting, which provides new evidence for the presence of Holocene activity of this fault section and new information for long-term seismic risk assessment in this area.

The Anqiu-Juxian Fault(F₅)in the middle part of Tanlu fault zone is the most important seismically active fault in eastern China. The Fault F₅ is divided into the Anqiu-Juxian section, the Juxian-Tancheng section and the Xinyi-Sihong section, each of which is an independent rupture unit. There are no historical records about earthquakes with magnitude above 5 in the Xinyi-Sihong section, but it is revealed that there are Holocene paleoseismic events, so this section is a significant gap segment of surface rupture of historical earthquakes. In recent years, an important progress in the study of neotectonic activity of Xinyi-Sihong section of F₅ is to find that it extends southward to the region between Huai River and Nüshan Lake in Anhui Province, with a length of about 20km. The fault spreads on the gentle slope on the edge of Cretaceous red sandstone uplift(hillock)along the line from Fushan to Ziyangshan, and the latest activity can date back to the early Holocene. At present, there is a clear understanding of the geometric distribution, structural characteristics and activity nature of the Huai River-Nüshan Lake section of F₅(F₅-HRNL), but the paleoseismic research is relatively weak, the revealed paleoseismic events are relatively sporadic, and the research results are from single trench, so there is a lack of comprehensive and comparative analysis from multiple trenches. In addition, the study on slip rate has not been carried out in this section, which affects the understanding of the overall activity level of the fault. Therefore, based on the previous work, paleoseismic research is carried out by excavating trenches in key locations, and more reliable paleoseismic events are determined through comprehensive comparative analysis of multiple trenches. The vertical slip rate of the fault is calculated by measuring the height of the fault scarp near the trench and combining with the dating data of relevant strata. Based on the paleoseismic research results of the F₅-HRNL and combined with the data of other disciplines, the seismic risk of this fault section is analyzed. The results of this study enrich the understanding of the overall activity characteristics of F₅ in the Tanlu fault zone in the Late Quaternary, and provide new data for medium- and long-term earthquake prediction in the border area of Jiangsu and Anhui Provinces.
In this study, a new trench was excavated at the foot of Fushan Mountain on the south bank of the Huai River, named Santangnan trench, for the special research on ancient earthquake events. The trench reveals that four paleoseismic events have occurred on F₅, and the latest event occurred since the late Late Pleistocene, that is, since(15.7±2.0)ka BP, but the trench failed to constrain the age of each event. Based on the trenching work and combined with the previously published trench research data, the paleoseismic events in the F5-HRNL are further constrained by using the progressive constraining method. The results show that at least five paleoseismic events have occurred in the F₅-HRNL since the late Middle Pleistocene. The first three events occurred in the late Middle Pleistocene to the late Late Pleistocene, all of which were thrust in nature and manifested as gently dipping thrust faults, reverse faulting colluvial wedges and structural wedges in the trench; the latest two events occurred since the late Late Pleistocene, both of which were extensional in nature and manifested as splitting wedges in the trench; the age of the latest two events are constrained at 20.36~(18.7±0.3)ka BP and 10.92~7.83ka BP respectively.
At present, the research on the slip rate of F₅ mainly focuses on the horizontal slip rate on the Shandong Province section, where the water systems are relatively developed and the deformation is obvious. The vertical slip rate of the fault is rarely reported. Stable and continuous fault scarps are developed in local segments of the F₅-HRNL, and trenches are excavated across the scarps, which provides support for the calculation of vertical slip rate of this section. Through UAV topographic mapping, a high-precision digital elevation model near the scarp is constructed, the topographic profile across the scarp is extracted, and the vertical displacement of the fault is discussed. Based on the results of Quaternary stratum dating and paleoseismic event analysis in the trench near the scarp, the starting time of vertical displacement of the scarp is determined. The calculation shows that the vertical slip rate of the F₅-HRNL is about 0.05mm/a in the Ziyangshan area and about 0.07mm/a in the Doushan area, indicating that this fault section is weakly active as a whole.
The Sihong-Mingguang section of F₅ is from the south of Chonggang Mountain in Sihong County, Jiangsu Province to the north of Nüshan Lake in Mingguang City, Anhui Province, with a total length of about 65km. The latest paleoseismic event revealed in this section is about 8 000 years ago. Based on the research results of paleoearthquakes and combined with the research data of other disciplines, it is considered that the F₅ Sihong-Mingguang section is the surface rupture gap section of historical earthquakes, a long time has elapsed since the latest ancient earthquake, and the current small earthquakes are not active, the locking degree is high, and it is likely to accumulate stress, and there is a risk of strong earthquakes of magnitude 7 or above.

Based on DEM data and ArcGIS software, we extract the geomorphic parameters of drainage basins and rivers that flow through the Huashan piedmont, which include stream length-gradient index (SL), stream-power incision model normalized channel steepness index (k_sn), hypsometric integral (HI), valley floor width to valley height ratio (Vf)and mountain front sinuosity (Smf). Study shows that all parameter indexes have obviously different distributions roughly bounded by Huaxian and Huayin. In the Huaxian to Huayin section, the stream length-gradient index has extremely high abnormal values near the fault, the values of river mean SL, mean k_sn, HI, Vf and Smf are concentrated in 500~700, 120~140, 0.5~0.6, 0~0.1 and 1.0~1.1, respectively. Between Lantian and Huaxian and between Huayin and Lingbao, the parameter indexes distributional characteristics are largely the same, with the values in 300~500, 100~120, 0.4~0.5, 0.2~0.6 and 1.2~1.5, respectively. Comprehensive analysis suggests that tectonic activity is the primary factor responsible for these differences. We divide each geomorphic parameter into three classes (strong, medium, and low)and calculate the relative active tectonics (Iat)of the Huashan piedmont. The results show that the Iat values in Huaxian to Huayin section are in 1.0~1.5, those at other places are in 1.5~3.0, indicating that the tectonic activity from Huaxian to Huayin is most intense, while that of other places are relatively weak. Field geological investigations show that the Huashan piedmont fault can be divided into Lantian to Huaxian section, Huaxian to Huayin section and Huayin to Lingbao section. In Huaxian to Huayin section the fault has been active several times since Holocene indicative of strongest activity, while in Lantian to Huaxian section and Huayin to Lingbao section the fault was active only in the late Pleistocene and its activity was weaker as a whole. Tectonic activity of the Huashan piedmont derived from river geomorphic parameters is consistent with field geological investigations, indicating that geomorphic parameters of rivers can be used to characterize activity of faults on a regional scale.

The east branch fault of Tan-Lu fault zone extends from Fengshan Town of Sihong County on the north shore of the Huaihe River in Jiangsu Province, into Fushan Town of Mingguang City on the south shore of Huaihe River in Anhui Province. The landform changes from Subei plain on the north of Huaihe River to Zhangbaling uplift area on the south of Huaihe River. The terrain rises gradually with larger relief amplitude. The Fushan section of the Tan-Lu fault zone is located in Ziyang to Fushan area of Mingguang City. The fault is shown in the satellite image as a clear linear image, and the fault extends along the east side of a NNE-trending hillock. In this section the Quaternary strata are unevenly distributed, which causes some difficulties in the study of recent fault activity.In recent years, the author has found that the fault of the Fushan section of the Tan-Lu fault zone on the south of the Huaihe River still has a certain control effect on the landform and the Quaternary strata. Based on satellite imagery and geological data, we select the appropriate location in the Fushan section to excavate the Santang trench Tc1 and Fushannan trench Tc2, and clean up the Fushannan profile Pm, which reveals rich phenomena of recent fault activity. Santang trench reveals three faults, and the faulting phenomenon is obvious. One of the faults shows the characteristic of right-lateral strike-slip normal faulting; Fushannan profile reveals one fault, with the same faulting behavior of right-lateral strike-slip normal fault. Comprehensive stratigraphic sample dating results indicate that the fault dislocated the middle Pleistocene strata, late Quaternary strata and early Holocene strata. All our work shows that the fault of Fushan section has intensive activity since late Pleistocene, and the latest active age can reach early Holocene. The latest earthquake occurred at(10.6±0.8)~(7.6±0.5)ka BP. The faults exposed by trenches and profiles show the characteristics of right-lateral strike-slip normal faulting, which reflects the complexity of the tectonic stress field in the area where the fault locates.

Anqiu-Juxian Fault(F₅) is the latest active fault in the eastern graben of the middle segment of the Tanlu fault zone. In recent years, the research results of F₅ in Jiangsu Province are abundant, and it is found that Holocene activity is prevalent in different segments, and the movement pattern is dominated by dextral strike-slip and squeezing thrust. The Anhui segment and the Jiangsu segment of the Tan-Lu fault zone are bounded by the Huaihe River. Previous studies have not discussed the extension and activity of F₅ in the south of the Huaihe River in Anhui Province. This paper chooses the Ziyangshan segment of Tanlu fault zone in the south of the Huaihe River as the breakthrough point, which is consistent with the linear image feature of extension of F₅ in Jiangsu Province. Through the remote sensing image interpretation, geological and geomorphological investigation and trench excavation, we initially get the following understanding:(1)The linear structural features of the Ziyang segment are clear, and the fault is developed on the gentle slope of the Mesozoic red sandstone uplift along the Fushan-Ziyangshan, which is the southern extension of the Anqiu-Juxian Fault(F₅); (2)The excavation of the Zhuliu trench reveals that the late Pleistocene clastic layers are interrupted, and the late late Pleistocene to early Holocene black clay layers are filled along the fault to form black fault strips and black soil-filled wedges, indicating that the latest active age of the fault is early Holocene; (3)The excavation of Zhuliu trench reveals that there are at least 3 paleo-earthquake events since the Quaternary, the first paleo-seismic event is dated to the early and middle Quaternary, and the 2nd paleo-seismic event is 20.10~13.46ka BP, the age of the third paleo-seismic event is(10.15±0.05)~(8.16±0.05)ka BP. These results complement our understanding of the late Quaternary activity in the Anhui segment of the Tanlu fault zone, providing basic data for earthquake monitoring and seismic damage prevention in Anhui Province.

Based on the 1︰50000 active fault geological mapping, combining with high-precision remote imaging, field geological investigation and dating technique, the paper investigates the stratum, topography and faulted landforms of the Huashan Piedmont Fault. Research shows that the Huashan Piedmont Fault can be divided into Lantian to Huaxian section (the west section), Huaxian to Huayin section (the middle section) and Huayin to Lingbao section (the east section) according to the respective different fault activity.
The fault in Lantian to Huaxian section is mainly contacted by loess and bedrock. Bedrock fault plane has already become unsmooth and mirror surfaces or striations can not be seen due to the erosion of running water and wind. 10~20m high fault scarps can be seen ahead of mountain in the north section near Mayu gully and Qiaoyu gully, and we can see Malan loess faulted profiles in some gully walls. In this section terraces are mainly composed of T₁ and T₂ which formed in the early stage of Holocene and late Pleistocene respectively. Field investigation shows that T₁ is continuous and T₂ is dislocated across the fault. These indicate that in this section the fault has been active in the late Pleistocene and its activity becomes weaker or no longer active after that.
In the section between Huaxian and Huayin, neotectonics is very obvious, fault triangular facets are clearly visible and fault scarps are in linear distribution. Terrace T₁, T₂ and T₃ develop well on both sides of most gullies. Dating data shows that T₁ forms in 2~3ka BP, T₂ forms in 6~7ka BP, and T₃ forms in 60~70ka BP. All terraces are faulted in this section, combing with average ages and scarp heights of terraces, we calculate the average vertical slip rates during the period of T₃ to T₂, T₂ to T₁ and since the formation of T₁, which are 0.4mm/a, 1.1mm/a and 1.6mm/a, and among them, 1.1mm/a can roughly represent as the average vertical slip rate since the middle stage of Holocene. Fault has been active several times since the late period of late Pleistocene according to fault profiles, in addition, Tanyu west trench also reveals the dislocation of the culture layer of(0.31~0.27)a BP. 1~2m high scarps of floodplains which formed in(400~600)a BP can be seen at Shidiyu gully and Gouyu gully. In contrast with historical earthquake data, we consider that the faulted culture layer exposed by Tanyu west trench and the scarps of floodplains are the remains of Huanxian M_S8½ earthquake.
The fault in Huayin to Lingbao section is also mainly contacted by loess and mountain bedrock. Malan loess faulted profiles can be seen at many river outlets of mountains. Terrace geomorphic feature is similar with that in the west section, T₁ is covered by thin incompact Holocene sand loam, and T₂ is covered by Malan loess. OSL dating shows that T₂ formed in the early to middle stage of late Pleistocene. Field investigation shows that T₁ is continuous and T₂ is dislocated across the fault. These also indicate that in this section fault was active in the late Pleistocene and its activity becomes weaker or no longer active since Holocene.
According to this study combined with former researches, we incline to the view that the seismogenic structure of Huanxian M_S8½ earthquake is the Huashan Piedmont Fault and the Northern Margin Fault of Weinan Loess, as for whether there are other faults or not awaits further study.

The relationship between the latest activity of active fault and seismic events is of the utmost importance. The Tan-lu fault zone in eastern China is a major fault zone, of which the active characteristics of the segments in Jiangsu, Shandong and Anhui has been the focus of research. This study takes the Dahongshan segment of the Tanlu Fault in Sihong County as the main research area. We carried out a detailed geological survey and excavated two trenches across the steep slope on the southwest side of the Dahongshan. Each trench shows fault clearly. Combining the comparative analysis of previous work, we identified and cataloged the late Quaternary deformation events and prehistoric earthquake relics, and analyzed the activity stages and behavior of this segment.
Fault gonge observed in the trench profiles shows that multiple earthquake events occurred in the fault. The faulting dislocated the Neogene sandstone, black gravel layer and gray clay layer. Brown clay layer is not broken. According to the relations of dislocated stratums, corresponding ¹⁴C and OSL samples were collected and dated. The result indicates that the Dahongshan segment of the Tanlu Fault has experienced strong earthquakes since the late Quaternary. Thrust fault, normal fault and strike-slip fault are found in the trenches. The microscopic analysis of slices from fault shows that there are many stick-creep events taking place in the area during the late Quaternary. Comprehensive analysis shows that there have been many paleoearthquakes in this region since the late Quaternary, the recent active time is the late Pleistocene, and the most recent earthquake event occurred in(12~2.5ka BP).
The neotectonic activity is relatively weak in the Anhui segment(south of the Huaihe River)of Tanlu fault zone. There are difficulties in the study of late Quaternary activity. For example, uneven distribution of the Quaternary, complex geological structure, larger man-made transformation of surface and so on. The progressive research may be able to promote the study on the activity of the Anhui segment of Tanlu fault zone.

Given the scarcity of research on the activity of Xinyi-Sihong segment of the Tanlu Fault zone, this paper focuses on the Zhangshan segment where there are quite evident geomorphic features to complement the shortage of the research on the northern part of Xinyi-Sihong segment. This study enriches evidences for the late Quaternary activity and paleoseismological events on the Xinyi-Sihong segment. The Zhangshan segment is located at Xiaodian Town to Jintou Village of Suqian City, stretching towards NE for 7 kilometers with a dip angle of 60~80. Research of tectonic geomorphology shows that gullies in northern part of Zhangshan segment were evidently displaced, while in the southern, two NE-trending right-stepped fault scarps are developed, with an average height of 3 meters, which generally suggests that the fault was dominated by thrust and dextral motion. Two trenches were excavated in the southern part of Zhangshan segment, numbered Mayao trench 1 and Mayao trench 2. Both trenches reveal that:(1)within this segment, Tanlu Fault shows periodic fault activity, that is, normal faulting during Pliocene epoch while thrust faulting in Quaternary period; (2)an event occurred between 15.12ka BP to 11.82 BP; (3)the latest event possibly took place around 3 500 a BP. Based on integrated results of previous studies, we identify the dates of paleoseismic events on the Xinyi-Sihong segment as follows:more than 960 thousands years ago, early to middle period of late Pleistocene, (15.12~11.82)ka BP, (11.76±0.05)ka~(10.53±0.05)ka BP, (10.15±0.05)ka~(8.16±0.05)ka BP and 4 960~3 510a BP.

The relationship between the latest displacement of active fault and seismic events is an important basic problem. Taking the Sihong segment of the Tan-Lu Fault as the main research area, we selected the Chonggangshan-Wangqian segment as the study section by Google map terrain analysis, excavated 6 large scale trenches, identified and cataloged the Late Quaternary deformation events and prehistoric earthquake relics, analyzed activity stages and behavior of the section. The result indicates that the Chonggangshan-Wangqian segment of the Tan-Lu Fault has undergone intense compression and thrust movement since the Late Quaternary. It is displayed that the brick-red sandstone of Upper Cretaceous thrust at high angle over the yellowish-brown clay of the Late Pleistocene in Chonggangshan segment, while the white or yellow sandstone of Oligocene in Wangqian segment thrust westward over the Late Quaternary sediments, accompanied by rifting activity. By the ¹⁴C dating, we get two paleoearthquake events, and their age is (11 755±45)～(10 525±45)a BP and about(10 135± 50)a BP, respectively. This suggests that the Chonggangshan-Wangqian segment of Tan-Lu Fault zone has undergone strong thrust movement since the late Pleistocene, and this activity had continued to the early Holocene.

Based on the 1: 50 000 geological mapping of active fault, the paper investigates the stratum, topography and faulted landforms of the northern marginal fault of Emei Platform, and preliminarily divides the northern marginal fault of Emei Platform into three sections by two stepovers near Tanjiazhuang Village and Nanliu Village according to different fault activity of each section.
At west of Tanjiazhuang Village is a loess platform, and the high terrain scarp can be seen from the northern margin. The height of scarp decreases progressively and the slope becomes gentle westwards at the place between Nanchi Village and Xikang Village, and to the place near Xiaoliang town, we cannot see obvious terrain scarps. The faulted sections can only be seen in the gullies which cross the terrain scarp at the south of Guozhuang Village and Tanjiazhuang Village. The fault dislocates the Pliocene red clay and the middle Pleistocene Lishi loess and covered by Malan loess; continuous paleosoil can be seen across the terrain scarp in some gullies. These indicate that in this section the fault was active in the early middle Pleistocene and its activity becomes weaker or no longer active after that.
The fault in the section between Tanjiazhuang Village and Nanliu Village can be divided into three parts by Shidian Village and Jinming Village, which are named, from west to east in sequence according to each faulted landform, the northern marginal fault of lacustrine terrace, the piedmont fault of Zijin Mountain and the northern marginal fault of loess platform. The fault transition area between each part is continuous and the fault is in linear distribution, so we see the whole fault section as having the same activity. In this section the Holocene diluvial fan is faulted. At least two plaeoearthquake events happened since Holocene, and the latest activity is in (2.00～1.29) ka BP according to Renzhuang trench and Jinsha trench, which can be well compared with former researches. The fault slip rate is over 0.33mm/a in the section south of Maguduo Village and is more than 0.36mm/a according to Renzhuang trench since the later period of the late Pleistocene.
In the section between Nanliu Village and Xizhangpo Village, the fault distributes along the frontal edge of the diluvial platform and is covered by thick loess. A 50～200m high linear terrain scarp formed due to the activity of fault can be seen along the frontal edge especially in the part between Xunwang Village and Xulu Village. At north of Wuzhai Village, the height of scarp decreases progressively and to the place near Xizhangpo Village, the terrain scarp cannot be seen clearly. In this section, Malan loess is faulted, which indicates that this fault section has been active since the late Pleistocene, but the evidence of Holocene fault activity has not been obtained yet due to the non-development of Holocene stratum. The fault slip rate is no less than 0.1mm/a since the late Pleistocene according to the faulted section at south of Xunwang Village.