The Tanlu fault zone is the most active fault zone in eastern China. It has been active mainly along the Anqiu-Juxian Fault(AJF)since the Quaternary. Predecessors have done a lot of research on the age, paleoearthquake and geometry structure of the AJF, but most of them focus on the exposed area of the fault, and relatively few studies on the buried section. Using field geological survey, shallow seismic exploration, drilling, and paleoearthquake trench, this paper focuses on the geometry structure of the Xinyi section(the buried section)of the AJF, and analyzes its geometry distribution characteristics in the plane and the structural relationship between the deep and the shallow parts, thus filling the gap of the activity characteristics of the Xinyi section of the AJF. The results show that the Xinyi section of the AJF can be divided into three sections from north to south: the Beimalingshan-Guanzhuang section, the Guanzhuang-Tangdian section and the Tangdian-Xindian section.
The Xinyi section of the AJF, mainly manifested as strike-slip and normal faulting, has a right-handed and right-step distribution. The step-over zone with~900m in width and~16km in length is dominated by extension, leaving a length-width ratio of 18:1, much larger than the traditional pull-apart basin ratio of 3:1. According to the shallow seismic profile, the shallow seismic line in the Guanzhuang-Tangdian section revealed the extensional fault depression basin on the north side of the terrace, and the bedrock top of the basin gradually became shallower toward the north. The top of the bedrock in the shallow seismic survey line on the north side of the Nanmalingshan suddenly became deeper, and the NNE-trending compressional near-EW basins of the Nanmalingshan and Tashan developed. The two basins were formed from different origin. With the activity of the Anqiu-Juxian Fault and the erosion and deposition of the Shu River, the two basins gradually developed and merged into a composite basin, and the basin structure was consistent with the Quaternary stratigraphic isopach.
The Xinyi section of the Anqiu-Juxian Fault presents the deformation characteristics of the same genesis and coordinated geometric structure in the deep and superficial layers, showing a single branch in the deep, cutting through the Cretaceous strata, extending and rupturing upward along the contact interface between the bedrock mountains and the Quaternary soft soil layer in the superficial layer. The fault is shown as a single branch in the north and south Maling Mountains, and ruptured to the surface in many places. In the pull-apart basin in the middle of the fault, the thickness of the Quaternary system is more than 300m. When the Anqiu-Juxian Fault ruptures to the upper part, it divides into two branches, the east and the west, which are concealed and stand opposite to each other in the shape of “Y”, forming the Anqiu-Juxian Fault. On the east-west boundary of the fault, the latest activity is along the west branch of the fault, which is a Holocene active fault. When it extends to the basement rock mass of the Maling Mountains in the north and south, the depth of the upper fault point gradually becomes shallower until it is exposed.
The vertical movement of the Xinyi section of the AJF shows the four quadrants characteristics of uplift and subsidence. The extensional area forms a pull-apart basin, while the compressive area constitutes an uplift. The vertical bedrock offset of the Guanzhuang-Tangdian section, with the maximum vertical offset of~230m, gradually decreases to both sides. It can be concluded that the Xinyi section of the AJF presents a spiral-like pivot movement.

The Gaoyou-Baoying M_S4.9 earthquake on July 20, 2012 occurred in the Gaoyou Sag in the Subei Basin. This earthquake was a relatively rare medium-strength earthquake in the weak seismicity region of eastern China. Although studies on the seismogenic structure of this earthquake have been conducted previously, the seismogenic structure itself is still under debate and needs to be further studied. This paper uses the methods such as distribution of seismic intensity, precise positioning of earthquake sequence, focal mechanism, regional tectonic stress, seismic exploration, etc. to comprehensively study the seismogenic structure of this earthquake.
The characteristics of earthquake sequence show that the seismic structure is a high dip-angle fault spreading along the NNE direction, dipping ESE. The result of focal mechanism solutions shows that the strike of one of the two nodal planes is NNE, and the fault plane shows high dip angle. The earthquake is mainly characterized by strike-slip motion. Through the seismic exploration lines(GYL1, GYL2)laid at the epicenter area of the earthquake, a fault structure is identified, which strikes nearly NNE and dips near ESE. This fault is located between the Linze sag and the Liubao low uplift, coinciding with the distribution of the Liuling Fault, the boundary fault in the northwest of the Gaoyou Sag, so it can be judged that all the detected breakpoints belong to the Liuling Fault. The “Y-shaped” breakpoints detected by the two seismic exploration lines are characterized by high dip angle. There is a very obvious wave group disorder area at the distance of 6 500~9 000m on the GYL1 seismic exploration line. This area is about 2.5km in width displayed on the post-stack migration profile and shows an uplifting trend. The disordered uplifting of wave group is caused by intrusion of soft material into the structural breakage and weakness, squeezed by horizontal stress. The GYL2 post-stack migration profile shows obvious uplift appearing in the reflection wave group(T_g)on the top of the bedrock. This arc-shaped uplift also reflects the effect of strong compression of horizontal stress.
In order to further discuss the seismogenic structure of the Gaoyou-Baoying M_S4.9 earthquake, we used the focal mechanism data to invert the modern tectonic stress field in the Northern Jiangsu-South Yellow Sea Basin where the earthquake occurred. The maximum principal stress in this area is NE-SW, while the minimum principal stress is NW-SE; both of them are nearly horizontal, and the intermediate principal stress is nearly vertical. According to Zoback's rule for dividing the types of dislocation in the direction of the force axis, the distribution of principal stresses in the Northern Jiangsu-South Yellow Sea Basin is equivalent to a strike-slip dislocation.
To sum up, the stress characteristics reflected by the Liuling Fault are consistent with the horizontal forces on the P-axis and T-axis shown by the focal mechanism solution results, and also consistent with the horizontal state of the stress in the tectonic stress field in this region. The above characteristics indicate that the development of the Liuling Fault is affected and controlled by modern tectonic activities. At the same time, the characteristics of the strike and dip of the seismic fault reflected by the methods of seismic intensity investigation, precise earthquake positioning, focal mechanism solution and seismic exploration, etc. are consistent with each other. Therefore, the occurrence of this earthquake may be the result of continuous stress accumulation and sudden instability and rupture of the NNE-trending Liuling Fault under the long-term compression of the NE-direction principal stress.

The Tan-Lu fault zone is the largest active tectonic zone in eastern China, with a complex history of formation and evolution, and it has a very important control effect on the regional structure, magmatic activity, the formation and distribution of mineral resources and modern seismic activity in eastern China. Xinyi City has a very important position as a segmental node in the Shandong and Suwan sections of the Tan-Lu fault zone. Predecessors have conducted research on the spatial distribution, occurrence and activity characteristics of the shallow crustal faults in the Suqian section of the Tan-Lu belt, and have obtained some new scientific understandings and results. However, due to different research objectives or limitations of research methods, previous researches have either focused on the deep crustal structure, or targeted on the Suqian section or other regions. However, the structural style and deep-shallow structural association characteristics of Xinyi section of Tan-Lu belt have not been well illustrated, nor its activity and spatial distribution have been systematically studied. In order to investigate the shallow crustal structure features, the fault activities, the spatial distribution and the relationship between deep and shallow structures of the Xinyi section of the Tan-Lu Fault, we used a method combining mid-deep/shallow seismic reflection exploration and first-break wave imaging. Firstly, a mid-deep seismic reflection profile with a length of 33km and a coverage number greater than 30 was completed in the south of Xinyi City. At the same time, using the first arrival wave on the common shot record, the tomographic study of the shallow crust structure was carried out. Secondly, three shallow seismic reflection profiles and one refraction tomography profile with high resolution across faults were presented. The results show that the Xinyi section of Tan-Lu fault zone is a fault zone composed of five concealed main faults, with a structural pattern of “two grabens sandwiched by a barrier”. The five main faults reveal more clearly the structural style of “one base between two cuts” of the Tan-Lu fault zone. From west to east, the distribution is as follows: on the west side, there are two high-angle faults, F₄ and F₃, with a slot-shaped fault block falling in the middle, forming the western graben. In the middle, F₃ and F₂, two normal faults with opposite dip directions, are bounded and the middle discontinuity disk rises relatively to form a barrier. On the east side, F₂ and F₁, two conjugate high-angle faults, constitute the eastern graben. The mid-deep and shallow seismic reflection profiles indicate that the main faults of the Xinyi section of Tan-Lu fault zone have a consistent upper-lower relationship and obvious Quaternary activities, which play a significant role in controlling the characteristics of graben-barrier structure and thickness of Cenozoic strata. The shape of the reflective interface of the stratum and the characteristics of the shallow part of the fault revealed by shallow seismic reflection profiles are clear. The Mohe-Lingcheng Fault, Xinyi-Xindian Fault, Malingshan-Chonggangshan Fault and Shanzuokou-Sihong Fault not only broke the top surface of the bedrock, but also are hidden active faults since Quaternary, especially the Malingshan-Chonggangshan Fault which shows strong activity characteristics of Holocene. The results of this paper provide a seismological basis for an in-depth understanding of the deep dynamics process of Xinyi City and its surrounding areas, and for studying the deep-shallow tectonic association and its activity in the the Xinyi section of the Tan-Lu Fault.

The NE-trending regional deep fault, i.e. the Jintan-Rugao Fault, is a boundary fault between the Subei depression and Nantong uplift, and its research has always received broad attention because of its importance and complexity. For the absence of definite proof, there is little consensus regarding the structure and spatial distribution of the fault among geoscientists, and its latest active time is ambiguous. The study of Quaternary activity characteristics of the Jintan-Rugao Fault is of great significance for earthquake trend prediction and engineering safety evaluation, and for earthquake prevention and disaster reduction in Jiangsu Province. In order to investigate the spatial location, characteristics and tectonic features and redefine the activity of the NE-segment of the Jintan-Rugao Fault, and on the basis of likely location and marker beds derived from petroleum seismic exploration sections, we collect and arrange 4 shallow seismic exploration profiles crossing the fault to conduct high-resolution seismic reflection imaging, following the working concept of ‘from known to unknown, from deep to shallow’. In this study, an observation system with trace intervals of 4~6m, shot intervals of 12~18m, and channels of 90~256 and 15~36 folds is used. In addition, by introducing different tonnage vibroseis to suppress the background noise, the raw data with high SNR(signal-noise ratio)can be obtained. By using the above working method and spread geometry, we obtained clear imaging results of the subsurface structure and fault structure in the coverage area of the survey lines. This exploration research accurately locates the NE-segment of Jintan-Rugao Fault, and further shows that it is not a single fault but a fault zone consisting of two normal faults with N-dipping and NE-striking within the effective detection depth. The shallow seismic profiles reveal that the up-breakpoint on the south branch with stronger activity is at depth of 235~243m, which offsets the lower strata of lower Pleistocene. Combining drilling data around the survey lines, we infer the activity time of this fault is early Pleistocene. The results of this paper provide reliable seismological data for determining the location and activity evaluation of the NE-segment of Jintan-Rugao Fault. In eastern China, where the sedimentary layer is thicker, the latest active age of faults can not be determined entirely according to the latest faulted strata. For a fault passing through the thicker area of new deposits, its latest active age should be based on the tectonic background, seismic activity, present tectonic stress field, topographic deformation, structural micro-geomorphological characteristics, sedimentary thickness of new strata, controlling effect of faults on new strata and the latest strata of faults, and combined with upper breakpoints, morphology, structure and occurrence of faults, the active state of the target concealed faults should be analyzed. If the activity of the fault is judged only by the upper faulted point, it may lead to overestimating the age of the fault activity.

The Tan-Lu Fault Zone(TLFZ), a well-known lithosphere fault zone in eastern China, is a boundary tectonic belt of the secondary block within the North China plate, and its seismic risk has always been a focus problem. Previous studies were primarily conducted on the eastern graben faults of the Yishu segment where there are historical earthquake records, but the faults in western graben have seldom been involved. So, there has been no agreement about the activity of the western graben fault from the previous studies. This paper focuses on the activity of the two buried faults in the western graben along the southern segment of Yishu through combination of shallow seismic reflection profile and composite drilling section exploration.
Shallow seismic reflection profile reveals that the Tangwu-Gegou Fault(F₄)only affects the top surface of Suqian Formation, therefore, the fault may be an early Quaternary fault. The Yishui-Tangtou Fault(F₃)has displaced the upper Pleistocene series in the shallow seismic reflection profile, suggesting that the fault may be a late Pleistocene active fault. Drilling was implemented in Caiji Town and Lingcheng Town along the Yishui-Tangtou Fault(F₃)respectively, and the result shows that the latest activity time of Yishui-Tangtou Fault(F₃)is between(91.2±4.4)ka and(97.0±4.8)ka, therefore, the fault belongs to late Pleistocene active fault.
Combined with the latest research on the activity of other faults along TLFZ, both faults in eastern and western graben were active during the late Pleistocene in the southern segment of the Yishu fault zone, however, only the fault in eastern graben was active in the Holocene. This phenomenon is the tectonic response to the subduction of the Pacific and Philippine Sea Plate and collision between India and Asian Plate. The two late Quaternary active faults in the Yishu segment of TLFZ are deep faults and present different forms on the surface and in near surface according to studies of deep seismic reflection profile, seismic wave function and seismic relocation. Considering the tectonic structure of the southern segment of Yishu fault zone, the relationship between deep and shallow structures, and the impact of 1668 Tancheng earthquake(M=8(1/2)), the seismogenic ability of moderate-strong earthquake along the Yishui-Tangtou Fault(F₃)can't be ignored.

The fault F₅ is considered as the most active fault in the Tanlu fault zone(Yi-Shu fault zone), which is located from Weifang of Shandong Province to Jiashan of Anhui Province, with a length of 360km. It has always been a focus of concern to many geoscientists because of its complexity and importance. But, for a long period of time, there exists biggish indetermination in the accurate position and active ages of the fault F₅ in Suqian section of Tanlu fault zone. Seismic reflection exploration is the main technique in present urban active faults detecting. In order to investigate the spatial distribution, characteristics and activities of the fault F₅ in covered terrains, we carried out a systematic survey to the fault with shallow seismic prospecting method and obtained the accurate position and development characteristics of the fault. The results show that the fault F₅ continues to develop toward south rather than ending at the Huancheng South Road of Suqian City. F₅ is mainly composed of two main faults, which dip in opposite directions and almost vertically. Near the Sankeshu town, F₅ is composed of three faults with right-stepping, forming a small pull-apart basin with length of 6km, width of 2.5km, controlling the deposition of Neogene and Quaternary strata. By combining the results of composite drilling section and trenching, we make a conclusion that the western branch of fault F₅ is a Holocene active fault, and the eastern branch is a Pleistocene active fault. Our general view is that fault F₅ is a Holocene active fault.