The Daxing Fault is an important buried fault in the Beijing sub-plain, which is also the boundary fault of the structural unit between Langgu sub-sag and Daxing sub-uplift. So far, there is a lack of data on the shallow tectonic features of the Daxing Fault, especially for the key structural part of its northern section where it joins with the Xiadian Fault. In this paper, the fine stratigraphic classifications and shallow tectonic features of the northern section in the main Daxing Fault are explored by using three NW-trending shallow seismic reflection profiles. These profiles pass through the Daxing earthquake(M6¾)area in 1057AD and the northern section of the main Daxing Fault. The results show that seven strong reflection layers(T₀₁—T₀₃, T_Q and T₁₁—T₁₃)are recognized in the strata of Neogene and Quaternary beneath the investigated area. The largest depth of strong reflection layer(T₁₃)is about 550~850ms, which is interpreted as an important surface of unconformity between Neogene and Paleogene or basement rock. The remaining reflection layers, such as T₀₁ and T_Q, are interpreted as internal interfaces in Neogene to Quaternary strata. There are different rupture surfaces and slip as well as obviously different structural features of the Daxing Fault revealed in three shallow seismic reflection profiles. The two profiles(2-7 and 2-8)show obvious rupture surfaces, which are the expression of Daxing Fault in shallow strata. Along the profile(2-6), which is located at the end of the Daxing fault structure, a triangle deformation zone or bending fracture can be identified, implying that the Daxing Fault is manifested as bending deformation instead of rupture surfaces at its end section. This unique structural feature can be explained by a shearing motion at the end of extensional normal fault. Therefore, the Daxing Fault exhibits obviously different tectonic features of deformation or displacement at different structural locations. The attitude and displacement of the fault at the shallow part are also different to some extent. From the southwest section to the northeast section of the fault, the dip angle gradually becomes gentler(80°~60°), the upper breakpoint becomes deeper(160~600m), and the fault displacement in Neogene to Quaternary strata decreases(80~0m). Three shallow seismic reflection profiles also reveal that the Daxing Fault is a normal fault during Neogene to early Quaternary, and the deformation or displacement caused by the activity of the fault reaches the reflection layer T₀₂. This depth is equivalent to the sedimentary strata of late Early-Pleistocene. Therefore, the geometry and morphology of the Daxing Fault also reveal that the early normal fault activity has continued into the Early Pleistocene, but the evidence of activity is not obvious since the late Pleistocene. The earthquakes occurring along the Daxing Fault, such as Daxing earthquake(M6¾)in 1057AD, may not have much relation with this extensional normal fault, but with another new strike-slip fault. A series of focal mechanism solutions of modern earthquakes reveal that the seismic activity is closely related to the strike-slip fault. The Daxing Fault extends also downwards into the lower crust, and may be cut by the steeply dipping new Xiadian Fault on deep seismic reflection profile. The northern section of the Daxing Fault strikes NNE, with a length of about 23km, arranged in a right step pattern with the Xiadian Fault. Transrotational basins have been developed in the junction between the northern Daxing Fault and the southern Xiadian Fault. Such combined tectonic features of the Daxing Fault and Xiadian Fault evolute independently under the extensional structure background and control the development of the Langgu sub-sag and Dachang sub-sag, respectively.

The Youshashan Fault lies in the south flank of Yingxiongling anticline, southwestern margin of Qaidam Basin. The Yingxiongling anticline is one of the most active neotectonics, situated at the front of folds expanding southward in the Qaidam Basin. Research on the paleoseimology and Late Quaternary slip rate of this fault is important for hazard assessment and understanding tectonic deformation in this area. We excavated a 27-m-long trench across the Youshashan fault where a pressure bridge formed on the Holocene alluvial fans, measured a profile of the fold scarp created by the fault west of the Youshashan mountain, and collected several samples of finer sands for luminescence dating. Analysis of these data shows that(1) The Youshashan Fault is a Holocene active feature. The fold scarp in the basin indicates that this fault has been active along a same surface trace since at least mid-late Pleistocene. At least two paleoseismic events are revealed by trenching, both occurred in Holocene. The latest event Ⅱ in the trench happened after 500a. The current information fails to confidently support that it is the 1977 Mangya M6.4 earthquake, but cannot excludes the possibility of it is related to this earthquake. The other event Ⅰ occurred about between 1 000a to 4 000a. Erosion after the event Ⅰ prevents us to constrain the event age and to identify more events further. (2)The vertical slip rate of the Youshashan fault is about(0.38±0.06)mm/a since mid-late Pleistocene. Comparing with relative speeds of GPS sites across the Yingxiongling anticline suggests that the Youshashan fault is an important structure which is accommodating crustal shortening in this region.

As a part of the north-south seismic zone in China, a lot of M6.0-7.2 earthquakes have occurred in the margin faults of the Minshan block in history. This work attempted to characterize the geometry and activity of the north section of the Minjiang fault in this region based on high-resolution satellite images, geologic and geomorphic investigations, micro-geomorphic surveys, and trench excavation. The results show left-lateral-slip and Holocene activity of this structure. Along it, the offset landform has a continuous linearity on Ⅱ terraces near the Chuanpan village. The vertical height of the fault scarp measures 3.1 meters, which is almost the same as the accumulative horizontal displacement of the gully. The accumulative horizontal shortening due to faulting is 3.0 meters. Calculation using the model of displacement-dependent characteristic earthquakes shows both the vertical and horizontal co-seismic displacements and the horizontal shortening amount are about 1.0 meter. While strata dating suggests that the vertical and horizontal slip rates are all about 0.7-0.9mm/a, and the horizontal shortening rate is approximately 1.0-1.1mm/a. The excavated trench, perpendicular to the fault trace, reveals low-angle thrust dipping in 260åt 29°. From the relationship of the fault, colluvial wedge and stratigraphy ages, three palaeoseismic events are identified from youngest to oldest at 0-295a BP, 1 405-1 565a BP, and 2 750-2 875a BP, respectively, with recurrence intervals 1 110-1 565 years and elapsed time about 0-295 years。According to the relationship between magnitude and active parameters, it is considered that the northern segment of the Minjiang fault is capable of generating M7 or greater earthquakes. Now it is in the process of stress accumulation, having a certain seismic risk.

Beijing plain area has been always characterized by the tectonic subsidence movement since the Pliocene. Influenced and affected by the extensional tectonic environment, tensional normal faulting occurred on the buried NE-trending faults in this area, forming the "two uplifts and one sag" tectonic pattern. Since Quaternary, the Neocathaysian stress field caused the NW-directed tensional shear faulting, and two groups of active faults are developed. The NE-trending active faults include three major faults, namely, from west to east, the Huangzhuang-Gaoliying Fault, Shunyi Fault and Xiadian Fault. The NW-trending active faults include the Nankou-Sunke Fault, which strikes in the direction of NW320°~330°, with a total length of about 50km in the Beijing area. The northwestern segment of the fault dips SW, forming a NW-directed collapse zone, which controls the NW-directed Machikou Quaternary depression. The thickness of the Quaternary is more than 600 meters; the southeastern segment of the fault dips NE, with a small vertical throw between the two walls of the fault. Huangzhuang-Gaoliying Fault is a discontinuous buried active fault, a boundary line between the Beijing sag and Xishan tectonic uplift. In the Beijing area, it has a total length of 110km, striking NE, dipping SE, with a dip angle of about 50~80 degrees. It is a normal fault, with the maximum fault throw of more than 1 000m since the Tertiary. The fault was formed in the last phase of Yanshan movement and controls the Cretaceous, Paleogene, Neogene and Quaternary sediments.There are four holes drilled at the junction between Nankou-Sunhe Fault and Huangzhuang-Gaoliying Fault in Beijing area. The geographic coordinates of ZK17 is 40°5'51"N, 116°25'40"E, the hole depth is 416.6 meters. The geographic coordinates of ZK18 is 40°5'16"N, 116°25'32"E, the hole depth is 247.6 meters. The geographic coordinates of ZK19 is 40°5'32"N, 116°26'51"E, the hole depth is 500.9 meters. The geographic coordinates of ZK20 is 40°4'27"N, 116°26'30"E, the hole depth is 308.2 meters. The total number of paleomagnetism samples is 687, and 460 of them are selected for thermal demagnetization. Based on the magnetostratigraphic study and analysis on the characteristics of sedimentary rock assemblage and shallow dating data, Quaternary stratigraphic framework of drilling profiles is established. As the sedimentation rate of strata has a good response to the activity of the basin-controlling fault, we discussed the activity of target fault during the Quaternary by studying variations of deposition rate. The results show that the fault block in the junction between the Nankou-Sunhe Fault and the Huangzhuang-Gaoliying Fault is characteristic of obvious differential subsidence. The average deposition rate difference of fault-controlled stratum reflects the control of the neotectonic movement on the sediment distribution of different tectonic units. The activity of Nankou-Sunhe Fault shows the strong-weak alternating pattern from the early Pleistocene to Holocene. In the early Pleistocene the activity intensity of Huangzhuang-Gaoliying Fault is stronger than Nankou-Sunhe Fault. After the early Pleistocene the activity intensity of Nankou-Sunhe Fault is stronger than Huangzhuang-Gaoliying Fault. The activity of the two faults tends to consistent till the Holocene.

The maximum magnitude of probable future earthquake in the target area is forecasted in this paper by synthetical analysis of tectonic structure,seismicity,stress and strain fields,based on data of the Chongqing urban active faults surveying. The maximum magnitude of probable future earthquake in the target area is 5.5≤M_S≤6.0. We try to use the spatially smoothed seismicity method in weakly active area. Two seismic hazard models in weakly active area based on spatially smoothed seismicity method are proposed using strong seismic catalog and small seismic catalog. The completeness of these two catalogs is analyzed based on EMR(Entire Magnitude Range)method. And the seismic hazard(probability)in the target area is calculated using Poisson model. The results show that the seismic probability value for a single fault is relatively low and the possibility of destructive earthquake of magnitude above 5.5 in the target area is small. There are still some uncertainty and reliability for the results because of the hypnosis of the spatially smoothed seismicity model. However it should be noticed that the technique of seismic hazard calculation described in this paper based on spatially smoothed seismicity method is a new try and has potential application for the weakly active area such as Chongqing.

Yuxian-Guangling Basin is a half-graben basin unit belonging to the basin-ridge structure zone in northwest Beijing area.The southern boundary of this basin is controlled by a normal fault belt called the Yuguang Basin South Margin Fault(YBSMF).The YBSMF is about 120km long,with a general strike of N70°E,and is an active fault zone.The YBSMF was evolved from the propagation,interaction or linkage of existing isolated segments and the forming of new fault segments,and there are actually many segments and places along the YBSMF where the faults propagate and grow.However,except the study on the fault growth at the Jiugongkou segment by Cheng Shaoping in 1998,which indicated that the fault has propagated several kilometers westwardly in the late Late Pleistocene alluvial fans,the research about the propagation and growth of the faults at other places and segments is quite limited.At these segments and places,in what ways or patterns does the fault propagate,grow,link and evolve？What on earth controls and affects the propagation and growth of the faults？All these questions still remain unanswered yet and deserve further analysis and study.Based on high-resolution remote sensing image interpretation,DEM 3D analysis,field geological investigation,trenching and so on,we made a research on the fault growth of the YBSMF.According to the fault geometry,fault activity and the difference of the faulted landforms,the YBSMF belt can be divided into five segments: Shangbaiyang segment,Tangshankou segment,Beikou segment,Songzhikou segment and Shanghupen segment.The faults grow and evolve both between adjacent segments and within each segment.Besides,some new faults also form in the proluvial fans in front of mountains.After a detailed comparison and analysis of all the sites of fault growth along the YBSMF,we find out several characteristics and rules about the growth of the fault.First,the faults often grow or evolve where the fault geometry is irregular,and the irregularity of fault geometry is a primary factor which determines whether the faults propagate and grow or not.The irregular segments where the faults propagate and grow can be divided into two categories.The first type mainly includes the uneven or unsmooth segments,such as the segments with convex or concave arcs,edges or corners,and so on; the second type mainly consists of two nearly parallel faults with a gap between them,which causes the discontinuity of the fault geometry along the strike.Second,fault growth leads to the "cut off" and elimination of the irregularity of fault geometry,such as cutting off the uneven or unsmooth segments,and linking the discontinuous segments along the strike.The elimination of the irregularity makes the fault geometry smooth and continuous,and reduces the roughness on the sliding surface,which contributes to the downward slip of the half-graben block inside the basin along the sliding surface.Third,the degree of "cut off" or elimination may be affected by the spatial scale of the irregular shape.As the scale of the irregularity increases,the fault will propagate a larger distance to overcome the hindrance of the roughness,so it will take more time for the irregular segments to be completely "cut off" or eliminated,and vice versa.Therefore,after the same period of time,the irregularity with a small scale has been completely "cut off" or eliminated,while the irregularity with a large scale may be still in the process of segment linkage or cutting off,so the degree of "cutting off" or elimination is lagging behind and relatively lower.

Early in the 60s-70s of the 20th century, Japanese and American scientists have successfully observed the variation of gravity associated with earthquake process. In China, many researchers have studied the gravity field of some typical earthquakes. The results showed that observable variations of gravity do exist during the development process of medium-strong earthquakes, and that different earthquakes have different features of gravity variation. It is considered that the density variation in the crust may be an important factor. In this paper a model of stress-failure coupling effect is proposed in the light of failure mechanism. In addition, the theoretical relation between the stress failure coupling effect and variation of gravity in the crust is derived according to strain equivalent hypothesis. Furthermore, the different features of gravity evolution during the development processes of the Tangshan earthquake (M_S7.8) in 1976 and the Kunlun earthquake (M_S8.1) in 2001 are simulated through rock mechanical test with various loading paths. The results are something in good agreement with the actual facts and show that the stress failure coupling effect may be a physical explanation of gravity change before earthquake.