High-precision and high-resolution topography are the basis of quantitative study of active tectonics. Traditional methods are mainly interpreted from the remote sensing image and can only obtain two-dimensional, medium-resolution DEM(5~10m grid unit)or local three-dimensional surface deformation characteristics. A combination of offset and micro-relief information is essential for understanding the long-term rupture pattern of faults, such as in seismic hazard evaluation. The recently developed high-resolution light detection and ranging(LiDAR)technology can directly carry out high-precision and omni-directional three-dimensional measurement of the landform, and provide fine geomorphologic data for the study of active tectonics, which is helpful to deepen the understanding of surface rupture process and fault activity characteristics. In this study, we take part of the Xiaohongshan Fault, the western segment of Xiangshan-Tianjingshan Fault located in Gansu Province(NE Tibet), as an example of how LiDAR data may be used to improve the study of active faults. Using the airborne LiDAR technology, we obtain the three-dimensional surface deformation characteristics with high accuracy and establish the three-dimensional topographic model of the fault geomorphic. A high-resolution digital elevation model(DEM)of the Jingtai-Xiaohongshan Fault was extracted based on high-precision LiDAR data. Then the faulted geomorphic markers(gullies, ridges and terraces)were measured in detail along the fault, and different offset clusters and long-term sliding vector of different segments of the fault were finally acquired. We obtained the 82 horizontal displacements and 62 vertical displacements of geomorphic markers. According to the offset amounts, we observed peaks in the histogram by using the method of cumulative offset probability density and interpreted that each peak may represent an earthquake that ruptured the Xiaohongshan Fault. The results show that the horizontal and vertical displacements fall into five clusters, and the smallest cluster may indicate the coseismic slip of the most recent earthquake, while the other clusters may represent the slip accumulation of multiple preceding earthquakes. The sliding vectors constrained by the horizontal and vertical displacement of several typical geomorphic markers show obvious differences on different segments of the fault. The results show that the fault segment is divided into three segments from west to east, which indicates that the fault activity is not uniform along the fault.

Due to the interaction between the Tibetan plateau, the Alxa block and the Ordos block, the western margin of Ordos(33.5°~39°N, 104°~108°E)has complex tectonic features and deformation patterns with strong tectonic activities and active faults. Active faults with different strikes and characteristics have been developed, including the Haiyuan Fault, the Xiangshan-Tianjingshan Fault, the Liupanshan Fault, the Yunwushan Fault, the Yantongshan Fault, the eastern Luoshan Fault, the Sanguankou-Niushoushan Fault, the Yellow River Fault, the west Qinling Fault, and the Xiaoguanshan Fault.
    In this study, 7 845 earthquakes(M≥1.0)from January 1st, 1990 to June 30th, 2018 were relocated using the double-difference location algorithm, and finally, we got valid locations for 4 417 earthquakes. Meanwhile, we determined focal mechanism solutions for 54 earthquakes(M≥3.5)from February 28th, 2009 to September 2nd, 2017 by the Cut and Paste(CAP)method and collected 15 focal mechanism solutions from previous studies. The spatial distribution law of the earthquake, the main active fault geometry and the regional tectonic stress field characteristics are studied comprehensively.
    We found that the earthquakes are more spatially concentrated after the relocation, and the epicenters of larger earthquakes(M≥3.5) are located at the edge of main active faults. The average hypocenter depth is about 8km and the seismogenic layer ranges from 0 to 20km. The spatial distributions and geometry structures of the faults and the regional deformation feature are clearly mapped with the relocated earthquakes and vertical profiles. The complex focal mechanism solutions indicate that the arc-shaped tectonic belt consisting of Haiyuan Fault, Xiangshan-Tianjingshan Fault and Yantongshan Fault is dominated by compression and torsion; the Yellow River Fault is mainly by stretching; the west Qinling Fault is characterized by shear and compression. The structural properties of the fault structure are dominated by strike-slip and thrust, with a larger strike-slip component. The near-north-south Yellow River Fault is characterized by high angle NW dipping and normal fault motion.
    Based on small earthquake relocation and focal mechanism solution results, and in combination with published active structures and geophysical data in the study area, it is confirmed that the western margin of Ordos is affected by the three blocks of the Tibetan plateau, the Alax and the Ordos, presenting different tectonic deformation modes, and there are also obvious differences in motion among the secondary blocks between the active faults. The area south of the Xiangshan-Tianjingshan Fault has moved southeastward since the early Quaternary; the Yinchuan Basin and the block in the eastern margin of the Yellow River Fault move toward the SE direction.

Most great(M≥8)earthquakes during modern times have occurred in interplate regions or major continental collision zones, such as Sumatra, the Japanese island arc or the San Andreas fault zone. Continental faults slip at a much lower rate than boundary faults, but they also have the potential of generating large earthquakes. For example, the 2008 Wenchuan earthquake with a magnitude of 7.9, the slip rate of seismic fault is less than 3mm/a. They also have the potential to be significantly deadlier than those on plate boundaries because of the long repeat times and lack of preparedness. The January 23rd 1556 Huaxian earthquake in Shaanxi Province, central China, is the deadliest in history with an estimated death toll of ~830 000 from building collapse, land-sliding, famine, and disease. The earthquake occurred in the graben of the Weihe River.
    The Weihe Graben in Shaanxi Province has recorded multiple earthquakes in history, whereas most active faults within the graben have a low slip rate over geological times (~1mm/a). The slip rate of faults is an important parameter for assessing the risk of earthquakes and the interval between major earthquake recurrences. In order to obtain the quantitative information of faults slip rate, traditional geological methods or geodetic observation techniques can be used. Interferometric synthetic aperture radar(InSAR), as a modern geodetic observation technology, has the characteristics of all-weather and day-and-night imaging capability, wide spatial coverage, fine resolution, and high measurement accuracy. InSAR offers the potential to measure interseismic slip rates on faults at a resolution of millimetres per year. In this study, we use InSAR data to analyze the present deformation of the Kouzhen-Guanshan, Weihe and North Qinling faults in the central part of the graben.
    We collected 32 European Space Agency(ESA's)Envisat ASAR images from descending track 161 between 2003 and 2010, and processed them using ROI_PAC. The precise orbit determination from the Delft Institute for Earth Oriented Space Research(DEOS)was applied to correct for orbital effects. The topographic contribution was simulated and removed using the 90m resolution Shuttle Radar Topography Mission(SRTM)Digital Elevation Model(DEM)from CGIAR-SCI. Each interferogram was downsampled to 64 looks in the range direction (1 280m). Before phase unwrapping, a weighted power spectrum filter was applied to improve the signal-to-noise ratio. The branch-cut method was used for phase unwrapping. Phase unwrapping errors were checked by summing around a closed loop. All the major unwrapping errors were identified and corrected manually. We obtained a total of 98 interferograms with a spatial baseline of smaller than 300m, and selected 33 interferograms whose coherence is well preserved for time-series analysis. The time-series analysis was implemented using the π-RATE software package. It uses the geocoded interferograms from ROI_PAC to create a minimum spanning tree(MST)network, from which the orbital and topographically-correlated atmospheric errors are estimated. The MST network connects all epochs with the most coherent interferograms,including no closed loops of interferograms. The network approach is able to improve the estimation of orbital error by ~9% compared to the independent interferograms approach. The orbital errors are empirically modelled as planar or quadratic ramps. The topographically-correlated atmospheric correction was applied to each interferogram after having corrected for the orbital errors. Following creating a minimum spanning tree network, correcting for orbital and topographically-correlated atmospheric errors, and calculating the covariance matrix, we obtained the 7-year average slip rate of the faults that we are focused on.
    Our results show that the faults across the Weihe graben all have a small slip rate of less than 2mm/a. The Kouzhen-Guanshan Fault does not show any evident deformation signal. The Weihe Fault seems to show 1mm/a normal faulting in the satellite line-of-sight direction. In addition, we find ~10mm/a surface subsidence of the Xi'an City between 2003 and 2010. We use the stable Ordos block as a reference to assess the accuracy of our InSAR time-series analysis. Assuming the Ordos block has no internal deformation, we calculated the error of the InSAR rate map to be (-0.1±1)mm/a, indicating that our result is reliable. This paper presents a preliminary result of the present deformation of the Weihe Graben. InSAR is a powerful technique for monitoring active faults on a timescale of tens of years, and can be used for seismic hazard assessment in the future.

The hypothesis that strong earthquakes in China mainland are controlled by the movement and interaction of active-tectonic blocks was advanced by Chinese scientists, with the remarkable ability to encompass geological and geophysical observations. Application of the active-tectonic block concept can illustrate 6 active-tectonic block regions and 22 active-tectonic blocks in mainland China and its neighboring regions. Systems of active-tectonic block boundaries are characterized by a zone of decades or hundreds of strong earthquakes. One of the greatest strengths of the modern active-tectonic block hypothesis is its ability to explain the origin of virtually all the M8 and 80% M7 earthquakes on the main continent in eastern Asia. In other words, active-tectonic block boundary stands in strong causal interrelation with recurrence behaviors of strong earthquakes and thus, it is possible to predict an earthquake occurrence in principle. After nearly two decades of development and improvement, the active-tectonic block hypothesis has established its theoretical foundation for the active tectonics and earthquake prediction, and is promoting the transition from probabilistic prediction to physical prediction of strong earthquakes. The active-tectonic block concept was tested by application to a well-documented, high-frequent earthquake area, and was found to be an effective way of describing and interpreting the focal mechanism and seismogenic environment, but there are still many problems existing in the active-tectonic block hypothesis, which confronts with rigorous challenges. Future progress will continue to be heavily dependent on the high-precision synthetic seismogram, especially of critical poorly documented settings. It is well known that strong earthquakes occur anywhere in the interactions among the active-tectonic block boundaries where there is sufficient stored elastic strain energy driving fault propagation, and then releasing the stored energy. Therefore, future studies will focus on the mechanism and forecast of the strong earthquake activity in the active-tectonic block boundary zone, with fault activity within the active-tectonic block boundary zone, quantifying current crustal strain status, upper crust and deep lithosphere coupling relation, strong earthquake-generating process and its precursory variation mechanism in seismic geophysical model as the main research contents, which are the key issues regarding deepening the theory of active-tectonic block and developing continental tectonics and dynamics in the modern earth science.

Slip rate is one of the most important parameters in quantitative research of active faults. It is an average rate of fault dislocation during a particular period, which can reflect the strain energy accumulation rate of a fault. Thus it is often directly used in the evaluation of seismic hazard. Tectonic activities significantly influence regional geomorphic characteristics. Therefore, river evolution characteristics can be used to study tectonic activities characteristics, which is a relatively reliable method to determine slip rate of fault. Based on the study of the river geomorphology evolution process model and considering the influence of topographic and geomorphic factors, this paper established the river terrace dislocation model and put forward that the accurate measurement of the displacement caused by the fault should focus on the erosion of the terrace caused by river migration under the influence of topography. Through the analysis of the different cases in detail, it was found that the evolution of rivers is often affected by the topography, and rivers tend to migrate to the lower side of the terrain and erode the terraces on this side. However, terraces on the higher side of the terrain can usually be preserved, and the displacement caused by faulting can be accumulated relatively completely. Though it is reliable to calculate the slip rate of faults through the terrace dislocation on this side, a detailed analysis should be carried out in the field in order to select the appropriate terraces to measure the displacement under the comprehensive effects of topography, landform and other factors, if the terraces on both sides of the river are preserved. In order to obtain the results more objectively, we used Monte Carlo method to estimate the fault displacement and displacement error range. We used the linear equation to fit the position of terrace scarps and faults, and then calculate the terrace displacement. After 100, 000 times of simulation, the fault displacement and its error range could be obtained with 95%confidence interval. We selected the Gaoyan River in the eastern Altyn Tagh Fault as the research object, and used the unmanned air vehicle aerial photography technology to obtain the high-resolution DEM of this area. Based on the terrace evolution model proposed in this paper, we analyzed the terrace evolution with the detailed interpretation of the topography and landform of the DEM, and inferred that the right bank of the river was higher than the left bank, which led to the continuous erosion of the river to the left bank, while the terraces on the right bank were preserved. In addition, four stages of fault displacements and their error ranges were obtained by Monte Carlo method. By integrating the dating results of previous researches in this area, we got the fault slip rate of(1.80±0.51)mm/a. After comparing this result with the slip rates of each section of Altyn Tagh Fault studied by predecessors, it was found that the slip rate obtained in this paper is in line with the variation trend of the slip rate summarized by predecessors, namely, the slip rate gradually decreases from west to east, from 10~12mm/a in the middle section to about 2mm/a at the end.

In tectonically active regions, geomorphic features such as fluvial terraces can be interpreted as the consequence of tectonic and climatic forcing. However, deciphering and distinguishing tectonic impacts and climate changes remain a challenge. In this study, we examine the terraces along the Hongshuiba river and Maying river, which flow across the Fudongmiao-Hongyazi fault in the northern margin of the Qilian Mountains. Our purpose is to analyze the relative roles of tectonics and climate in shaping orogenic topography in this area. 8~9 levels of river terraces were identified through field observations, interpretation of satellite images and using DEMs. According to relative heights and ages of T₅ of the Hongshuiba river and T₆ of the Maying river, the incision rates are calculated to be (10.2±2.0)mm/a and (12.2±2.8)mm/a, respectively. Furthermore, the thrust rate along the Fodongmiao-hongyazi fault was determined based on offset terraces and OSL dating, which are ten times less than river incision rates approximately. Comparing the uplift rate and incision rate in the northern margin of the Qilian Mountains and adjacent areas, we inferred that climate change is the most plausible controlling factor in the evolution of the river terraces, while tectonics plays a minor role in this process.

The 2008 M_S8 Wenchuan earthquake occurs on a high angle listric thrust fault. It is the first time that the near and far field strong ground motion was observed for such special type thrust earthquake. This paper jointly interprets the distribution of peak acceleration of ground motion data with seismogenic structure and slip propagating process to investigate how high angle listric thrust fault controls the pattern of strong ground motion. We found that the distribution of peak acceleration of strong ground motion during the Wenchuan earthquake has four distinctive features: 1)The peak acceleration of ground motion inside the Longmenshan fault zone is large, that is, nearly twice as strong as that outside the fault zone; 2)This earthquake produces significant vertical ground motion, prevailing against horizontal components in the near field; 3)The far field records show that the peak acceleration is generally higher and attenuates slower versus station-fault distance in the hanging wall. It is doubtful that the attenuation of horizontal components also has the hanging wall effect since no evidence yet proving that the unexpected high value at long distance need be omitted; 4)As to the attenuation in directions parallel to the source fault(Yingxiu-Beichuan Fault), the far field records also exhibit azimuthal heterogeneity that the peak acceleration of horizontal components decreases slower in the north-northeastern direction in which the co-seismic slip propagates than that in the backward way. However, the attenuation of vertical component displays very weak heterogeneity of this kind. Synthetically considered with shallow dislocation, high dip angle, and prevailing vertical deformation during co-seismic process of the Wenchuan earthquake, our near and far field ground motion records reflect the truth that the magnitude of ground motion is principally determined by slip type of earthquake and actual distance between the slipping source patches and stations. As a further interpretation, the uniqueness of high angle listric thrust results in that the ground motion effects of the Wenchuan earthquake are similar to that due to a common thrust earthquake in some components while differ in the others.

Field investigation and damage evaluation of the Lushan M7.0 earthquake have revealed that the seismogenic fault of this earthquake is a typical blind fault with thrust component and there is no distinct surface rupture or deformation zone. The earthquake caused severe damage and failure such as mountain landslide,bedrock collapse,sand liquefaction on near-fault region and tensional fractures. In order to estimate the influence of strong ground motion on damage distribution,based on the inversion of slip distribution and rupture process on the source fault of the Lushan 7.0 earthquake,strong ground motion simulation is carried out with finite-fault model and three-dimension crust model of Lushan area and its adjacent region. In the finite-fault model,the rupture source is characterized as a low-angle fault plane with inhomogeneous slip distribution. The maximum slip on fault plane is up to 150cm. For the three-dimension crust model,deep fault structure,steep terrain and basin have been taken into account and described by different physical parameters. In this paper,the numerical simulation results of strong ground motion about Lushan main earthquake reveal two following major characters. The first is that the distribution characteristics of peak acceleration values,peak velocities and peak displacements on the ground surface shows good consistency with the seismic damage investigation. On the hanging-wall of the causative fault,high intensity of strong ground motion mainly concentrates on Baosheng,Longmen and the northern area of Lushan,which are located within the IX meizoseismal area. Around the area of Longmen town,the maximum acceleration of UD component reaches up to 350gal and the maximum transient displacement is up to 110cm,which are consistent with recordings and investigations. The second conclusion from simulation results is that the strong ground distribution and propagation process are influenced by basin effect and steep terrain. Seismic wave propagated back and forth in intermountain basins,combining with the amplification of thin soil layers,which directly caused and increased the earthquake damage.

Besides the site condition,the distribution of sub-sources and expansion of sub-source rupture on finite fault effect the near-field ground motions strongly,too.In order to simulate strong ground motion effectively,we develop a simplified numerical simulation method by numerical Green's function's computation.The whole process can be divided into two steps.At first,in infinite uniform elastic medium,take the finite fault as an exterior source under asymmetry layers,the displacement time histories of overburden layers' bottom caused by all sub-sources can be synthesized by analytic solution.The results can be used as the input for the finite element numerical simulation.Secondly,in asymmetry medium,with local artificial transmitting boundary condition,the explicit decoupled finite element simulation approach is applied to model the near-field strong ground motions.Comparing with former source studies,two ideas are added into the source model.One is that,instead of simple couples source,the earthquake moment tensor is introduced to describe fault attitude and the rupture mechanism,which could express the relationships among force,slip direction and slip-time function of every sub-sources in detail.The dislocation could by equivalent to three-order earthquake moment tensor with 9 force couples and every couple expression includes source parameters,such as fault strike,dip angle and rake angle.The other idea is about the slip-time function.On a finite fault,the inhomogeneous characteristics can be expressed by the nonuniform dislocation distributions and the nonuniform rupture expanding in temporal and spatial space.In this paper,the Brune model is used to characterize the slip-time function.For a certain sub-source,three parameters can define the function,average dislocation,rising time and the rupture time delay.It is feasible of the simplified method of strong ground motion simulation by an example test.According to the source parameters and velocity structures of the 1994 Northridge M_W 6.7 earthquake,long period ground motions of four rock stations are simulated and compared with their records.The simulation acceleration histories and velocity histories are coincident well with the records in durations,amplitudes and envelops.The large acceleration pulse and velocity pulse in the simulation are clear.The simulation results also express engineering characters of hanging wall effect and rupture directivity effect distinctly.All above are well in accord with the ground motion records and earthquake investigation which prove that the approach is reasonable and applicable for seismic hazard evaluation and prediction.