Strike-slip fault are the active faults that are most closely related to large earthquakes. The study on how a large earthquake develops and occurs on strike-slip faults is an issue much concerned with the seismologists. As it is shown by structural geology studies, strike-slip faults are a complex tectonic system, which represents combination of various types of deformation under the shearing forces. Based on the research cases of various strike-slip fault zones both at home and abroad, this paper investigates and summarizes the geometry, kinematics and evolution processes of continuous or discontinuous strike-slip faults and analyzes the hinge role of the strike-slip faults. It is found that the hinge axis area is subject to intense compression, and the area is locked, where stress is concentrated, strain is localized, and earthquakes nucleate and develop. When the locked hinge axis is broken through, unstable sliding will occur along the strike-slip fault, producing sudden big displacement, accompanied with large earthquake. In the stepover zones of discontinuous strike-slip faults, earthquakes of corresponding size and type will develop and occur according to the relevant stress fields and rupture mechanics.

This paper studies the temporal and spatial distribution of global great earthquakes(M_W≥8.0)since 1900.We compare two periods of upsurges of great earthquakes occurring in the middle of last century and beginning of this century. The former took place between 1950 and 1965 when 13 great earthquakes(M_W≥8.0)occurred,including 3 events with moment magnitude greater than 9.0.The maximum magnitude in this period reached 9.6.The later period starts since the beginning of this century. In less than 12 years,15 great earthquakes have attacked the world with the largest magnitude of M_W 9.1.On the basis of comparison of these two upsurges of global earthquake activities,we postulate that the ongoing high level of earthquake activity may continue for another 5 years or so. Numerous great earthquakes(M_W≥8.0)and many large earthquakes(M_W 6～7)will occur globally in this 5 years. In addition,this paper also discusses the relationships between earthquake activities along the Sumatra segment of the Indian-Australia plate boundary and that in the Bayankala block in the middle of the Tibetan plateau as well as the plateau itself. The results indicate that Tibetan plateau,especially its middle and southern parts are the likely places for future earthquakes of magnitude over 7.

The great Haiyuan earthquake occurred at 20: 06: 09 on December 16,1920 in the southern Ningxia Autonomous Region.The magnitude of this earthquake is 8.5 which is listed as one of the three greatest earthquakes occurring in continental China.This devastating earthquake killed about 230,000 people according to previous reports.Recent studies show that total casualty might have reached 270,000. Study on this earthquake using modern scientific and technological methods is the first in the history of earthquake research in China that sets up the monument in China's earthquake science.Significant breakthrough took place in the 80s of last century.The earthquake surface rupture,with 200km in length and prominent left-lateral strike-slip displacement,was discovered.The first monograph on the Haiyuan earthquake was published.The innovative technology of large-scale geological mapping of active faults was developed during studies on the Haiyuan earthquake surface ruptures with the publication of the first large-scale map of the Haiyaun active fault.Quantitative studies were carried out on fine structures and geometry of the fault zone,Holocene slip rate,co-seismic displacement,paleoearthquake and recurrence interval of great earthquake,and future earthquake risk assessment.The innovative studies also include rupture propagation along strike-slip fault,evolution of pull-apart basin,determination of total offset across the strike-slip fault,transition equilibrium between strike-slip displacement along its major strand and crustal shortening at its end of the strike-slip fault,and the mechanism of deformation on the Liupanshan Mountains.On the occasion of the 90th anniversary of the tragic Haiyuan earthquake,the careful retrospect in scientific progresses achieved during past 20 years would lead to further directions in the studies of active faults and earthquake hazard reduction,so that we can console those killed by the Haiyuan earthquake and put forward our efforts toward earthquake prediction and seismic hazard reduction.

The M_S 8.0 Wenchuan earthquake caused tremendous disaster to the country and people,and meanwhile it also gives us many lessons worth thinking about.The severe earthquake disaster tells us the necessity of evaluating and surveying the seismic and geological environment in urban and township construction so as to avoid the unfavorable terrain conditions and keep away from the instable sites prone to landslide,soil liquefaction and mud-rock flow and etc,complying stringently the standards for disaster fortification regulated by the state and the relevant technical code for seismic design of buildings,including the houses in rural area,and intensifying the study on active tectonics to avoid the active faults and possible seismogenic tectonics.Since the historical earthquake record is too short and the recurrence interval of earthquakes is long within the continent,we should be cautious when using the principle of recurrence of historical earthquake in seismic hazard assessment,but promote the study on paleoearthquakes on active tectonics,so as to fully understand the history of earthquake recurrence.Big earthquake may also occur on low-slip-rate active fault,but with longer recurrence interval.So special attention be paid to the elapsed time of the latest event on it.The source rupture of the M_S 8.0 Wenchuan earthquake is characterized by a multi-point rupturing process,resulting from the expansion of deep fault from the detachment zone to the front-end ramp,and shown as double-ramp faulting on the surface.In the source rupture propagating process,the nature of the rupture changed.The southwest section is characterized with thrust faulting,and the northeast section turns to be dominated by strike-slipping.The initial thrust-type rupture occurring in the middle of the Longmenshan tectonic belt ran through its northeast section and became a new fault.The 1997 Mani,the 2001 west pass of Kunlun Mountains and the 2008 Wenchuan earthquakes with MS 7～8 are all the products of the movement of the Bayahar block.After the Wenchuan earthquake,a series of earthquakes with magnitude around 6 occurred in the Qinghai-Tibetan Fault block region,which characterizes the new episode of seismic activity of the Qinghai-Tibet Plateau block region.

We present the results of selected balanced cross-sections across four active reverse fault-fold belts at the front margins of the Chinese Tianshan,an intracontinental mountain belt formed in response to the India-Eurasia continental collision. It consists of Paleozoic,Mesozoic and Cenozoic strata in active fold-and-thrust belts at two sides of Tianshan, and the stratum in active fold belt changes younger from west fold-and-thrust belt to east. The middle part's structure is complicated in Kalpin active fold-and-fault belt,the width gradually diminishes towards east. At the west of Akesu city,the Kalpin active fold-and-thrust belt vanishes, but at the east part of Akesu city,the Kuche active fold-and-thrust belt emerges on the Tarim desert. In the middle section of the Kuche active fold-and-thrust belt near Baicheng County,the structure is most complicated,and the width is also maximal. The width of Kuche fold-and-thrust belt gradually decreases towards east,and disappears near 85° longitude line. But at the northern piedmont of Tianshan,the Manas active fold-and-thrust belt presents itself approximately at the same longitude. The longitude of the west end of Turpan centre uplift active fold-and-thrust zone is approximately the longitude of the east end of the Manas active fold-and-thrust belt. The four active fold-and-thrust belts which we studied are located at the north and south sides of Tianshan Mountains. Two balanced cross-sections traverse the Kalpin active fold-and-fault belt and one crosses the Kuche fold-and-fault belt. Two balanced cross-sections cross the Manas active fold-and-thrust belt,and one balanced cross-section crosses the Turfan centre uplift zone. The crustal shortening of Kalpin fold-and-thrust belt is 40～45km,that of Kuche fold-and-thrust belt about 27～37km,and of Manas,Turpan centre uplift active fold-and-thrust belt are 8.5～10.5km and 6～7km,respectively. The four active fold-and-thrust belts at south and north Tianshan are not superposed in longitude. So their crustal shortenings approximately represent the minimal crustal shortening of Tianshan at different segments,and also reflect the reducing trend of crustal shortening from west to east. It is very difficult to calculate the crustal shortening across entire Tianshan,owing to deficiency of data about active reverse fault and strike-slip fault within Tianshan Mountains. If the time of initial deformation is the starting time of Xiyu conglomerate deposition(since 2.5Ma),and considering the shortening component on south-north direction of Bo-A NW strike-slip fault,the minimum crustal shortening rates at four segments of Tianshan would be 15.4～17.3mm/a,12.7～16.5mm/a,3.8～4.5mm/a and 2.3～2.7mm/a,respectively.

This paper reviews the history and progresses of the research on active tectonics in China and overseas. By giving a brief introduction on histories of active tectonic researches in China and some other countries,the paper sums up the process and development on the quantitative investigation of active tectonics since the 1980's,and puts the focus on the main efforts and progresses that have been made in China on some aspects of the research,such as the basic survey and research and the applied investigation of active tectonics,the study on theories about regional active tectonics and their kinematics and geodynamics,the survey on the coupling relations between deep and shallow structures,the project on active fault survey and prospecting and seismic hazard assessment in urban areas,as well as the efforts on using the Quaternary geochronology. Further,the paper looks back to the Chinese efforts on the quantitative investigation of active tectonics,sums up those cognitions from studies on the determination of several basic and measurable parameters of active tectonics,such as the length of fault and fault-segment,coseismic slip and cumulative slip,fault slip rate,sequence of paleo-earthquake events,and the time elapsed from the latest event. At the same time,efforts and progresses in China on assessing long-term seismic potential for active fault and evaluating potential risk from potential active fault movement have been reviewed by summarizing researches on developing theories,models,methods and on application to the probabilistic assessment of time-dependent seismic potential,magnitude estimation for potential earthquake on active fault,and forecasting the potential risk caused by potential active fault movement. Finally,considering the realities and problems in the research of active tectonics in China,the authors put forward several suggestions for issues worthy of paying more attention to for further investigation in the future.

According to the practice of urban active fault exploration and associated activity assessment conducted in recent years,this paper summarizes the problems encountered in geological,geomorphological,geochemical and geophysical explorations,and proposes the following means and suggestions to solve these problems.To determine the most recent faults or fault zones,emphases should be placed on iden～tifing the youngest active faults and offset geomorphology.To understand the history of faulting and to discover the last offset event,it is suggested to combine geophysical prospecting,drilling and trenching into one profile.Because of significant uncertainties in late Quaternary dating,it is advised to sample systematically and to use multiple dating methods.Shallow seismic reflection has been proven to be the most useful method in urban active fault exploration.However,it is a pressing need to increase the quality of data acquiring and data processing to obtain high resolution images,so that the abi～lity of identifying active faults could be enhanced.Combination of seismic P-wave reflection and S-wave reflection methods is proved to be a powerful means to investigate tectonic environments of deep crust.

This paper deals with the degradation of fault scarp and the identification of paleo-earthquake in the light of theoretical and mathematical analysis of scarp development processes. We propose some models for fault scarp degradation in unstable and diffusion stages, and give out their mathematical solutions. From these solutions, the quantitative relation between geometrical features and/or sedimentary structures of fault scarp is analyzed. Furthermore, the precise expressions of the isochronous form-line of the scarp profile, the maximum gradient of scarp, and the sectional area of scarp deposits are established quantitatively for the use of paleo-earthquake research. Based on the field investigation, as well as experimental and mathematical analysis, the comprehensive indicators for identifying paleo-earthquakes associated with fault scarp, including geometrical, sedimentary, structural and experimental features, are summarized. Case studies on the fault scarp of the eastern Helanshan Fault at Hongguozi and fault scarp of the F 201 Fault at Mengjiawan were carried out. The results of these two studies reveal 2 series of 4 paleo-earthquake events that had occurred on the two faults respectively. The characteristics and recurrence intervals of these events have been studied in details. It is determined that the recurrence interval of paleo-earthquke events on the eastern Helashan Fault is 2.2～2.6ka, and that on the F 201 Fault is 3.3～4.0ka.

Displacements along active faults buried directly beneath major cities create devastating earthquakes that seriously threaten the safety of human lives and properties. Exploration and seismic hazard assessment of active faults in urban areas are thus an important systematic engineering for disaster mitigation in major cities. It is also a new field for active tectonic studies. The kernel of this work includes determining of the exact location of active faults, dating the ages of last tectonic activity, relating the shallow level faults to structures in the crustal depth, assessing seismic hazard and potential for surface offsets, and formulating countermeasures for disaster mitigations. We use the following simple phrases to express these key scientific problems: Where are the faults? Are they active? How deep are they? Will they create earthquake? Will they form surface offsets? What are the countermeasures? This paper explains these key scientific problems in detail.

Jianghan-Dongting Lake basin is a large-scale Meso-Cenozoic basin in South China. Its geological structure, deep-seated structure, tectonic evolution, neotectonic activity and seismicity are analysed basing on the data of geology, geophysis and earthquakes in the basin region.The basin consists of Jianghan and Dongting Lake depressions and Huarong uplift. WNW-EW-trending secondary segs and positive land forms controlled by the faults developed in Jianghan depression, but NEN-NE striking secondary positive forms and segs controlled by the faults developed in Dongting Lake depression. The basin has undergone a complex process o'f tectonic evolution. There is intensive neotectonic activity in the basin region. Mid-intensity earthquakes are closely related to the structures with differential activities in Quaternary.

En echelon shear rupture zone is of a common type of ruptures in the crust. Several models which contain one,two and three fanlts arranged in en echelon arrays are calculated by twe-dimensional finite element method. The result indicates that a shear rupture zone acts just as a single shear fault in kinematics. There exist tensile and compressive areas at both ends of a shear rupture zone. They are the products of the relative movement of two blocks along the shear rupture zone. The structure of rock bridge (pullapart basin or push-upward) and the meterial mechanical coefficient are two main factors affecting the concentration of the tensile and compressive stresses at the ends of a shear rupture zone. Moreover,the structural features and faulting types in tensile and compressible areas are discussed in the paper.

A great earthquake(M=8)occurred at southwest Danxung,Tibet Autonomous Region,on September 29,1411. A surface ruprure zone as long as 136 km was formed during the event. It consists of two subparallel secondary fractures which are different in their strikes and modes of motion. The maximum vertical displacement is 8-9m.,while the maximum horizontal one 11-13m. The surface rupture is a discontinues fracture characterized by tensile shearing. The geometry and characteristics of the displacement of the rupture zone are summarized,and the seismogenic structure of the 1411 earthquake is discussed in this paper as well.

The piedmont fault of Mt. Huoshan is the main boundary fault in the northern part of Linfen basin. On the basis of geological mapping (scale: 1:10000) along the fault,the authors have studied in detail the geological history,segmentations and structural features,the right-lateral dislocations and slip-rates of the fault since the later quaternary as well as their relation to the 1303 Hongdong earthquake (M_s=8). Finally,the first order right-lateral movement on both sides of Shanxi graben system has been discussed according to the structural peculiarities of NNE-trending major faults.

A great earthquake with a magnitude of M_s =8.0 occured in the vicinity of Bengco, northern Tibet, on November 18, 1951. Surface rupture zone as long as 91 km was formed by the event. The geomtry and displacement features of this surface rupture zone as well as the seismogenic structural conditions of the Bengco earthquake are discussed in this paper.

Haiyuan active fault zone is a main active fault in the northeast boundary of Qing-hai-Xizang plateau. Geologically, mapping of 1:50,000 has been made along the fault zone. The results show that Haiyuan active fault zone, 237km long, striking WNW in its west segment and striking NW in the east segment, has been a strike-slip fault sense since middle-late early Pleistoncene. The horizontal offset is 12-14.5km from middle-late early Pleistocene, with the slip rate of 11.7-19.2mm/a. The strong activity has been going on since the begining of the Holocene, with the sinistral strike slip rate of 6-10mm/a.Haiyuan strike slip fault zone consists of 11 secondary shear faults, mostly left stepping pinnate, partly right stepping pinnate. 8 pull-apart basins and 2 pushed-up blocks were formed along the fault zone. Pull-apart basins showing 2 types of romb and elongated, big ones began to develop in middle Pleistocene, small ones in late Pleistocene. The largest thickness of sediments is greater than 750m. A tensile-shear fault with the sinistral strike-slip was formed in some pull-apart basins.The tensile-shear fault links up two secondary shear faults which control pull-apart basin, intersecting them with low angle. After the formation of this kind fault, pull-apart process of the basin decreased and pull-apart basin gradually became feeble and die. The thrust faults intersecting the secondary shear faults with high angle was formed in the pushing area, appeared as an uplift in topography.A great earthquake of magnitude 8.6 occurred in Haiyuan, in December 16, 1920. The surface ruptures of earthquake can be divided as 15 fracture segments, which developed along secondary shear faults, tensile shear faults on pull-apart basin and boundary normal fault at both ends of pull-apart basin. The horizonal dislocation of 268 values have been measured. The maximum displacement of left-lateral strike-slip is 10-11m.At the southeast end of Haiyuan strike slip fault, a rare end compressional area, which consists of thrust fault zone of east pediment of Liupan Mountain, Madong Mountain fold zone and Xiaoguan Mountain thrust fault zone, striking nearly NS, was formed. The detail mapping and studying have been made on the deformation features of these compressional structure belts and calculation on the amount of crust shortening have been made, and the value is about 12.4-16.7km, which is about equal to the sinistral offset amount of Haiyuan active fault zone.