The current and conventional fault-crossing short baseline measurement has a relatively high precision, but its measurement arrays usually fail to or cannot completely span major active fault zones due to the short length of the baselines, which are only tens to 100 meters. GNSS measurement has relatively low resolution on near-fault deformation and hence is not suitable for monitoring those faults with low motion and deformation rates, due to sparse stations and relatively low accuracy of the GNSS observation. We recently built up two experimental sites on the eastern boundary of the active Sichuan-Yunnan block, one crossing the Daqing section of the Zemuhe Fault and the other crossing the Longshu section of the Zhaotong Fault, aiming to test the measurement of near-fault motion and deformation by using fault-crossing arrays of one-kilometer-long baselines. In this paper, from a three-year-long data set we firstly introduce the selection of the sites and the methods of the measurement. We then calculate and analyze the near-field displacement and strain of the two sites by using three hypothetical models, the rigid body, elastic and composed models, proposed by previous researchers. In the rigid body model, we assume that an observed fault is located between two rigid blocks and the observed variances in baseline lengths result from the relative motion of the blocks. In the elastic model, we assume that a fault deforms uniformly within the fault zone over which a baseline array spans, and in the array baselines in different directions may play roles as strainmeters whose observations allow us to calculate three components of near-fault horizontal strain. In the composed model, we assume that both displacement and strain are accumulated within the fault zone that a baseline array spans, and both contribute to the observed variances in baseline lengths. Our results show that, from the rigid body model, variations in horizontal fault-parallel displacement component of the Zemuhe Fault at the Daqing site fluctuate within 3mm without obvious tendencies. The displacement variation in the fault-normal component keeps dropping in 2015 and 2016 with a cumulative decrease of 6mm, reflecting transverse horizontal compression, and it turns to rise slightly(suggesting extension)in 2017. From the elastic model, the variation in horizontal fault-normal strain component of the fault at Daqing shows mainly compression, with an annual variation close to 10^-5, and variations in the other two strain components are at the order of 10^-6. For the Longshu Fault, the rigid-body displacement of the fault varies totally within a few millimeters, but shows a dextral strike-slip tendency that is consistent with the fault motion known from geological investigation, and the observed dextral-slip rate is about 0.7mm/a on average. The fault-parallel strain component of the Longshu Fault is compressional within 2×10^-6, and the fault-normal strain component is mainly extensional. Restricted by the assumption of rigid-body model, we have to ignore homolateral deformation on either side of an observed fault and attribute such deformation to the fault displacement, resulting in an upper limit estimate of the fault displacement. The elastic model emphasizes more the deformation on an observed fault zone and may give us information about localizations of near-fault strain. The results of the two sites from the composed model suggest that it needs caution when using this model due to that big uncertainty would be introduced in solving relevant equations. Level surveying has also been carried out at the meantime at the two sites. The leveling series of the Daqing site fluctuates within 4mm and shows no tendency, meaning little vertical component of fault motion has been observed at this site; while, from the rigid-body model, the fault-normal motion shows transverse-horizontal compression of up to 6mm, indicating that the motion of the Zemuhe Fault at Daqing is dominantly horizontal. The leveling series of the Longshu site shows a variation with amplitude comparable with that observed from the baseline series here, suggesting a minor component of thrust faulting; while the baseline series of the same site do not present tendencies of fault-normal displacement. Since the steep-dip faults at the two sites are dominantly strike-slip in geological time scale, we ignore probable vertical movement temporarily. In addition, lengths of homolateral baselines on either side of the faults change somewhat over time, and this makes us consider the existence of minor faults on either side of the main faults. These probable minor faults may not reach to the surface and have not been identified through geological mapping; they might result in the observed variances in lengths of homolateral baselines, fortunately such variations are small relative to those in fault-crossing baselines. In summary, the fault-crossing measurement using arrays with one-kilometer-long baselines provides us information about near-fault movement and strain, and has a slightly higher resolution relative to current GNSS observation at similar time and space scales, and therefore this geodetic technology will be used until GNSS networks with dense near-fault stations are available in the future.

Study on structure of earthquake source regions is one aspect of earthquake geology, which has relatively small spatial-scale but needs to know more details about seismogenic structures of specific historical events and potential events. Most of shallow-focus and tectonio-genic earthquakes on continents usually nucleate at 10 to 30km below the earth's surface. Such a depth is not able to be reached from studies only using data and methods of geology, geomorphology, or only from analyses of remote sense imagery. It needs studies of coupling deep and shallow structures by combining data from geological investigations and seismological survey, as well as from inspections of multi-geophysical techniques. The study region of this article is the middle to western portion of the Hetao graben system in Inner Mongolia, China, which is an active boundary belt separating the Ordos and Yanshan blocks, and a large-scale seismogenic tectonic zone dominantly under tensional-stress in the northwestern North China. Along the system, at least two great earthquakes occurred in the historical time, and reportedly some lines of evidence of paleo-earthquakes have been found. Since the 20th century 4 strong events with magnitudes 6.0 to 6.4 have taken place in the Hetao graben system. The most recent two of the 4 are the M_S 6.0 Wuyuan earthquake on Aug. 25, 1979 and the M_S 6.4 Baotou earthquake on May 3, 1996, respectively. For seismogenic structures of the two, although relevant studies were made mainly based on analyzing the relation between aftershock's and seismic intensity's distributions and surface faults or "faults" inferred just from remote sensing images, queries still remain in the corresponding conclusions because information used in these studies are limited to the surface and far from the event's nucleation depths. Based on the previous studies, this study collects and combines more information available of active tectonics, petroleum seismic survey and relocated earthquake distribution, as well as seismic intensities and focal mechanism solutions of the mainshocks, further constructs and analyzes two seismo-tectonic profiles across the individual source regions of the two events, and then re-determines seismogenic faults of the two events. The author concludes that the M_S 6.0 Wuyuan earthquake of 1979 occurred as a result of normal faulting along the main one of the Sertengshan piedmont fault zone, which trends in near west-east and dips southward, and the M_S 6.4 Baotou earthquake of 1996 was produced by oblique-slip normal faulting along an unnamed blind fault that hides beneath the Wulashan horst, strikes west-northwest and dips south-southwest. The new conclusion is able to march and explain, to the maximum extent, the relevant information and phenomena in the individual source regions, including surface and subsurface active tectonics, aftershock, seismic intensity distributions and focal mechanism solutions of the mainshocks, and coseismic macro ground damages (as cracks yielded in bedrocks of the Wulashan horst during the 1996 mainshock). The only one phenomenon that cannot be completely explained is that about 2/5 of the area of the meizoseismal zone(with intensity Ⅷ) of the 1996 Baotou earthquake lies north(the foot wall)of the seismogenic fault determined in this study. In addition, case study of the seismogenic structure of the Baotou earthquake suggests that secondary active normal faults or oblique-slip normal faults may exist beneath horsts within large-scale active grabens, and they would have ability to produce strong earthquakes.

An earthquake rupture zone is defined as the vertical projection area on the ground surface of the plane(s) on which coseismic slip(s)/rupture(s) occurred along a seismogenic fault zone, and indicates both the position and the extension of a seismic source fault zone/rupture zone. Determination of rupture zones of strong and major earthquakes occurring in the long-past history is an important foundation for identifying seismic gaps, studying and forecasting seismic potentials. Several modern techniques and methods are available to determine rupture zones of modern strong and major earthquakes. However, the relevant methods need to be developed if we hope to determine the rupture zones of the historical events. Taking North China(108°～124°E, 32°～42°N)as a research region, this paper tries to establish the empirical criterion that can be used to determine both positions and extensions of the rupture zones of strong and major earthquakes using data of modern seismic intensities/coseismals and seismotectonics and surficial geologic environments of seismogenic regions, and then makes case studies applying the method developed in this paper. To establish the regional empirical criterion, we systematically collect and analyze the relational material and the research results of the modern earthquakes whose maximum intensities are equal or greater than Ⅶ occurring in North China since 1966, including the surficial geologic-environments, the seismogenic faults and their movement styles, intensity distributions, the inversion of seismic fracture process, the aftershock distributions and the coseismic deformation. Based on those data, we determine the rupture positions and extensions among the seismic intensity spans of eight modern strong and major earthquakes occurring in North China. To compare with the other regional empirical criterions and making up for the inadequacy of the data in North China, we also determine the rupture zones and their extensions among the seismic intensity spans of the 2008 Wenchuan earthquake, Sichuan and the 2010 Yushu earthquake, Qinghai. Our researches show that a close correlation exists among the seismic intensity spans in which the rupture zones extend, with respect to the highest seismic intensities and the surficial geologic-environments of seismogenic regions. Based on the correlations, we establish two empirical criterions that can be used to determine both positions and extensions of rupture zones of historical strong and major earthquakes in two kinds of surficial geologic-environments, regions of basement rocks and regions covered by thick loose sediments of the Quaternary, respectively, in North China, from seismic intensity distributions. In this paper, we also put forward relevant thinking and methods for determining the widths of rupture zones of historical events by combining information from seismotectonics and distributions of modern small earthquakes/aftershocks. As an application test, we use the method presented in this study to determine rupture zones of five historical events, and the results prove that the empirical criterions and the corresponding methods developed in this study are effective to determine rupture zones of historical strong and major earthquakes in North China.

The April 20,2013,M_S 7.0 Lushan earthquake occurred along the southwestern part of the Longmen Shan Fault zone. Tectonics around the epicenter area is complicated and several NE-trending faults are developed. Focal mechanisms of the main shock and inversions from finite fault model suggest that the earthquake occurred on a northeast-trending,moderately dipping reverse fault,which is consistent with the strike and slip of the Longmen Shan Fault zone. NE-trending ground fissures and soil liquefaction along the fissures,heavy landslides along the Dachuan-Shuangshi and Xinkaidian Faults were observed during the field investigations. No surface ruptures were found in the field work. GPS data indicate that the fault on which this earthquake occurred is a fault east of or near the Lushan county and the earthquake also triggered slip on the fault west of the Lushan county. Field observations,GPS data,focal fault plane,focal depth,and distribution of the aftershocks suggest, that the seismogenic structure associated with the M_S 7.0 Lushan earthquake is the décollement beneath the folds of the eastern Longmen Shan. Slip along this decollement generated the earthquake,and also triggered the slip along the Dachuan-Shuangshi and Xinkaidian Faults.

Based on seismic data of the regional network of the last 34 years,we have analyzed current faulting behaviors of major fault zones in Mabian area,southern Sichuan,and preliminarily identified the risky fault-segments on which potential strong and large earthquakes may occur in future,with the method combining the spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1)The spatial distribution of b values displays significant heterogeneity in the study area,which reflects the spatial difference of cumulative stress level along various fault zones and segments in the area;(2)Three anomalously low b-value areas with different sizes exist on Mabian-Yanjin Fault zone,these anomalies can be identified as asperities under relatively high cumulated stress levels,in which,two asperities,located at north of Mabian county and Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone,respectively,may be the potential seismogenic sources of large earthquakes in Mabian area in the near future,and the third asperity with a small size located at southern Suijiang may be the potential strong-earthquake source;(3)An asperity at south-western segment of Longquanshan Fault may be the site of potential moderate to strong earthquakes;and(4)the asperity on the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu Fault has potential for moderate to strong earthquakes.

The paper carries out geometric segmentation of the eastern segment of East Kunlun Active Fault(Maqin-Maqu segment)based on the interpretation of ETM and QuickBird satellite images and using segmental signs of strike-slip fault,such as geometric and tectonic discontinuities in combination of geomorphic features and fault zone materials. In this study,we divide the Kunlun Active Fault(Maqin-Maqu segment)into eight segments,geometrically,namely,Dongqinggou,Dawutan,Kendingna,Xikehe,Tangdi,Maqu,Moxi and Luocha. Faults in the former seven segments are developed en echelon,of which,that in the Tangdi and Maqu segments are of right-stepping en echelon,and the rest are of left-stepping. The space between steps is small and well-connected. Beside the stepover,the segmentation indicators also include the bend of fault strike and the intersection of fault with others. Luocha segment is divided mainly by the strike bend. The east Kunlun Fault intersects with the Awancang Fault and Xikehe-Maqu Fault at the south side of Mohatang,which is taken as an indicator of segmentation of the Xikehe segment. The extensive landform and evidence of paleo-seismic rupture indicate that the Maqin-Maqu segment has experienced numerous movements. The fault splays out from northwest to southeast,which has good correlation with the slip rate gradients along the Eastern Kunlun Fault.

To investigate the mechanism of seismicity due to impoundment of a reservoir,we propose a method for integrated study on regional/local seismicity and stress by both surface loading and pore pressure diffusion.As an example,possible role of Zipingpu Reservoir on nearby seismicity was studied in detail.The epicenter of the M_S 8.0 Wenchuan earthquake on May 12 of 2008 is very close to the Zipingpu Reservoir.On one hand,several statistical properties including event rate(n),cumulative energy release(E),seismic b-value in the magnitude-frequency relation,and spatial correlation length(SCL)of earthquakes occurring in Zipingpu area from 2004.8 to 2008.5.11 were estimated in detail.On the other hand,we quantitatively examined change of Coulomb Stress(ΔCFS)due to the impoundment of the reservoir.Both weight loading and pore pressure diffusion resulted in significant ΔCFS on the underlying Yingshu-Beichuan and Guanxian-Mianzhu Faults,which are considered as the source faults of the Wenchuan earthquake.Some clear correlations were verified between the local seismicity and stress change,thus we concluded that the impoundment of Zipingpu Reservoir clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake.

A 240km-long co-seismic surface rupture was produced along the Beichuan-Yingxiu Fault during the 2008 Wenchuan earthquake.We made a detailed survey at representative sites along the surface rupture and analyzed the data based on the geometry between the benchmark and deformation.The co-seismic vertical slip,horizontal dip-slip shortening,strike slip and moving direction of the hanging-wall were calculated based on the survey data of these sites.Results show that the spatial distribution of the co-seismic deformation of the fault varies a lot along the fault.The maximal horizontal slip,as we got till now,is located in the Shenxigou site of Hongkou with a value of 4.98m,and the maximal strike slip is also located in the same site with a value of 4.5m.The maximal vertical displacement is located to the northeast of Shenxigou with a value of 5.7～6.7m.The average horizontal slip for the NE trending fault is 1～2m,and the average vertical slip is 3m.But horizontal and vertical slip for the NW-trending branch from Xiaoyudong to Caoba is only 0.5～1.5m.The data from Leigu town show that the gravity deformation resulting from the fault-related landslide was perhaps superimposed on the tectonic one.The dip angles of the fault at the surveyed sites calculated from the horizontal shortening and vertical displacement indicate that the Beichuan-Yingxiu Fault is a steep dipping reverse fault with some strike-slip.From the comparison between field results and geophysical inversion,we believe that the spatial distribution of co-seismic fault-slip is related to the barriers and rupture process along the fault plane.

The Wenchuan earthquake(M_S=8.0)of May 12,2008 in Sichuan Province,China produced surface ruptures along the seismogenic reverse faults.It is very important to analyze the co-seismic surface ruptures quantitatively for the comprehension of tectonic behavior of the great earthquake.We discuss several issues about the quantitative analysis of co-seismic surface ruptures produced by reverse faults with some examples from Wenchuan earthquake.With the geometric relations between the parameters of deformation and survey data from planar and linear geomorphologic benchmark,solution methods for those parameters are provided.The vertical slip,horizontal dip-slip and dip angles on survey profiles were measured to calculate the actual deformation of the fault.The relationship between the apparent deformation and the real value was provided quantitatively for oblique profiles and linear benchmarks.We also made some discussions on the application of the survey data to the analysis of regional structural geometry and kinematics.

Field investigations show that the M_S8.0 Wenchuan earthquake of 12^th May 2008 ruptured two NW-dipping imbricate reverse faults along the Longmenshan Fault zone at the eastern margin of the Tibetan Plateau.This earthquake generated a 240km long surface rupture along the Beichuan-Yingxiu Fault characterized by right-lateral oblique faulting and a 90km long surface rupture along the Guanxian-Jiangyou Fault characterized by dip-slip reverse faulting.Maximum vertical and horizontal dispacements of 6.2m and 4.9m,respectively,were observed along the Beichuan-Yingxiu Fault,whereas a maximum vertical displacement of 3.5m occurred along the Guanxian-jiangyou Fault.This co-seismic surface rupture pattern,involving multiple structures,is among the most complicated of recent great earthquakes.Its surface rupture length is the longest among the co-seismic surface rupture zones for reverse faulting events ever reported.Aftershocks recorded by local network clearly outline the hanging wall of the Beichuan-Yingxiu Fault and indicate that the fault dips about 47? to the west.Industry seismic lines,in addition to surface ruptures and aftershocks,allow us to build a 3D model for the rupture geometry that shows crustal shortening is the dominant process along the Longmen Shan to accommodate long-term deformation.Oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of Tibet Plateau accommodates,in part,the continuing penetration of the Indian plate into the Eurasian plate,and this extrusion is transformed at the eastern margin of the Tibetan Plateau into crustal thickening and shortening along the Longmenshan Fault zone that is responsible for the growth of high topography in the region.

It is a fundamental study for the research of evolution of Qinghai-Tibetan Plateau to resolve the kinematics of active boundary belt around the plateau quantitatively. With the measuring technology of photogrammetry and chronological method of analysis among regional climate,geomorphology and tectonics,we get a set of quantitative kinematical data for the main faults along the northern-eastern active boundary belt of Sichuan-Yunnan block. There were centre-symmetrical dip-slips locally in many fault segments while they were mainly strike-slip. With vector computation within a regional system including the north-east active boundary belt of Sichuan-Yunnan block and related blocks,we analyzed the longitudinal kinematical transformation and transversal slip partitioning quantitatively based on the parameters of main faults kinematics. From these data,there were a vertical uplift rate of 6.2mm/a in the Gonggashan area in distributed deformation form,and a mainly dip-slip fault at least with a rate of 1.45mm/a along the inner side of eastern terrace of Anninghe valley. We set up quantitative slip partitioning models for the northeast corner and eastern belt of Sichuan-Yunnan block. A quantitative kinematical model is set up for the regional system which consists of the northern-eastern active boundary belt of Sichuan-Yunnan block and related blocks.

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.

This paper briefly reports our newly findings on remains of earthquake surface ruptures along and near the transition segment(between Lizhou and Xichang)of the Anninghe and Zemuhe Fault zones in western Sichuan and analyzes their relations to historically documented large earthquakes.During geological surveys in recent years we find unknown-aged surface ruptures at three sites on and near the transition segment of the Anninghe and Zemuhe Fault zones.Through analyzing both characters of the surface ruptures and their locations in heavily damaged areas of relevant historical earthquakes in the studied region,we distinguish that ruptures at two of the three sites,north of Yangfushan and west of Dapingzi,should be remains of the surface rupture zone of the large 1536 earthquake.The findings of the remains at these two sites suggest not only that the southern portion of the 1536 rupture zone was yielded along the transition segment of the Anninghe and Zemuhe Fault zones,but also that the southernmost end of the 1536 rupture zone would have arrived at or been very close to Xichang.While,ruptures at and near the other site,east of Lijinbao,north of and not far from Xichang,should be remnants of the surface rupture zone of another historical large earthquake occurring in 1850.These remnants further prove that the most northwestern end of the 1850 rupture zone would have arrived somewhere at least several kilometers north of Xichang.It thus can be inferred from the evidence given in this paper that the major fault at and near Xichang was once ruptured by both the 1536 and 1850 earthquakes.Such an inference is compatible with the described phenomena in literal historical records that very heavy destruction and disaster happened in the city of Xichang during both the 1536 and 1850 earthquakes.

In order to analyze the feasibility using seismicity parameters to identify current faulting behavior and hence to assess long-term seismic potential of active fault zones,we introduced the technique of spatially scanning and mapping b values along fault zones,as well as the procedure of data processing,calculating and result analyzing of multiple-seismicity-parameter-value combinations(i.e.b,√E,n and a/b)for various fault-segments in this paper.We then brought forward several reference criterions for fault segmentation,that is,the calculation of multiple seismicity parameters.Moreover,we generalized the approach and thinking of synthetically identifying spatial differences of current faulting behavior from the calculated parameters by combining histories of strong and major earthquakes and spatial distributions of modern seismicity.As an application,we took 5 active fault zones with various current seismicity levels and seismic detectabilities as the experimental targets to analyze synthetically the spatial differences of current faulting behaviors and identify fault segments with potential risk of strong earthquakes for each of the 5 fault zones basing on the generalized approach or methods.Meanwhile,the issues,such as the relation between the postseismic fault healing and relatively low b-value,the proper use of high-accurate re-located seismic data in the parameter calculating,and etc.have also been discussed.The results mainly suggest that the spatial distribution of multiple seismicity parameter values,among which the b-value is the most important one,can be effectively applied to analyzing current faulting behaviors and hence to identifying fault segments with potential risk of strong or major earthquakes.The two methods,the scanning and mapping of b-values along fault zones and the analysis of multi-parameter-value combinations for various fault-segments,can be applied to active fault zones in areas of high or moderate seismic detectability,respectively.And if the two methods are employed to areas with higher seismic detectability,more reliable results will be obtained.Fault-segments with activity of late-stage aftershocks or with postseismic fault healing may have relatively low b or b-values too.We believe that once a catalog of earthquakes accurately relocated is used in calculation of seismicity parameters,it must contain the complete data of earthquakes occurring during the period and in the area studied,rather than only a part of data selected.

In this paper,we make an effort to study the feasibility to assess magnitudes of maximum potential earthquakes in sub-areas of moderately and weakly active faults in eastern China mainland by using parameters of frequency-magnitude relationship,and develop the corresponding methodology.Data suggest that frequency-magnitude relationships of fault sub-areas in eastern China mainland accord with the characteristic earthquake model,meaning that for a fault sub-area,the ratio a/b of the constants,which is also called as the maximum intercept magnitude of the exponential portion(i.e.the G-R relationship)of the frequency-magnitude relationship,is obviously less than that of the characteristic magnitudes M_C.In order to make the ratio a/b be usable in indirectly assessing the magnitudes of maximum potential earthquakes in fault sub-areas,we develop a method to establish long-duration frequency-magnitude relationships for fault sub-areas by combining data of both historical and modern seismicity.In this method,event's numbers from the two sources of data are all normalized to a duration of t=500years.We then calculate a_t/b values of the normalized G-R relationships for 130 fault sub-areas.Our analyses reveal that maximum magnitudes,M_max,of earthquakes occurring and recorded in the studied fault sub-areas are positively correlative with sizes of a_t/b values,and with the increase of a_t/b values the upper-limits of the maximum magnitudes,M_max,show the feature of monotonously rising and relatively smooth variation.Based on such feature we develop three empirical formulae of the relations between the upper-limits M_mu of the maximum magnitudes,and a_t/b values,for the three regions,i.e.North China,Central and East China,and South China and the southeastern coastal area,respectively,and take them as empirical models to estimate magnitudes of maximum potential earthquakes in fault sub-areas.By using the newly developed method and empirical models we estimate magnitudes of maximum potential earthquakes in several fault sub-areas.Our research also suggests that several types of the abnormal seismicity,such as the swarms of moderate and small size earthquakes,aftershocks and triggered earthquake sequences,and artificially induced seismicity,as well as the determination of minimum complete magnitudes,have influences to the calculation results of a_t/b values,and that severely influenced a_t/b values are overestimated and show deflecting to the right on M_max-a_t/b diagrams.The empirical models and method developed in this study can be applied to the assessment of magnitudes of maximum potential earthquakes for sub-areas of moderately and weakly active faults in eastern China mainland.

Estimation of sizes of potential earthquakes is required in long-term seismic hazard assessment.Such estimation is usually related to a single active fault segment with specific scale,and can be carried out by using an empirical relationship between magnitude and rupture-scale of earthquake.This type of empirical relationships published already for China mainland is established on data of surface ruptures along seismogenic active faults in West China.While,the special active tectonic environment in the region of North China makes it impossible to establish the same type of empirical relationships that are suitable for the region by using the surface-rupture data.Therefore,for North China,we should make an effort to develop the empirical relationships between earthquake magnitude and(sub-surface)source rupture-scale.In the latest 40 years,4 major earthquakes of magnitudes ≥7.0,7 strong ones of magnitudes 6.0 to 6.9 and tens of moderate ones(including aftershock events)of magnitudes 5.0 to 5.9,occurred in North China.For a part of these earthquakes,data or distribution patterns of aftershock sequences with good or high precision locations are available,and for many of these earthquakes,studies on source rupture processes and parameters have been carried out.These make it possible to establish preliminarily the empirical relationships between magnitude and rupture-scale for North China.For this purpose,from data of earthquakes that occurred in North China since 1965,we systematically collected and compiled the relevant rupture-scale parameters obtained from methods of analyzing seismic wave spectrum,coseismic crustal deformation and aftershock distribution,or available from published researches,including rupture length L,downdip rupture width W,and rupture area A(A=L×W).We also re-determined the rupture-scale parameters for part of these earthquakes basing on aftershock distributions.In order to deal with uncertainties in the source rupture-scale parameters due to the use of various methods,and to take the main factors influencing the uncertainties into account,we broungt forward 8 criterions for determining the reliable parameters through further analyses and identification.With synthetic analyses basing on these criterions,we obtained reliable rupture-lengths of 34 earthquakes and reliable rupture-areas of 20 of the 34 earhtquakes.Focal mechanism data suggest that the absolute majority of the 34 ruptures are of strike-slip type.By using the least-square method,we further established two regression relationships between surface-wave magnitude M_S and rupture-length L,and between magnitude M_S and rupture-area A for seismogenic active faults.They are M_S=3.821+1.860lg(L)and M_S= 4.134+0.954lg(A),respectively.A comparative analysis with previous empirical relationships of the same type suggests that the two new empirical relationships developed in this research are suitable very well for estimation of sizes of potential earthquakes on seismogenic active strike-slip faults in North China and in the urban area of Beijing and its surroundings.

There exist several groups of seismogenic active faults at the conjoined areas of the Chuandian,Bayankala and Huanan active blocks along the eastern margin of the Qinghai-Tibetan Plateau. Owing to existence of transverse secondary active faults,the Chuandian Block can be further divided into the Middle Yunnan and Northwestern Sichuan sub-blocks,and the Longmenshan sub-block at the easternmost end of the Bayankala Block. Joint exploration of the crustal structure shows that low-velocity layers exist in the crust of the Chuandian and Bayankala Blocks. These low velocity layers correspond also to high conducting layers and they are the cause of frequent earthquake occurrence in the upper crust. Geologic study and GPS surveying indicate that the tectonic motion of the blocks in this region is accounted to be a complex or superimposition of three basic types of motions: southeastward sliding,rotating on vertical axis,and uplifting,but there is difference in the geologic slip rate and GPS rate. Besides,this paper collects the database of the geologic slip rates and GPS slip rates for the active faults in the region and major scientific problems are also discussed at last.

The parameters given by the available earthquake catalogs for the 1067 AD Chaoshan,Guandong,earthquake can't be used to explain reasonably the geographical distribution of the earthquake damages recorded in the historical documents. Comparing the recorded damages caused by this event with those caused by the 1918 AD Nan-ao large earthquake at the same places,we find that they are almost the same,and prove that these two earthquakes should be the characteristic earthquakes occurred on the same source. These imply that the 1067 earthquake should be located at just west of the Nanpeng Islands,and its magnitude should be up to 71/4. In view of that there have been diverse recognitions of the 1574 AD earthquake occurred in the coastal region of northeastern Fujian,we have reanalyzed the distributions of the damaged and felt areas of this event,and made a comparison with that of the 1906 AD earthquake off-shore of Xiamen. As a result,it is confirmed that the epicenter of the 1574 AD earthquake should be located at the sea area east of the shore between Fuzhou and Putian,but its magnitude is possibly up to 61/4. Moreover,it is believed that the 1574 Qingyuan,Zhejiang,earthquake recorded in relevant historical document is unlikely a destructive earthquake that occurred at Qingyuan,but more probably is just the one occurred at the sea area east of the shore between Fuzhou and Putian in the same year. Both the 1067 and the 1574 earthquakes were produced possibly along the NE-trending Off-shore Fault zone.

Comprehensive prospecting in Fuzhou basin has revealed that all the main faults in the basin have not been active since Holocene time. Therefore, how to assess the long-term seismic risk of such a type of fault has become an important problem. Taking the Fuzhou basin as an example, we seek after a possible way to evaluate the long-term seismic risk on non-Holocene active faults in eastern China. Firstly this paper demonstrates the position of the Fuzhou basin in the regional earthquake environment by analyzing regional crustal dynamics and seismic background, and then finds out the fault-segments of relatively high seismic risk and estimate the maximum magnitude of potential earthquakes on these fault-segments through the comparison of seismotectonic conditions and the statistic analysis of seismic activity levels in individual seismotectonic zones. The main results are as follows: The Fuzhou basin lies in the area that has been influenced slightly by the action of the Taiwan Dynamic Wedge (TDW), but is close to the border between the areas that are slightly and strongly influenced by the TDW, and is located at the transition zone between regions with and without strong earthquake (of M_S≥6.0) occurrence. As compared with the seismotectonic background in Fujian northeast Guangdong areas, where strong earthquakes have occurred, the main faults in Fuzhou basin and its neighboring area have older ages of the latest activities, while the crust beneath the region contains less distinct low-velocity layers. Moreover, among 12 zones that are taken as statistic units for seismicity in Fujian, northeastern Guandong and the border region among Fujian, Guangdong and Jiangxi Provinces, the unit in Fuzhou basin shows the lowest level of seismicity. It is suggested that the Minhou-Nanyu Fault along the western border of the Fuzhou basin and the Wuhushan northern-foot fault along the southern border of the basin are the two fault-segments with relatively high potential of moderate to strong earthquakes. The maximum magnitudes of the potential earthquakes on the two fault segments have been estimated to be M_S6.0 and 5.6, respectively, on the basis of the worldwide empirical relationship between subsurface rupture scale and magnitude of earthquake.

The inner part of the Sichuan-Yunnan rhombic block is dissected by the Lijiang-Xiaojinhe Fault, and hence can be subdivided into Northwest Sichuan sub-block in the north and Central Yunnan sub-block in the south. The eastern boundary faults of these two sub-blocks are regularly characterized by left-lateral strike-slip, while the western boundary faults are characterized by right-lateral strike-slip. The slip rate of both the eastern and western boundary faults are significantly different. All these phenomena may indicate the composite movement of these sub-blocks characterized by southeastward horizontal slipping associated with clockwise rotation around a vertical axis during the Cenozoic time. Among them, the horizontal slip rate of the Southwest Sichuan sub-block is 5mm/a, and the angular velocity of clockwise rotation is about 1 4°/Ma, while those of the Central Yunnan sub-block are 3.5mm/a and 1 5°/Ma, respectively. About 90 oriented samples have been collected from Paleogene strata in Yaoan, Dayao, Yongren and Beimajie of Kunming within the Central Yunnan sub-block. The vectors of remanent magnetism of each sample (measured magnetic declination and inclination) have been obtained through alternating field demagnetization and thermal demagnetization. The comparison between the measured magnetic declination and the expected value shows that the accumulated clockwise rotation of the Central Yunnan sub-block of the Sichuan-Yunnan rhombic block since early Miocene has reached up to 30°～48°. The feature represented by the entire rotation of the sub-blocks accompanied by left-lateral slipping along the boundary active faults is consistent with the kinetic model of clockwise rotation of the block in left-lateral strike-slip faulting region.