Minshan active block is located in Bayan Har block of Qinghai-Tibet Plateau. It is bounded by the Huya Fault and Minjiang Fault on the east and west sides of the block. In less than 100 years, there have been four earthquakes with M_S≥7.0 occurring along the eastern and western boundary faults, namely, the Diexi earthquake with M7.5 in 1933, two Songpan earthquakes with M_S7.2 in 1976, the Jiuzhaigou earthquake with M_S7.0 in 2017, and several earthquakes with M6.0~6.9. Such intensity and frequency of seismicity on either side of a relatively small intraplate active block is rare. Because the landforms along the active fault are mostly relatively gentle valleys with dense population and there is large terrain difference between the two sides of the valleys, each of the major earthquakes and the large-scale landslides it triggered were liable to cause serious casualties and property losses.
Therefore, how does the destructive seismic activity around the active block migrate in space, and is it closely related to the segmentation and coalescence of active faults?And what are the temporal development characteristics of major earthquake activities and earthquake sequences?The discussion of these questions will not only deepen our understanding of the location and time of future destructive earthquakes, but also promote the development of the hypothesis of active block theory. Compared with the Bayan Har block, the Minshan active block located in the eastern margin of the Qinghai-Tibet Plateau provides a unique experimental field for studying the temporal and spatial regularity of earthquake occurrence in the active block.
In this paper, 39 076 small earthquakes in Minshan active block and its adjacent areas from 2000 to 2019 were relocated using the double-difference location method, and 48, 110 seismic events in the study area were obtained by combining the earthquake catalogues recorded by instruments in the same area from 1972 to 1999. For the major earthquakes since the 1933 Diexi M7.5 earthquake, a thorough analysis was made on the spatial distribution characteristics of earthquake sequences in different periods, especially on the basis of formation of small earthquake bands, and the results show that: Since the Diexi M7.5 earthquake in 1933, the four M≥7.0 earthquake sequences are all distributed along the boundary zone of Minshan active block in space, indicating that the active block plays a controlling role in the process of large earthquake preparation. In terms of the determination of seismogenic structure, the strike of the seismogenic fault of the 1976 Songpan M_S7.2 earthquake is basically the same with that of the 2017 Jiuzhaigou M_S7.0 earthquake, but differs by 60°~70° with that of the 1976 Pingwu M_S7.2 earthquake. So, it is more reasonable that the seismogenic faults of these three major earthquakes belong to two earthquake rupture segments, among them, the seismogenic fault of Jiuzhaigou M_S7.0 earthquake in 2017 and Songpan M_S7.2 earthquake in 1976 is the NW-trending Shuzheng Fault, and that of the 1976 Pingwu M_S7.2 earthquake is the north segment of the Huya Fault. From the perspective of seismicity, the seismogenic fault of the 1933 Diexi earthquake should be the southern segment of Minjiang Fault. The 2017 Jiuzhaigou M_S7.0 earthquake occurred in the gap between the 1976 Songpan M_S7.2 earthquake and the Minjiang Fault. There are probably two seismic hazard areas around Minshan active block, which are located in the southern segment of Huya Fault and the middle segment of Minjiang Fault. The large earthquakes around Minshan block probably belong to foreshock-main shock-aftershock type. Therefore, from the perspective of earthquake prediction, it is suggested to strengthen monitoring of these two seismic gaps.

Historical records with time information are useful for determining the time of earthquake events, while the investigation of historical damage phenomena such as earthquake-triggered landslides can help determine the magnitude of historical earthquakes by analyzing the correlation among historical earthquake-caused landslides, historical earthquakes and related active faults. A series of small basins were developed along the southern segment of the Xiaojiang Fault(XJF), with relatively flat and open topography and concentrated human activities. In most of the southern segment of the XJF, the terrain is relative flat, but some landslide accumulations are still clear, which are obviously different from the surrounding settings and are easy to be identified. Based on remote sensing interpretation and field investigations, landslides with different scales have developed in more than 10 locations along the southern segment of the XJF. Some of them are large with a volume of more than 1 million m³, and some are small with a volume of less than 100 000m³. They are the ancient landslides with a stable state. These landslides are mainly distributed in basins and their border areas with gentle terrain slopes. They are likely to be earthquake landslides rather than rainfall induced. The main scarp angles of these landslides are relatively concentrated, most of which are between 29~31 degrees, indicating that these landslides are caused by one geological event. We use light detection and ranging(LiDAR) measurement technology to obtain the digital elevation model(DEM)data of the landslide development section. The generated three-dimensional topographic shadow map presented in this paper suggests that there is a close relationship between these landslides and the latest surface ruptures of the southern segment of the XJF, indicating that these landslides should be triggered by the latest seismic event along the southern segment of XJF. The fault section was faulted in the latest earthquake events on the surface, triggering clusters of landslides. Based on the age test results of samples from the trench on the landslide body and historical literature data, the co-seismic landslides were triggered in 1606AD. According to the latest research results of the earthquake surface rupture zone in the southern segment of the XJF and empirical formula, combined with the comparative analysis on the intensity of geological disasters and the number of casualties of different earthquake cases, the authors re-assess the magnitude of the 1606 Jianshui earthquake and find that the magnitude of this historical earthquake could not be less than 7½(≥7.5). It means that the southern segment of the XJF, as a part of Xianshuihe-Xiaojiang fault(XSH-XJF) system, shows strong activity and has the ability to generate large earthquakes. GPS observations have verified that the crustal material on the southeastern margin of the Tibetan plateau rotates clockwise around the Eastern Himalaya Syntaxis(EHS), which requires a continuous left-lateral strike-slip fault system as the eastern boundary. The results presented in this paper are useful for deeper study of such an eastern boundary.

On October 17, 2014, a M_S6.6 earthquake occurred in Jinggu, Yunnan. The epicenter was located in the western branch of Wuliang Mountain, the northwest extension line of Puwen Fault. There are 2 faults in the surrounding area, one is a sinistral strike-slip and the other is the dextral. Two faults have mutual intersection with conjugate joints property to form a checkerboard faulting structure. The structure of the area of the focal region is complex. The present-day tectonic movement is strong, and the aftershock distribution indicates the faulting surface trending NNW. There is no obvious surface rupture related to the known fault in the epicenter, and there is a certain distance from the surface of the Puwen fault zone. Regional seismic activity is strong. In 1941, there were two over magnitude 7.0 earthquakes in the south of the epicenter of Jinggu County and Mengzhe Town. In 1988, two mainshock-aftershock type earthquakes occurred in Canglan-Gengma Counties, the principal stress axes of the whole seismic area is in the direction of NNE. Geological method can be adopted to clarify the distribution of surficial fracture caused by active faults, and high-precision seismic positioning and spatial distribution characteristics of seismic sequences can contribute to understand deep seismogenic faults and geometric features. Thus, we can better analyze the three-dimensional spatial distribution characteristics of seismotectonics and the deep and shallow tectonic relationship. The focal mechanism reveals the property and faulting process to a certain extent, which can help us understand not only the active property of faults, but also the important basis for deep tectonic stress and seismogenic mechanism. In order to study the fault characteristic of the Jinggu earthquake, the stress field characteristics of the source area and the geometric parameters of the fault plane, this paper firstly uses the 15 days aftershock data of the Jingsuo M_S6.6 earthquake, to precisely locate the main shock and aftershock sequences using double-difference location method. The results show that the aftershock sequences have clustering characteristics along the NW direction, with a depth mainly of 5~15km. Based on the precise location, calculations are made to the focal mechanisms of a total of 46 earthquakes including the main shock and aftershocks with M_L ≥ 3.0 of the Jinggu earthquake. The double-couple(DC)component of the focal mechanism of the main shock shows that nodal plane Ⅰ:The strike is 239°, the dip 81°, and the rake -22°; nodal plane Ⅱ, the strike is 333°, the dip 68°, and the rake -170.31°. According to focal mechanism solutions, there are 42 earthquakes with a focal mechanism of strike-slip type, accounting for 91.3%. According to the distribution of the aftershock sequence, it can be inferred that the nodal plane Ⅱ is the seismogenic fault. The obtained focal mechanism is used to invert the stress field in the source region. The distribution of horizontal maximum principal stress orienation is concentrated. The main features of the regional tectonic stress field are under the NNE-SSW compression(P axis)and the NW-SE extension(T axis)and are also affected by NNW direction stress fields in the central region of Yunnan, which indicates that Jinggu earthquake fault, like Gengma earthquake, is a new NW-trending fault which is under domination of large-scale tectonic stress and effected by local tectonic stress environment. In order to define more accurately the occurrence of the fault plane of the Jinggu earthquake, with the precise location results and the stress field in the source region, the global optimal solution of the fault plane parameters and its error are obtained by using both global searching simulated annealing algorithm and local searching Gauss-Newton method. Since the parameters of the fault plane fitting process use the stress parameters obtained by the focal mechanism inversion, the data obtained by the fault plane fitting is more representative of the rupture plane, that is, the strike 332.75°, the dip 89.53°, and the rake -167.12°. The buried depth of the rupture plane is 2.746km, indicating that the source fault has not cut through the surface. Based on the stress field characteristics and the inversion results of the fault plane, it is preliminarily believed that the seismogenic structure of the Jinggu earthquake is a newly generated nearly vertical right-lateral strike-slip fault with normal component. The rupture plane length is about 17.2km, which does not extend to the Puwen fault zone. Jinggu earthquake occurred in Simao-Puer seismic region in the south of Sichuan-Yunnan plate. Its focal mechanism solution is similar to that of the three sub-events of the Gengma earthquake in November 1988. The seismogenic structure of both of them is NW-trending and the principal stress is NE-SW. The rupture plane of the Jinggu main shock(NW direction)is significantly different from the known near NS direction Lancang Fault and the near NE direction Jinggu Fault in the study area. It is preliminarily inferred that the seismogenic structure of this earthquake has a neogenetic feature.

How to survey fault activity and determine seismogenic structures in a relatively stable and bedrock-distributed region is a challenging research work. Ruichang-Tonggu Fault and Yifeng-Jingdezhen Fault, distributed mainly at the pre-Cenozoic bedrock region, show the geological evidences of activity in the Quaternary and they are two important tectonic belts for the occurrence of moderate earthquakes in the central-northern Jiangxi Province. Fault gouge stripes can be found on the outcrop sections of the two faults. The imageries of the microstructures of fault gouge show abundant phenomena about the structural deformation, and it is clear that the fault gouge was formed by structural activity. As to the deformation modes, there are not only the Y-shears and R1-shears, which represent the localized-brittle deformation, but also the P-foliations, which reflect the ductile deformation in the microstructures of fault gouge. These features demonstrate that the micro-scale rapid deformation can exist in the seismogenic structure at the near-surface in the occurrence process of moderate earthquakes very possibly. The microstructures of soft material from the fault outcrop section at the southern segment of Hukou-Xingan Fault, which is inactive in the Quaternary, reflects that the soft material from the fault zone can also be the products of rainwater leaching and filling, or weathering in the later periods. Based on the macroscopic observation in the field, it is sometimes difficult to distinguish the differences of structurally-formed or non-structurally formed soft gunk in the fault zones, however, their differences in the microstructure on the slices grinded indoor are obvious. The relatively stable bedrock areas in South China often are not only favorable for the siting of major projects, such as nuclear power plant in China, but also the economically-developed, densely populated, urban agglomeration areas. The study of microstructure of fault gouge provides a technical reference approach for the identification of fault activity and the determination of seismogenic structure of moderate-strong earthquakes in assessing the seismotectonic environment in these regions.

The Chaohu-Tongling area in Anhui Province is a typical moderate-to-strong earthquake active area in the mainland of China. Four earthquakes occurred in this area, displayed as a NNE-trending zonal distribution, including the 1585 M5(3/4) Chaoxian earthquake and the 1654 M5(1/4) Lujiang earthquake, which formed a striking moderate-to-strong seismic activity zone. Field survey, shallow geophysical prospecting, drilling data, collection and dating of chronology samples and comprehensive analysis of fault activity indicate that the Fanshan, Xiajialing and Langcun faults are not active since Quaternary. The NNE-trending Tongling Fault is a buried middle-Pleistocene fault, but it can produce moderate-to-strong earthquakes and control the evolution and development of three en echelon geologic structures. The intensity of the four earthquakes is characterized by southward progressive decrease, which is in accordance with the characteristics that the subsidence range of Wuwei Basin is obviously larger than that of Guichi Basin to its south since late Cenozoic. In terms of deep structure, the characteristics of spatial distribution of Tongling Fault indicate that it corresponds to a NNE-striking Bouguer gravity anomaly gradient belt. So there is a spatial correspondence between the middle-Pleistocene Tongling Fault, the en echelon structures, the differential movement of the neotectonics, the Bouguer gravity anomaly gradient belt and the moderate-to-strong seismic activity belt in the Chaohu-Tongling area, indicating that they should be the tectonic indications of occurrence for moderate-to-strong earthquakes.

The surface ruptures produced by the 2016 M_W7.8 Karkoura earthquake, New Zealand are distributed in a belt with~170km long and~35km wide, trending generally in the NE-SW direction. There are at least 12 faults on which meter-scale displacements are identified and they were formed across two distinct seismotectonic provinces with fundamental different characteristics(Hamling et al., 2017; Litchfield et al., 2017). Although the trending directions of the seismic surface ruptures vary greatly at different locations, the ruptured faults can be generally divided into two groups with the NE to NEE direction and the NNW to N direction, respectively. The faults in the NNW-near NS direction are nearly parallel with 40~50km apart and featured by reverse movement with the maximum displacement of 5~6m. The faults in the NE-NNE direction, with the maximum of 25~30km apart are not continuous and featured by the dextral strike slip with the largest displacement of 10~12m. Even if some faults along the NE-NEE direction are end to end connected, their strikes differ by about 30°. The combination styles of the strike-slip fault surface ruptures along the NE-NEE direction can be merged into 3 categories, including en-echelon, bifurcation and parallel patterns. The scales of the fault surface ruptures with the same structural style could be obviously different in different areas, which results in significant changes in the widths of deformation zone, from tens of meters to hundreds of meters. En-echelon distributed surface rupture(section)can appear as a combination belt of meter-scale to dozens of meter-scale shear fracture with bulge and compressional shear fractures, and also can be characterized by the combination of the left-step en-echelon tensile shear fractures with a length of more than one hundred meters. The step-overs between surface rupture sections are clearly different in sizes, which can be dozens of meters, hundreds of meters to several kilometers. The spacing between parallel surface ruptures can be several meters, dozens of meters to several kilometers. Besides, as one of the prominent characteristics, the seismic surface ruptures caused by the Karkoura earthquake broke through the known distribution pattern of active faults. The surface ruptures can occur either on the previously thought inactive or unmapped faults, or break through the distribution range of previously realized active faults in the striking or lateral direction. The basic features about the distribution and widths of the surface ruptures induced by the 2016 M_W7.8 Karkoura earthquake, New Zealand presented in this paper might be helpful for understanding some seismic problems such as complex corresponding relationship between the active faults and the deep seismogenic structure, and the necessary measurements for engineering crossing active faults.

On January 21 2016, an earthquake of M_S6.4 hit the Lenglongling fault zone(LLLFZ)in the NE Tibetan plateau, which has a contrary focal mechanism solution to the Ms 6.4 earthquake occurring in 1986. Fault behaviors of both earthquakes in 1986 and 2016 are also quite different from the left-lateral strike-slip pattern of the Lenglongling fault zone. In order to find out the seismogenic structure of both earthquakes and figure out relationships among the two earthquakes and the LLLFZ, InSAR co-seismic deformation map is constructed by Sentinel -1A data. Moreover, the geological map, remote sensing images, relocation of aftershocks and GPS data are also combined in the research. The InSAR results indicate that the co-seismic deformation fields are distributed on both sides of the branch fault(F₂)on the northwest of the Lenglongling main fault(F₁), where the Earth's surface uplifts like a tent during the 2016 earthquake. The 2016 and 1986 earthquakes occurred on the eastern and western bending segments of the F₂ respectively, where the two parts of the F₂ bend gradually and finally join with the F₁. The intersections between the F₁ and F₂ compose the right-order and left-order alignments in the planar geometry, which lead to the restraining bend and releasing bend because of the left-lateral strike-slip movement, respectively. Therefore, the thrust and normal faults are formed in the two bending positions. In consequence, the focal mechanism solutions of the 2016 and 1986 earthquakes mainly present the compression and tensional behaviors, respectively, both of which also behave as slight strike-slip motion. All results indicate that seismic activity and tectonic deformation of the LLLFZ play important parts in the Qilian-Haiyuan tectonic zone, as well as in the NE Tibetan plateau. The complicated tectonic deformation of NE Tibetan plateau results from the collisions from three different directions between the north Eurasian plate, the east Pacific plate and the southwest Indian plate. The intensive tectonic movement leads to a series of left-lateral strike-slip faults in this region and the tectonic deformation direction rotates clockwise gradually to the east along the Qilian-Haiyuan tectonic zone. The Menyuan earthquake makes it very important to reevaluate the earthquake risk of this region.

The Lenglongling Fault(LLLF) is a major active left-lateral strike-slip fault along the northeastern margin of the Tibetan plateau. Fault slip rate is of great significance for researching the dynamics of tectonic deformation in NE Tibetan plateau and understanding the activity and seismic risk of the fault. However, slip rate of the LLLF, which remains controversial, is limited within~3~24mm/a, a relatively broad range. Taking Niutougou site(37.440 2°N, 102.094 0°E)and Chailong site(37.447 3°N, 102.063 0°E) in the upstream of Talihua gully in Menyuan County, Qinghai Province as the research objects, where faulted landform is typical, we analyzed the displacement evolution model and measured the slip amounts by back-slip of the faulted landform using high-resolution DEM from Terrestrial LiDAR and high-precision satellite images of Google Earth, and by collecting and testing samples from stratigraphic pit excavated in the faulted landform surface and stripping fresh stratigraphic section, we determined the abandonment age of the surface. Holocene slip rate obtained from Niutougou site and Chailong site is(6.4±0.7)mm/a and(6.6±0.3)mm/a, respectively, which have a good consistency. Taking into account the error range of the slip rate, the left-lateral slip rate of the LLLF is(6.6±0.8)mm/a since Holocene, which is between the previons results from geological method, also within the slip rate range of 4.2~8mm/a from InSAR, but slightly larger than that from GPS((4.0±1.0)mm/a). Late Quaternary slip rate of Qilian-Haiyuan fault zone, which displays an arc-shape distribution, turns to be the largest in LLLF region. The most intensive uplift in the LLLF region of the NE Tibetan plateau confirms the important role of the LLLF in accommodating the eastward component of movement of Tibetan plateau relative to the Gobi-Ala Shan block from one side.

The southern segment of the Xiaojiang Fault (SSXF) is located at the intersection of the Xianshuihe-Xiaojiang Fault and Red River-Ailao Shan fault systems in the southeast margin of the Tibetan plateau. Based on the interpretation of remote sensing image, the SSXF clearly shows the linear feature and continuous distribution as a single, penetrating fault. It has a total length of about 70km, trends generally about 20° to the northeast and protrudes slightly in the middle to the east. A typically geomorphologic phenomenon about the synchronous left-lateral dislocation of ridges and gullies can be found at Liangchahe, Longtan Village along the SSXF. The distribution of faults, the sedimentary features, attitude variance and the primary dating results of the offset strata in the trench section across fault sag ponds reveal three paleoseismic events rupturing obviously the surface, which demonstrates that the SSXF has the ability of recurrence of strong earthquakes. High-precision topographic map about two gullies and the platform between them with synchronous dislocation is acquired by using the Trimble 5800 GPS real-time difference measurement system. The dislocation is (18.3±0.5)m. As the top geomorphologic surface between the above two gullies and their adjacent area, the terrace surface T₂ stopped accepting deposits at ~2606a, based on the linear regression analysis of three dating data. According to the geological method, a sinistral strike-slip rate of (7.02±0.20)mm/a on the SSXF in the Holocene is obtained, which has a good consistency with the results provided by using GPS data. The preliminary results about the Holocene activity and slip rate of the SSXF demonstrate that the southward or south-southeast motion of the Sichuan-Yunnan block in the SE Yunnan region has not been absorbed by the possible shortening deformation and the sinistral strike-slip rate of the SSXF has not been drastically reduced. The SSXF is a Holocene fault with obvious activity. This preliminary understanding provides some basic geological data for the seismic risk evaluation of the SSXF in the future, and for the establishment and inspection of the seismotectonic model about the Sichuan-Yunnan block.

On April 20,2013,a strong earthquake of M_S 7.0 struck the Lushan County,Sichuan Province of China. In this paper,basic information of the April 20,2013 Lushan earthquake,historical earthquakes in the Lushan earthquake struck area and associated historical earthquake-triggered landslides were introduced firstly. We delineated the probable spatial distribution boundary of landslides triggered by the Lushan earthquake based on correlations between the 2008 Wenchuan earthquake-triggered landslides and associated peak ground acceleration(PGA).According to earthquake-triggered landslides classification principles,landslides triggered by the earthquake are divided into three main categories: disrupted landslides,coherent landslides,and flow landslides. The first main category includes five types: rock falls,disrupted rock slides,rock avalanches,soil falls,and disrupted soil slides. The second main category includes two types of soil slumps and slow earth flows. The type of flow landslides is mainly rapid flow slides. Three disrupted landslides,including rock falls,disrupted rock slides,and soil falls are the most common types of landslides triggered by the earthquake. We preliminary mapped 3883 landslides based on available high-resolution aerial photographs taken soon after the earthquake. In addition,the effect of aftershocks on the landslides,comparisons of landslides triggered by the Lushan earthquake with landslides triggered by other earthquake events,and guidance for subsequent landslides detailed interpretation based on high-resolution remote sensing images were discussed respectively. In conclusion,based on quick field investigations to the Lushan earthquake,the classifications,morphology of source area,motion and accumulation area of many earthquake-triggered landslides were recorded before the landslide might be reconstructed by human factors,aftershocks,and rainfall etc. It has important significance to earthquake-triggered landslide hazard mitigation in earthquake struck area and the scientific research of subsequent landslides related to the Lushan earthquake.

The co-seismic surface rupture signs of the "4.20" Lushan M_S7.0 earthquake are found at Longmen Township,Lushan County. The sites of rupture signs have a linear distribution with a 2～3km length and N40°～50°N strike. The maximum shortening of the rupture is about 8cm,uplifting is about 1～2cm. Strike-slip component is not observed,but the dynamic process of the earthquake is characterized by compression from northwest to southeast. The observed co-seismic surface ruptures can be oblique shear-fissure,or thrusting crack,however most of them are extensive fissures,which can be explained by the local extensive stress-field on the top of the thrust bending. Although these ruptures have different geometric shapes or variant mechanic features,they similarly reflect the northwest-southeast compression and the surface lift-bending on the top of a thrusting seismogenic structure. Comparing with Dachuan-Shuangshi Fault(frontal fault)and Dayi-Mingshan Fault(piedmont fault),Lushan-Longmen presumed blind fault is more likely the seismogenic fault,which is also consistent with the results of the Lushan earthquake sequence relocations and the seismic intensity contours. As the seismogenic fault of the Lushan earthquake has surpassed the frontal fault of Longmen Shan,it may be a new-generated tectonics,which implies that it is important to re-evaluate the seismic risk at the piedmont area of the Longmen Shan. However,the conclusions are still very primary and geophysical survey is needed to demonstrate the existence of the Lushan-Longmen presumed blind fault.

The accurate relocations of small earthquakes during 1990-2011 on the arcuate tectonic belt in southeast Yunnan (23°N to 25°N,101°E to 103°E) are obtained by hypo2000 and double difference algorithm using Pg and Sg phase readings of 721 earthquakes recorded by Yunnan Earthquake Network Center.The RMS residual decreases to 0.45 from 1.43 with an average precision of about 1.4km horizontally and 1.9km vertically after relocation. Then we collected the waveform data of the study region during 2007-2012 and calculated the focal mechanisms of 148 small earthquakes with the method of the maximum amplitude ratio between Pg and Sg. Focal mechanism result shows that the number of nodal planes representing normal strike-slip is almost twice of that of the reverse strike-slip,which demonstrates that the dominant movement on the arcuate tectonic belt is normal strike-slip currently. Relocation of small earthquakes greatly improves the definition of the seismic images on the faults that generate them. According to the relocated focal depths,the Qujing Fault and Shiping-Jianshui Fault dip southwest,and the Honghe Fault dips northeast,which are in accord with the fault geometric characteristics revealed by crustal velocity profiles. According to this,the rollback of Sumatra-Myanmar trench has already brought a deep effect on the arcuate tectonics in southeast Yunnan,the compression caused by the slip of Sichuan-Yunnan rhombic block towards SSE may be weakened. So the tensile stress in SWW-NEE direction plays a more important role in present-day activities of the arcuate tectonics compared to the compressive stress in SSE-NNW direction,which can also be deduced from the characteristics of focal mechanism parameters. A transtensional zone may be emerging from the arcuate tectonics in southeast Yunnan.

According to the detailed shallow geophysical survey,drilling,dating,geological geomorphic investigation along the central segment of the Changde-Yiyang-Changshan Fault at the southern margin of Dongting Basin,features about its geometry,dynamics,active epochs and basin structural type are revealed.The central segment of the Changde-Yiyang-Changshan Fault(buried fault),a normal fault,dips to NNE.It not only offset the Eogene top boundary,but also displaced the Huatian group(Q_p¹ht),Miluo group(Q_p¹m)of Lower-Pleistocene and Xinkaipu group(Q_p²x)of lower section of Middle-Pleistocene.The Baishajing group(Q_p²b)in the middle section of Middle-Pleistocene overlays the fault without any deformation and disruption.The latest active age is the early Pleistocene,but no active evidences after the mid-late Pleistocene are found.Vertical displacement of the top of basal rock(or bottom of Lower Pleistocene),that is, the total offset at the Quaternary,is 16.10m,but it becomes smaller towards the surface.The fault was coetaneous with the sedimentation of Huatian group(Q_p¹ht)and Miluo(Q_p¹1m).Based on the comparison of fault-controlled subsidence and sediment thickness,it can be concluded that the Anxiang-Hanshou depression was formed mainly by deformation,not by faulting.Both faults and moderate earthquakes are concentrated along the zones between Dongting Basin and surrounding uplifting mountains.It can be looked as an important evidence for determining the seismotectonics of moderate earthquake,whether a fault offsets the lower or middle Pleistocene.

The kinematic property of the Wenchan earthquake's surface rupture changes from strike-slip with slightly smaller dip-slip component to the dominant dextral strike-slip at the northeasternmost region between Shikan in Pingwu county and Woqian in Qingchuan county,where,the dip-slip component is reverse between Shikan and Pingxi,normal at Kuangpingzi and its north,with no compressive deformation observed,and it turns to dextral strike-slip near Woqian.The width of the surface deformation zone is less than 10m on this segment.At Dongjia,a village north of Woqian in Qingchuan,the seismic surface rupture zone mainly exhibits as extensional fissures and graben-like negative landform,which are the products of accommodation of the heterogeneous stress and strain at the tip portion of the seismic surface rupture.The width of the surface deformation zone is about 10～12m.No surface rupture evidence was found at Donghekou village in Qingchuan county,so we infer that the surface rupture zone didn't spread through the Qingshui River flowing along the three villages of Donghekou,Guanzhuang and Liangshuijing in Qingchuan county and the structural geomorphology reflects variant vertical motion.Evidence for dextral strike-slip motion was hardly found.The whole length of the rupture zone on the central fault is around 240km.In the course of the Wenchuan earthquake,the tectonic deformation on the surface along the central fault was adjusted within the range of the central fault,and didn't transfer to the external regions.

Primary evidence of the latest tectonic deformation style and the activity age of the Dayi Fault in the Sichuan Basin are obtained by field geological mapping,and surveying with the help of 3D scanner and total station.In the region of Wenshangou-Longfengchang,northeast of Dayi county on the northwestern wall of the fault,the time of Cretaceous and Neogene strata's gentle folding was measured to be between late Neogene and early Pleistocene.And obviously its forming time is later than the Longmen Shan Fault which controls the northwest boundary of Sichuan Basin's Mesozoic strata and shows a thrust-nappe structure.With the help of oil prospecting data,fault plane inclines to northwest,dip angle changes from 10°～20° in the deep to 70° at near-surface,and depth of the upper faulted point is about 200～300m.Geological and geomorphologic evidence shows that Holocene is its latest active time.The latest tectonic deformation on the Dayi Fault is represented by Holocene fold.Topographically,the deformation appears as continuous piedmont hillocks with traceable length of 2.5km,distributed en echelon in the piedmont of the Longmen Mountains and with the plane shape of single hillock being ellipse.Accordingly the Dayi Fault is inferred as a Holocene active blind fault on the basis of hillocks' section shape.

Combined with the consideration of relative codes,this paper mainly presents a preliminary analysis on the engineering application of Yuwangshan Fault zone based on the results achieved in the project of active fault exploration and seismic hazard assessment in the city of Ningbo. The conclusions we get from it are as follows. We don't have to take safety distance into account as to the west section of Yuwangshan Fault in engineering and construction because it has not been active since mid-Pleistocene,whereas,as to the east section of this fault,there's no need to consider the safety distance about Ⅲ-type and Ⅳ-type buildings,nevertheless,the case is different for Ⅰ-type and Ⅱ-type buildings because the potential surface deformation to some extent will occur with the scenario earthquake of 6.0,even if no direct surface offset. Due to the existence of about 6km² area where near-fault peak ground accelerations lightly exceeds the present-day standard,the authors put forward a package of aseismatic solutions which consist of the identification of aseismatic capability to buildings in the area and consideration of this factor in new buildings' construction.There's a potential area of seismic settlement of 30km² which may sink 5 cm or above according to the characteristics of distribution of soft soil as well as the assessment of near-fault strong ground motion in the east of the west section of Yuwangshan Fault. We should not ignore this important result.Given the importance of the results to the policy-decision for the mitigation of seismic disasters,we should seek for the trade-off between the safety and economic rationality in engineering application analysis.

Few large earthquakes(M_S≥7)have ever occurred in the historical records in eastern China,especially in southeast continental region of 105°～120°E,20°～35°N. However,many moderate-strong earthquakes with magnitude between 5 and 6 occurred there,such as the 1962 Heyuan M_S 6.4 earthquake and 1969 Yangjiang M_S 6.7 earthquake in Guangdong province,the 1974 Liyang M_S 6.0 earthquake in Jiangsu province,1979 Guzhen M_S 5.0 earthquake in Anhui province,and the 2005 Jiujiang M_S 5.7 earthquake in Jiangxi province and so on. These earthquakes have no significant earthquake ruptures,and few late Pleistocene active faults were discovered. So,the seismogeological background of the above moderate-strong earthquakes is still unclear up to the present,that is to say, it is very necessary to study the seismogeologic evidence for moderate earthquakes. In the paper,by analysis of the seismogeologic evidence of the 1979 Liyang M_S 6.0 earthquake in Jiangsu province,the 2005 Jiujiang-Ruichang M_S 5.7 earthquake in Jiangxi province,the 1710 Xinhua M_S 5(1/2) earthquake in Hunan province,the 1917 Huoshan M_S 6(1/4) earthquake in Anhui province and the moderate-strong earthquakes around Dongting Lake,some conclusions are obtained as follows: 1)most moderate-strong earthquakes in east China occurred at regions near the early Quaternary active faults,and regions with early-middle Pleistocene active faults; 2)most moderate-strong earthquakes in East China are related to the development and distribution of the Quaternary down-faulted basins. 3)moderate-strong earthquakes in East China may occur at regions with evident active tectonic geomorphological characteristics in Quaternary,such as the areas with linear fault geomorphology or geomorphologic surface; 4)moderate-strong earthquakes may occur in the seismic gaps along the seismic zones where earthquakes with magnitudes 4～5 occurred in the history.

The Kalpintag nappe is located at the southwestern foot of the Tianshan Mountains,consisting of several rows of NE-to EW-trending fold-reverse fault zones.This paper demonstrates the activities and slip rate of the frontal faults of the first to third row fold-reverse fault zones located to the west of the Piqiang-Bachu phosphorite mine.The newly-found evidence shows that the front of each fold-reverse fault zone is composed of several faults of typical reverse fault type.The faults with newest activity are located at the forefront of the fold-reverse fault zones,and the active period of the faults is late Pleistocene-Holocene.They dissect the T₀,T₁,T₂ and T₃ terraces,resulting in fault scarps of different heights.According to the in-situ measurement of the fault scarps and the dating data of the relevant samples,it is estimated that the amount and rate of vertical displacement along the faults since the formation of the T₀ terrace are 0.9～1.1m and 0.53～0.65mm/a,respectively,while the amount and rate of the corresponding crustal shortening are 1.93～2.56m and 1.14～1.52mm/a,respectively.Similarly,it is estimated that the amount and rate of vertical displacement since the formation of the T₁ terrace are 1.4～1.8m,and 0.36～0.46mm/a,with the corresponding crustal shortening of 3.00～3.86m,and shortening rate of 0.77～0.99mm/a.The vertical displacement amount and rate since the formation of the T₂ terrace are about 2.1～3.0m and 0.31～0.45mm/a,respectively,and the amount and rate of the corresponding crustal shortening are 4.50～6.98m and 0.67～1.04mm/a.The amount and rate of vertical displacement since the formation of the T₃ terrace are 3.4～4.2m and 0.28～0.35mm/a,and the amount and rate of the corresponding crustal shortening are 7.29～9.22m and 0.61～0.77mm/a,respectively.Based on the obtained amount and rate of crustal shortening since the formation of T₀ terrace,the total amount and rate of crustal shortening of the Kalpintag nappe since 1.7ka can be estimated to be 9.65～12.80m and 5.68～7.53mm,respectively.

The Zhonggu Fault,located in the Yuanjiang-Yuanyang basin and active strike-slip fault since Neogene,is part of the southern segment of the Red River Fault.Its neotectonic movement resulted in the separation of the Red River basin(a Miocene basin)into two sub-basins and a dextral slip extending to the Guotoushan-Damanmi region.The concomitant mountain frontal fault,which was the dominant fault in Oligocene,is of normal faulting.Its activity resulted in the accumulation of red continental clastic sediments in the eastern and northeastern Yuanyang.The mountain frontal normal fault extended to the northwest of Honghe County and formed the deposition of conglomerate in early Miocene.Along the Zhonggu Fault,which offset Miocene sediments with high angles,the geological features,including the compressive fold with axis trending NE,the compressional deformation landforms,the distribution the Miocene to Quaternary sediments migrating from SE to NW successively and their delayed distribution at the northeastern wall of the Zhonggu Fault,all suggest that the southern segment of the Red River fault has been expanding from SE to NW and there has been the dextral strike-slip faulting since Miocene.The geological evidences,such as thick lower-mid Miocene conglomerate deposited in mountain front,the shear deformation involved in Zhonggu Fault which was more intense in Miocene than that in Pliocene,the shear zone of Zhonggu Fault mainly located in lower-mid Miocene sediments,indicate the large scale dextral slip occurring at mid Miocene,and its fission track age is 13.7Ma.The magnitude of dextral offset on the south segment of Red River Fault since Miocene was calculated by multiple means,e.g.the horizontal dimensions of the offset of Miocene sediment,the length of foreland basin and deformation width relating to the Zhonggu Fault slip,and the relationship between the offset and width of fault,with the result ranging from 62～69km(the mean is 65km).The data also suggest that dextral slip of the Red River fault experienced the process of transition(N₁),initial dextral slip(N₁²),large scale dextral slip(N₁³—N₂¹),and dextral slip propagation(N₂²—Q_p¹)phases.The related activity of the Red River fault altered between shear slip and extensional slip.

The Kalpintag Fault locates at the most forefront of Kalpintag nappe tectonics, and can be separated into eastern and western segments by Piqiang Fault. Six large trenches are excavated along the western segment and four paleoearthquakes can be distinguished in the three trenches of them. The first paleoevent occurred about 12ka BP, the second event occurred about 8.6ka BP, the third event occurred about 5ka BP,and the last event occurred after (1.73±0.15) ka BP, which probably is the Xike'er M6.8 earthquake in 1961 AD. The four paleoevents are characterized by 3～4ka quasi-periodic recurrence interval. The Kalpintag nappe structure are composed of 5～6 rows of fold-reverse fault zones. The faults with the latest activity are located at the forefront of the fold-reverse fault zones with 10km spacing between each fault. The north-dipping and listric style fault surfaces merged into the detachment surface in the deep along the bottom of Cambrian at 6～10km depth. The field investigation discovered that earthquake ruptures and paleoearthquake traces can be found not only along Kalpintag Fault but also along other faults, but the rupture length and seismic slip are smaller than that formed by an M≥7 earthquake. Although five paleoearthquakes since 14ka BP are obtained along western segment of Kalpintag Fault, some events are probably missed because of less trenches and dating samples. Many problems such as magnitude of these events, seismogenic fault and their rupture zones formed by one or several events await study in the future.

The Kalpintag fold nappe is located at the northwestern foot of the Tianshan Mountains. Since Cenozoic, owing to the Indian-Eurasian collision, the Mesozoic fold structures of Tianshan have been rejuvenated, uplifted and pushed northward and westward. As a result, several rows of fold-reverse fault zones have been progressively formed within the foreland basins. This paper describes in detail the Cenozoic deformation features and propagation of the fold-reverse fault zone on the west of the nearly south-north-trending Piqiang fault zone. The results show that the Cenozoic deformation of the nappe was characterized by wavy differential uplift, and this has caused the successive formation of the fold-reverse fault zone from the southern side of the Tianshan Mountains to the Tarim Basin. Among them, the early-formed folds are close to the Tianshan Mountains, while the latter-formed folds are close to the Tarim Basin, indicating the general tendency of northward propagation of the fold-reverse fault zone during their formation process. The distance of propagation may reach up to 76km. Moreover, the front of individual fold-reverse fault zone consists of several fault strands, which are associated with folds and have different ages of formation and time of recent activity. The early-formed faults are close to the Mountain side and the latter-formed close to the basin, indicating the northward propagation of the frontal faults of the individual fold. The distance of propagation is about 100～500m. The mechanism of the propagation of the fold-reverse fault zone is discussed in this paper as well.

Kalpin thrust tectonic is an active reverse fault fold zone at the southwestern Tianshan front piedmont, it consists of five to six rows of arc fold zones which are formed by Cambrian-Quaternary sedimentary rocks. The major morphology of the anticline is multiple box-shaped or asymmetry inclined, mostly, and it is similar to that of fault-bend fold and fault-propagation fold. Depending on the seismic reflection surveys data, the reverse faults on the front of the nappe in Kalpin thrust tectonic zone form an integrate detachment surface in the deep along the gypsum stratum in Cambrian. The depth of the detachment is shallower in the southeast and deeper in the northwest. The depth of the detachment fault is deeper in the west part (about 9km deep) and shallower in the east part (about 5km deep) of the Piqiang fault. In the middle part of the Kalpin reverse fault-fold zone, we have made two balanced cross-sections at the two sides of Piqiang fault. On the two geological cross-sections, we construct the structure mode at depth using fault-bend fold or fault-propagation fold model. The length of the two sections is 73km and 78km, respectively. The restored sections yield a crustal shortening of 40km to 45km, the shortening rate is 33% and 37%, respectively. Calculating the long-term shortening rate from these two across-sections is difficult, because the time of initiation of deformation is poorly known. Geological evidence suggests that most of the shortening began in the beginning of the deposition of the thick conglomerate unit in lower Quaternary. If the initiation time is about 2.5Ma, the shortening rate of Kalpin thrust tectonic zone is 15.4～17.3mm/a.

Owing to strong and permanent Cenozoic re-orogenic processing, a lot of EW-striking active thrusts and folds have been developed in Tian Shan, resulting in crustal shortening in NS direction. There also exist NW-striking transform-like strike-slip faults that cut the Tian Shan and accommodate uneven crustal shortening larger in the west and smaller in the east. The seismogenic structures in and around the Tian Shan mainly include EW-striking thrust ramps or blind thrusts and NW-striking transform-like strike-slip faults. The 2003 AD Bachu-Jiashi earthquake is located at south of the Kalpintag nappe. A NE-trending deep seismic reflection profile about 50km long across the epicenter has been conducted after the earthquake. From this reflection profile four blind faults are identified. Together with earthquake relocation, these identified blind faults are used in the paper to interpret the seismogenic structures of the 1997 AD Jiashi strong earthquake swarm and 2003 AD Bachu-Jiashi earthquake. The 1997 AD Jiashi strong earthquakes were generated mainly by a NW-striking buried transform-like strike-slip fault, while the 2003 AD Bachu-jiashi earthquake by blind thrusts in front of the Kalpintag nappe.

Geological, geomorphic and geophysical methods, including drilling and chronological dating, are applied to determine whether the suggestively buried Taoyuan fault exists. Stratum units at two sides of the fault are continuous. The “normal fault”, found at east side of Yuanjiang River, Yaohe town, is actually a fissure caused by un-loading. Weak cementation of the Eogene breccia is also favorable to formation of the extensive fissure. One meter of offset along the fissure is not the result of tectonic movement, but comes from up-plate falling under the gravitational action. Quaternary thicknesses at the two sides of Yuanjiang River show no abrupt change, but a normal sedimentation. According to the geomorphic feature and the fault location, suggested by previous work, three lines of shallow seismic survey are arranged at the two sides of Yuanjiang River. The sections of shallow seismic survey show that boundaries at different layers are clear and can be pursued continuously. There are no evidences to demonstrate that the suggestively buried Taoyuan fault may exist. Two drilling bores are arranged at Xunyangping Quaternary-covered area and they are located at two sides, 20m away from the suggestively buried fault. The drilling results show that the Quaternary layers are crossed almost at the same depths and the lower basal rocks are revealed, which are red clay-sand rocks of the Eogene. Thermoluminescence dating samples are collected respectively at depths of 4.5m, 9.8m and 16.8m. The experiment results are (2.2±1.8)×10⁴a, (2.9±2.5)×10⁴a and (13.1±11.0)×10⁴a, which shows that the Quaternary strata, formed at the end of mid-Pleistocene, are continuous at two sides of the suggestively buried fault. There is no neo-activity. In conclusion, there is no evidence to support the existence of the suggested Taoyuan fault. The Taiyangshan fault zone, on which the 1631 M6³/₄ earthquake occurred, is bounded at the north of Changde-Yeyang-Changsha fault zone. It provides a reasonable geological background for division of seismic belts and potential seismic sources and is also important for better understanding of structural conditions of the 1631, M6³/₄ Changde, Hunan, earthquake.

Field investigation has revealed that the large-scale dextral strike-slip movement and the associated tectonic deformation along the Honghe (Red River) Fault zone have the following features. Geometrically,the whole Red River fault system can be divided into three deformation regions:the north,central and south deformation regions. On the eastern side of the north region lies the northwest Yunnan extensional taphrogenic belt,which is characterized by three sets of rift-type faulted basins striking NNW,NNE and nearly N-S since the Miocene time. From Miocene to Quaternary epoch,the faulted basins spread or migrated southwestwardly,but the large-scale and intensive rift-depression mainly occurred in the late Pliocene to Quaternary. The extension amount of the basin since Quaternary is about 5.6 km. On the western side of the north segment lies the Lanping-Yunlong Tertiary compressive deformation region. The deformation in the central segment is characterized by dextral strike slip or shearing,resulting in a dextral displacement of about 7.4km since Quaternary. The east Yunnan Miocene compressive deformation region lies on the eastern side of the fault in the south,and the extensive fault-depressed region is located on its western side. In tectonic-geomorphology,the afore-mentioned deformation features appear as basin range tectonics in the north,linear fault valley depressed basins in the central part and compressive (or extensive) basins in the south. Among them,the great variance of the planation surfaces on both sides of the fault in Cangshan to Erhai area is the prominent expression of the normal faulting along the Red River fault zone since Pliocene time. From the view-point of spatial-temporal evolution,the main active portion of the Red River fault zone from Eogene to Pliocene was the southern segment,which was characterized by tearing from south to north. The main active portion of the fault has migrated to the north segment since Pliocene,especially in late Quaternary,and was characterized by extensional slipping from north to southeast. The range of deformation region and the magnitude of deformation show that the eastern plate of the Red River Fault zone is always the active plate for the relative movement of the fault blocks.

Okada's(1992)elastic half-space theory is applied to calculate the surface displacement and the amount of differential displacement produced by buried active faults. The theory has been verified to some extent by the 1992 Landers earthquake, United State, the 1994 Northridge earthquake, United State, the 1995 Kobe earthquake, Japan and the 1999 Izmit earthquake, Turkey. According to the criteria given in 《The Regulations of Seismic Design of Building》(GB50011-2001),the threshold value of differential displacement of buried seismogenic fault for producing surface rupture is estimated to be 0.1m for two adjacent points of 5m distance. This means that if the surface displacement produced by any buried fault is larger than 0.1m, then surface rupture zone will occur. If it is smaller than 0.1m, then the hazard that the buried fault may cause can be neglected. How the width and displacement of surface rupture zones vary with the buried depth, dip angle, motion mode and offset of the active fault is analyzed and discussed by applying the same theory and regulations. The results show that: 1)for buried normal fault, with increasing of the buried depth of the fault the width of surface rupture zones increases gradually to a peak value, and then decreases again. The point of peak value migrates toward the footwall of the fault(downfaulted block); 2)when the dip angle of the fault(normal fault)becomes small and the other parameters remain unchanged, the surface rupture zone will distribute mainly on the downfaulted block and the width becomes narrower; 3)as compared to the case of buried normal fault, the differential displacement produced by buried strike-slip fault attenuates more quickly when the buried depth of the fault becomes greater. It may imply that the normal fault is more risky; 4)the width and displacement of the surface rupture zone increase significantly with increasing displacement on the buried active fault. These primary results provide scientific basis for city planning and the seismic design of lifeline engineering and constructions across buried active faults. However, how to decide the threshold value of surface rupture and how to build up a more reasonable model for the middle and shallow crust need far more detailed study.

A large number of observed data about the widths of earthquake surface ruptures produced on diverse types of active faults, as well as the widths of intense deformation zones in trench logs across active faults are presented in this paper. Combining these data with the close relation between the damage zone of surface construction and the spatial position of active fault, the authors propose that a minimum distance(safety distance)of 30m away from active fault is essentially required for constructions to prevent the effect of faulting. The more accurate safety distance required for various types of active faults can be tested and modified through the analysis of the deformation features of strata in trench logs across the fault. The required safety distance in some specific sites, such as the step-overs of active faults, as well as the area defined by sub-parallel secondary faults on both sides, should be the sum of the width of the area and 15m from area boundaries. It is suggested that this "safety distance" should be taken as a legal regulation for constructions and buildings. In addition, further attention should be paid to the identification of active fault and precise determination of the geometric structures of the surface fault strand, so that earthquake hazards can be positively and effectively reduced.

The mathematical expression of the original horizontal plane can be obtained by coordinate transformation,provided that the following conditions are fulfilled:1. The block where the volcanic rocks outcroped has not been displaced in north-south direction; 2. The statistical average vector of the remanent magnetization follows the axial geocentric dipole magnetic field. In that case,it will enable us to get more reliable amounts of the rotation of the block around the vertical axis,and that of the tilting of the strata around the horizontal axis,based on paleomagnetic method. In addition,it will be possible to apply paleomagnetic method to the study of the tectonic movement of volcanic rocks with ambiguos attitute.

The neotectonics and seismicity in Jianchuan-Eryuuan area, northwestern Yunnan exhibit a model of the geometric barriers of the faults relative to the earthquake rupturings.Field investigations have revealed that the modern Jianchan-Eryuan extensional zone is made up of two longitudinal zigzag rupture belts which experienced two important stages of development in the past: pulling-apart within the overlapping area of NE-trending strike-slip faults in early Quaternary, and zigzag extension in late QuaternaryEarthquake data from 1982 to 1986 show that five earthquake events occurred along the extensional belts corresponding with the trend of the zigzag ruptures. First event happened in July, 1982 at the northern end of the zone, and then the following seismic swarms occurred one by one towards the south, each of which was located at the geometric barrier of the faults constructed by the intersections of faults with different behaviors.In order to describe the mechanism of the seismicity like this, we adopt the concept of barrier proposed by Aki (1979), King and Yielding (1984), and set a numerical model to calculate the change of stresses along the zigzag rupture. The results reveal that the. maximum shear stress and the displacement of the faults usually focus on the geometric barriers, which were formed by the intersections of faults with different behaviors or trends and broken most easilly. When the stress on a barrier is released, a mu-tuation of stress at next barrier must be induced and so it will be broken following the first in a short time which will cause the third barrier fractured after that, and so on. Thus, the earthquake events appear along the zone one by one untill the induced strain energy can not produce a new rupture.By comparision, the seismicity of extensional zones in other places of China is similar to that of Jianchuan-Eryuan zone. We can conclude that the migration and magnitude of earthquakes in modern structures are not random but depend on the geometry of the structures.