The vertical component of surface afterslip of the 2008 Mw7.9 Wenchuan Earthquake, China, has been observed since 2011 using repeated levelings at the Yingxiu site, which spans the southwestern segment of the 2008 rupture. We present the geological, co-seismic rupture, and aftershock features around the Yingxiu site and employ the rate- and state-dependent friction theory to analyze the friction mechanism and depth of the afterslip. The logarithmically increasing afterslip series with a cumulative amount of -30 mm obeys a velocitystrengthening friction law, from which the afterslip depth is estimated to be 2.6-6 km. This depth is further constrained to -5 km that probably represents the thickness of a shallow velocity-strengthening layer of the fault, from information of sedimentary strata and aftershock distribution. The coseismic reverse vertical slip was <3 m around the Yingxiu site during the 2008 mainshock, whereas that on the adjacent rupture section to the northeast was >= 8 m above -15 km depth, including the largest slip of 12.5 m at -13 km depth (Zhang et al., 2012). These features suggest that an apparent deficit of coseismic slip existed on the rupture section at the Yingxiu site, compared with coseismic slip on the adjacent section. Furthermore, the surface-observed afterslip is uncorrelated with the aftershock slip on the fault in the underlying velocity-weakening layer. Therefore, the surface afterslip at the Yingxiu site is due to post-2008 relaxation that compensates for the coseismic slip deficit in the shallow layer of the fault and is probably governed by velocity-strengthening friction.

To understand why the 130 km-long restraining bend segment (Big Bend) of the Red River fault (RRF) in Yunnan lacks historical ruptures and has a low geologic slip-rate, deep seismic profiles, relocated earthquakes and GPS-measured velocities are integrated to investigate the fault zone architecture and deformation associated with this segment. The Middle to Late Cenozoic southward extrusion of the Chuan-Dian Fragment, along with the northeastward blocking of the Yunxian Convex, have curved the RRF and its sub-parallel faults towards the southwest, forming the Big Bend. Branch faults split off the Big Bend RRF at depths between the upper lithosphere-mantle and the lower middle crust; splaying upward and sideward, they either connect/reactivate preexisting faults or become blind faults in the middle to upper crust, resulting in seismically active Lancang, Blind Wuliangshan, Qujiang and similar faults on both sides of the Big Bend. Together these faults form a complex fault system that shows a trans-crustal flower structure with a -350 km width at the surface. Across this fault system GPS-measured dextral-shearing at-8 mm/a and horizontal-shortening at-3.5 mm/a represent the deep deformation rate of the Big Bend RRF under the flower structure. 85% +/- of the deep shearing and almost all of the deep shortening are partitioned onto branch and subsidiary faults bounding and within the flower structure, and only 15% +/- of the deep shearing to the Big Bend RRF in the brittle crust, due to the complex fault system architecture and the existence of a large-scale asperity on the main RRF in the brittle crust. Across this fault system the deformation partitioning mode is nearly distributed. Active faulting and prehistoric surface rupture suggest that the Big Bend RRF has the capability to produce large earthquakes at recurrence intervals of millenniums, much longer than the local written earthquake history of 300-500 years.

Earthquake prediction practice and a large number of earthquake cases show that anomalous images of small earthquake belts may appear near the epicenter before strong earthquakes. Through the research of earthquake cases, researchers have a relatively consistent method to determine the clarity of an identified seismic belt, but there is still a lack of method on seismic belt identification from the distribution of scattered points. Due to the complexity of exhaustive algorithm, the rapid automatic identification technique of seismic belts has been progressing slowly. Visual recognition is still the basic method of seismic belt identification. Based on the algorithm of distance correlation, this paper presents a fast automatic identification method of seismic belts. The effectiveness of this method was proved by 100 random earthquakes and an example of seismic belts of magnitude 4.0 before the 2005 Jiujiang M5.7 earthquake. The results show that: ① the automatic identification of seismic belts should first identify the “relational earthquake”, then identify the “suspected seismic belt”, and finally use the criterion of seismic belt clarity to determine; ② random earthquakes and real earthquakes identification results show that the distance correlation method can realize the fast automatic identification of seismic belts by computer.

现行的基于数十至百m长的跨断层短基线的测量方法精度较高,但往往不能有效跨越大型活动断裂带进行观测;而GNSS目前受站点密度及观测精度所限,对断层近场尤其是运动速率偏低的断层开展形变观测的分辨率较差.基于上述现状,在川滇块体东边界构造带新布设2个实验场地,分别跨越则木河断裂大箐梁子段和昭通断裂的龙树分支进行测量.实验利用km尺度跨距的基线测量活动断裂带的近场运动与变形,获得3a的实验观测数据.文中首先介绍了场地选建、 监测断裂、基线测量方法以及实验观测结果,然后利用测量资料,基于刚体、弹性和组合模型3种假设条件计算分析这2个场地的断层近场位移和应变.则木河断裂大箐梁子段在刚体模型下2盘近场平行断裂走向的位移分量在±3mm内波动,无明显趋势变化;垂直断裂走向的位移分量在2015—2016年持续下降,反映断裂呈横向水平压缩,累计降幅达6mm,但2017年出现近2mm的横向水平拉张;弹性模型下该断裂段的横向水平应变分量εy以挤压为主,年变化幅度接近1×10-5,另外2个应变分量均为10-6量级.昭通断裂龙树分支2盘近场的相对位移虽然变化较小,但表现出与该断裂地质活动一致的右旋走滑特征,位移速率约0.7mm/a;沿该分支断层走向的应变分量εx为挤压状态,量值不超过2×10-6;垂直断层走向的应变分量εy则以拉张为主.文中还讨论了应用组合模型的效果与问题.

Earthquake prediction practice and a large number of earthquake cases show that there may be abnormal images of small earthquake belts near the epicenter before strong earthquakes occur. For a static small earthquakes spatial distribution, due to the complexity of exhaustive algorithm, the fast automatic identification method of seismic belts has not yet been realized. Visual identification is still the main method of seismic belt discrimination. Based on the Delaunay triangulation, this paper presents a fast automatic identification method of seismic belts. The effectiveness of this method is proved by a 1000 random points test and an actual example of the 4-magnitude belts before the 2005 Jiujiang M 5.7 Earthquake. The results show that: (1) Using Delaunay triangulation method, we can fast get the spatial relationship between two neighboring points; (2) using the two neighboring relationships, it can automatically extend to cluster, which carries the key information of seismic belt; (3) using the technology of minimum enclosing rectangle (MER) for the identified cluster, we can get the shape and structural information of the MER, which can be called the “suspect seismic belt”; (4) after using the other restrictions to sort and filter the suspect seismic belt, we complete the identification of seismic belt; (5) the random and actual earthquakes trial calculation shows that the Delaunay triangulation method can realize a fast automatic identification of seismic belts; and (6) this automatic identification method may provide a research basis for earthquake prediction.

Based on fundamental mode Rayleigh waves from distant earthquakes recorded at 245 broadband seismic stations within Sichuan Basin and surrounding areas, we imaged phase velocity and azimuthal anisotropy of Rayleigh wave at periods from 20 to 120 s. Together with previous studies, we discussed the coupling between the crust and the upper mantle. Image of phase velocity at short periods (20 similar to 30 s) shows good coincidence with geological structures of the Sichuan Basin. Longmenshan Fault and Xianshuihe Fault, which are surficial tectonic boundaries between Chuan-Dian block, Songpan-Garze block, and Sichuan Basin, have a role controlling the velocity structure of the crust. The Songpan-Garze and the Chuan-Dian blocks show lower velocity in the middle and lower crust, which is a sign of relatively soft lithologies. Whereas, the middle and lower crust beneath the Sichuan Basin is relatively strong with higher velocity. In the southern Sichuan Basin, areas close to the boundary of north Yangzi block and south Yangzi block show higher velocity anomaly. The southern Sichuan Basin and its southeast surrounding areas demonstrated stronger anisotropy with an almost uniform fast wave direction for different periods, indicating strong coupling between different depths. In the western Sichuan Basin and its north and northeast surroundings, direction of the fast wave varies with period, indicating weak coupling. All these features agree with the understanding that the strong Yangzi block plays a role resisting the clockwise rotational flow around the east Himalaya syntaxis.

Based on records of regional and mobile seismic stations (near the studied epicentral area), we have relocated the M(s)7.0 mainshock and M-L >= 3.0 aftershocks in the 2017 Jiuzhaigou, Sichuan, earthquake sequence by using first arriving P-phase only. Then, we analyzed the relationship between aftershocks distribution and seismotectonic structure by using the high-precision relocation result, and further interpret the structure of seismogenic fault zone. Our study has yielded some new conclusions: (1) The Jiuzhaigou mainshock is now relocated near the southern end of the middle segment of the aftershock zone, with a focal depth of 16 km; most of the aftershocks are relocated at depths of 4 to 17 km. (2) Along the aftershock zone, the distribution density of aftershocks is obviously related to the size of co-seismic slip of the mainshock. On the middle segment of the aftershock zone, an area with sparse aftershocks at depths of 8 to 16 km coincides with the location with relatively large co-seismic slip, and therefore it is inferred as the position of the main asperity of the seismogenic fault zone or as the place where the strain released thoroughly during the mainshock. On the southeastern segment of the aftershock zone, an area with dense aftershocks exists at depths of 10 to 18 km, where all the three M-L >= 5.0 aftershocks occurred, and coincides with one of the areas of co-seismic slip deficit. On the northwestern segment of the aftershock zone, an area at the depth range below 5 to 10 km is not only one place lacking aftershocks but also one of the areas of co-seismic slip deficit, and this situation would be related to the structural complexity due to the intersecting or merging of several faults there. On the middle and northwestern segments of the aftershock zone, no aftershocks occurred at the depth between 3 and 5 km, which coincides with a shallow area of co-seismic slip deficit. (3) It is confirmed in this study that the seismogenic structure of the Jiuzhaigou earthquake is the northern segment of the Huya fault zone. At the same time, it is newly revealed that the seismogenic fault zone has an upward-splay flower-like structure composed of the main fault and several branch faults, with a scale of similar to 4. 5 km (at the largest) in width and similar to 35 km in length, and with the main fault dipping steeply toward SW. These reflect that the mainshock rupture may not only be controlled by a single fault, but probably the main rupture occurred along the main fault and the secondary ruptures along several branch faults. In addition, in this study the interpretation of seismogenic faults for every segment is the result using the basic principle of structural geology to analyze hypocentral alignment, focal mechanism solutions, positions of surface faults and interpretations for adjacent segments, rather than inferring only from dense distribution of aftershocks.

This study examines the ~200-km-long northwestern Xianshuihe fault zone (NWXFZ), southwestern China, using more than three decades of geodetic observations from fault-crossing short-baseline and short-leveling surveys at seven sites. These data enable estimates of creep rates and depths, and examination of the long-term slip behavior. The surface motion of the NWXFZ is dominated by sinistral creep, although sinistral, transverse, and vertical slip components show spatio-temporal variations. Combining these slip variations with data of earthquake rupture, coseismic slip, seismicity, fault geometry, and far-fault movement velocity, and using the velocity-and-state friction theory, our analysis indicates that the surface slip behavior of the NWXFZ is segmented along strike. The 1973 rupture section of this fault zone exhibits spatio-temporally variable slip behavior, showing time-decaying post-1973 afterslip on the northwestern and southeastern parts of the rupture at depths above 5.8−7.0km with average sinistral-creep rates of 1.3 and 3.5mm/yr, respectively, but being relocked in the central part of the rupture. The 1923/1981 rupture section is generally in locking state, with postseismic and interseismic sinistral-creep at 1.1mm/yr on its central part at depths above 2.0–2.8km. The 1893 rupture section has been tightly locked without creep since at least the early 1980s. The thickness of the shallow velocity-strengthening (or creep) layer and the restraining bend geometry of the NWXFZ are the key factors that control spatio-temporal variations in surface creep rates. Two surface-observed locked fault portions are located within two different restraining bends in the NWXFZ, both of which act as compressive asperities and hence have enabled the long-term locking of these portions. Creep along the NWXFZ has also been affected to varying degrees by M6.5−Mw9.2 earthquakes at distances of 50−3800 km from the fault zone. Most of these effects have been removed from our estimates of the long-term average creep rates.

This work was based on integrated multidisciplinary data, including active tectonics, focal mechanism solutions, relocated small earthquakes, historical and modern seismicity, as well as GPS velocity fields. The authors have studied the Liupanshan fault zone (LPSFZ) in China, analyzing its tectonic-dynamic conditions and deformation style, cross-section structure, historical ruptures, GPS deformation, as well as modern micro-seismicity. Furthermore the authors discussed major earthquake hazard associated with the LPSFZ and one of its adjacent fault zones. The study suggests that the NNW-trending LPSFZ has been moving and deforming under the horizontal compression due to the resistance of the stable western part of the North China block, the Ordos block, to the eastward extrusive motion of the northeastern Tibetan block. Besides, lying at the right stepover or restraining bend between the NW-trending and left-lateral strike-slip Haiyuan and Longxian-Baoji fault zones, the LPSFZ absorbs and transforms the locally horizontal convergence of the two strike-slip fault zones. On the cross sections, the LPSFZ shows its characteristic as a large-scale reverse fault thrusting to the east, with a deep detachment zone at depth of 25 km. A seismic gap with a total length of 120 similar to 140 km exists along the middle-southern segment of the LPSFZ and the farther southeastern Longxian-Baoji fault zone (LXBJFZ), in which no M >= 61/2 events have occurred during the last similar to 1400 years at least. GPS deformation suggests that the two fault zones along the seismic gap have accumulated significant amount of elastic strain. Furthermore, that modern small earthquakes along several parts of the two fault zones are either sparsely distributed or absent also suggests that some fault patches are locked. In addition, an area with long-term low b-values is present along the middle-southern segment of the LPSFZ and the northernmost segment of the LXBJFZ, probably indicating high stress built up there. Therefore, the middle-southern segment of the LPSFZ and the LXBJFZ should be two fault sections that have high potential of major earthquakes in the future. The estimated maximum moment magnitudes for the potential events are M(w)7. 3 +/- and M(w)7.2 +/-, respectively.

在前人研究的基础上,收集了相对丰富的GPS、InSAR、强震动等同震观测数据,借助重新构建的发震断层模型开展了汶川地震同震位错分布的反演工作,分析了位错分布特征及可能的影响因素,并就目前汶川同震模型研究中存在的一些争议进行了讨论分析.

The electrical structure of western Yunnan is revealed by a NW-SE trending Mouding-Shangri-La magnetotelluric (MT) sounding profile. It shows a huge sub-horizontal high conductivity layer (HCL) in crust beneath northwestern Yunnan. This high conductivity layer extends from Shangri-La to east of the Yongsheng basin for at least 200 km with thickness more than 40 to 50 km and a depth of the bottom about 80 to 90 km. The HCL can be divided into 2 sections. The west section extends from Shangri-La to the east of the Lijiang basin, steeping upward in east Lijiang with depth less than 10 km from the earth surface. The east section extends from the east Lijiang basin to the Yongsheng basin and steepens upward near the Yongsheng basin. The two HCL sections form a composite shape in the profile like two bananas linked each other. The two high resistivity bodies (HRB) on the top of the HCL look like two HRB bowls. A comprehensive analysis of the tectonics is made based on 2-D inversion of MT data in combination with geology and relocation of small earthquakes. The tectonic pattern of the study area is characterized by a system of imbricate thrust nappes, which slip up and down stretching at least a total length more than 250 km. The first main nappe stretches more than 150 km, and rises up in the east of Lijiang close to the surface. Of this nappe, the Xiaojinhe fault is the main fault which extends to the surface. The second main nappe is more than 120 km long, extending nearly to the earth surface in the east of Yongsheng after crossing the Chenghai fault. According to the reverse low resistance images, we infer the existence of back thrusting structure that forms pop-ups in two regions. One lies at the east margin of Jade Dragon Mountain to the east of Lijiang, while the other is at the western margin of the Yongsheng basin to the Pingchuan basin. Listric faults in the profile reveal a gentle-steep-gentle-steep shaped ladder-like structure.

leveling and strong motion records to constrain the inversion. Our interpretation result of fault geometry shows that there exist 5 relevant faults named F1 to F5. We tested all of the possible combination models of different faults and the inversion results indicated that the combination of faults F1, F3, F4 and F5 can fit the observation data best, and may be the most probable seismogenic fault model. The inverted geodetic moment of Lushan mainshock in our research is about M(w)6. 5, and most of the coseismic slip are distributed around the ramp of fault F1, which is dipping to the NW, and the maximum value is 0. 86 m, as a thrust faulting with the rake of 92. 88 degrees. The slip on fault F3, which is one of the back-thrust secondary faults located above fault F1, is dominated by thrust motion with a slight dextral component, of which maximum value is 0. 37 m with the rake of 119. 92 degrees, while the slip on the other one named fault F4 is almost pure thrusting with the max value of 0. 40 m and the rake of 97. 98 degrees. A shallow decollement named F5 in this paper is identified existing in the seismogenic zone at the depths of 5 to 8 km, dipping to the NW gently. It separates the shallower sedimentary cover from the deeper metamorphic basement, and prohibits the coseismic rupture of F1, F3 and F4 spreading to the shallower layer. When the mainshock occurred, the pop-up structure, which is a wedge-shape rock restricted by the faults F1 and F3, moved upward, leading to the responding movement and deformation of the shallower layer and ground surface, and also caused the layer under the NE and SW segments of fault F5 to slip to the NE and the SWW, respectively, with the maximum value of 0. 25 m. In conclusion, the inversion result based on the combined seismogenic model of faults F1, F3, F4 and F5 in this paper can fit various kinds of the surface observation data very well, and also explain the "incompatibility" between the "sinistral" motion observed by GPS and the pure thrust faulting confirmed by seismological outcomes.

Quantitative analysis of the kinematics of the active faults distributed around the Qinghai-Tibetan Plateau is critical to understand current tectonic processes of the plateau. Chronological analysis, based on the comparison among regional climate and geomorphology, digital photogrammetry, offset landforms, and the tectonics were adopted in this study on the Xianshuihe fault in the eastern Tibetan plateau. Two or more offset-age data were obtained for each segment of the Xianshuihe and the Yunongxi faults. The offset landforms, including river terrace, alluvial fan and glacial moraine, provide constraints for the late Quaternary slip rate of the Xianshuihe fault. The left-lateral strike slip rate of the Xianshuihe fault decreases from 17 mm/a on the northwest segment to 9.3 mm/a on the southeast segment. Regarding the Xianshuihe fault zone and its adjacent blocks as a regional tectonic system, vector analysis was used to quantitatively analyze the longitudinal kinematical transformation and transversal slip partitioning on the fault zone in terms of the kinematical parameters of the main faults within the zone. The results show that there is a distributed vertical uplift at a rate of 6.1 mm/yr caused by shortening across the Gongga Mountains region. Based on these results, we established a model of the slip partitioning for the southeastern segment of the Xianshuihe fault zone.

通过对滇西NW-SE向牟定—香格里拉大地电磁测深剖面的反演并结合地质构造、小震精定位资料,综合分析了剖面经过地区的深部构造.在滇西北地区深部发育一规模巨大的近水平产出地壳高导层,此高导层从香格里拉一直延伸到永胜盆地以东,沿剖面水平延伸超过200 km.地壳高导层又可分为两段,两段高导层在剖面上形成两个香蕉型相连的复合形态,高导层之上的高阻体则形成两个碗型相连的复合结构,碗型内部有向东倾斜的相对低阻带存在.滇西北地区整体构造格架可以解译为一套两个主推覆面构成的叠瓦式推覆系统,两个主推覆面高低起伏形成总长度超过250 km的底部滑脱带.第一个主推覆面的长度超过150 km,并在丽江东部上翘到接近地表,小金河断裂是其延伸到地表的主要破裂带.第二个主推覆面长度超过120 km,过程海断裂后上翘并在永胜以东地区出露地表.根据反向低阻带影像,可以推断推覆体反冲构造发育,在玉龙山至丽江以东和永胜盆地西缘至平川盆地东缘形成两处冲起构造.推覆面在剖面上呈现出平缓-陡峭-再平缓-再陡峭的断坪和断坡相间的阶梯状结构.

利用南北地震带北段的区域GPS资料，结合区域的地震构造以及中小地震活动等信息进行分析，研究六盘山断裂带区域的地壳形变特征，探讨引起区域地壳形变的断裂带构造活动，并由此认识六盘山断裂带地震危险背景。

On 22 Nov. 2014, an M6.3 earthquake hit Kangding County in Sichuan Province, southwestern China. 3 days later, another M5.8 earthquake occurred in the same region, about 10 km southeastwards from the epicenter of the M6.3 mainshock. Both earthquakes were on the NW-striking Xianshuihe fault zone where no M >= 6.0. earthquakes were reported after the 1982 M6.0 Ganzi event. From the relocated aftershock distribution and focal mechanism solutions, we aim to analyze the seismogenic structure of the 2014 M6.3 Kangding earthquake sequence. Along with the analysis on the characteristics of the strong earthquake ruptures and the spatial distribution of the relocated small earthquakes, we will further discuss the future strong-earthquake risk between Daofu and Kangding on the central segment of the Xianshuihe fault zone. Based on the digital waveform data from China National Seismic Network and Sichuan Regional Network, the Kangding earthquake sequence was relocated by a multi-step locating method developed by Long et al. The focal mechanism solutions and the centriod depths of the M6.3 and M5.8 earthquakes were inverted simultaneously by the gCAP (generalized Cut and Paste) moment tensor inversion method. The spatial-temporal distribution of the M >= 6.5 strong earthquake ruptures since 1725 and the focal depth distribution of relocated small earthquakes from Jan. 2001 to Oct. 014 along the central-southern segment of the Xianshuihe fault zone were used to identify the potential seismogenic region of the next strong earthquakes on the segment between Daofu and Kangding. The epicentral relocation of the M6.3 mainshock is at 101.69 degrees E 30.27 degrees N, and its initial rupture depth is about 10 km, while the centroid depth is 9 km; the relocated M5.8 earthquake is at 101.73 degrees E, 30.18 degrees N with the initial rupture depth at about 11 km and the centroid depth of 9 km. The moment tensor solutions from gCAP method show that the two events are dominated by the double-couple component. The parameters of the best double-couple solutions are as follows, strike 143, dip 82, rake 9 for nodal plane I, and strike 234, dip 81, rake 172 for nodal plane II for the M6.3 earthquake. For the M5.8 earthquake, the parameters are listed as, strike 151, dip 83, rake 6 for the nodal plane I, and strike 242, dip 84, rake 173 for the nodal plane II. Most aftershocks during the first 3 days were distributed on the NW side of the M6.3 mainshock, and majority of the aftershocks after M5.8 earthquake were concentrated around the epicenter of the M5.8 event. The average focal depth of the 459 relocated earthquakes of the sequence is about 9 km. Focal depth distribution reveals that the sequence mainly concentrated in the depth range of 6 similar to 11 km and most of the aftershocks are shallower than the M6.3 and M5.8 earthquakes. The seismic source scale is estimated to be about 30 km in length and 4 km in width with 6 km in depth according to the aftershock distribution. On the space-time diagram of the historical M >= 6.5 strong earthquake ruptures, we observe a gap on the Selaha fault of the central Xianshuihe fault zone, where no M >= 6.5 earthquakes occurred since the 1748 M61/2 event. Aseismic gap below the depth 7 km between Kangding and Tagong and a low-seismicity region below the depth 2 km between Tagong and Songlinkou were identified on the vertical cross-section of the relocated small earthquakes since 2001 along the central-southern segment of the Xianshuihe fault zone. The nodel plane I of the focal mechanism solution was interpreted as the coseismic rupture. plane for the M6.3 and M5.8 earthquakes based on the aftershock distribution and the fault strike. Both earthquakes are of left-lateral strike-slip faulting with some normal component. The relocated M6.3 earthquake and its aftershocks during the first three days are on the NW-striking Selaha fault with a near-vertical dip angle of 82 degrees, while the M5.8 earthquake and its adjacent aftershocks are on the northern portion of the NW-striking Zheduotang fault with a. dip angle of 83, implying that the M5.8 earthquake on the Zheduotang fault may be induced by the M6.3 earthquake in the adjacent Selaha fault. The scarce aftershock region around the M6.3 mainshock may belong to a relatively large asperity where the accumulated energy was totally released as the M6.3 mainshock occurred. The 2014 M6.3 Kangding earthquake sequence occurred within the Selaha strong-earthquake rupture gap between Qianning and Kangding. Due to the duration of quiescence longer than the estimated average recurrence interval for the M7 earthquakes, we propose that the ruptures of the M6.3 and M5.8 earthquakes are too limited to fill up the gap, posing future M7 earthquake risk on the Selaha and its adjacent Qianning segments along the central segment of the Xianshuihe fault zone. Since most area of the previous seismic gap below the depth 7 km between Kangding and Tagong along the central-southern Xianshuihe fault zone was filled by the 2014 M6.3 Kangding earthquake sequence, and the most likely place of future strong earthquake occurrence will be below the segment between Tagong and Songlinkou where low seismicity is observed.

Using the seismic data recorded by the Yunnan Regional Seismic Network from Jan.1,1973 to Nov.5,1988,we obtained the spatial distribution of several seismicity parameters( including the a and b values in the frequency-magnitude relationship,and the local recurrence intervals TL) in seismogenic and its surrounding area before Lancang-Gengma M7.6 and M7.2 earthquakes in 1988 to reveal the stress state in the seismogenic area before the earthquake.The main results show that the spatial distribution of seismic activity parameters was significantly heterogeneous in the study area before the Lancang-Gengma earthquake.The seismogenic area of the Lancang M7.6 earthquake had the characteristic of the abnormal low b-value that were much lower than the regional average( b 0.7),low a-value,and the shortest recurrence intervals TL.All these implied that the fault where the focal located might have been locked under the high stress and more likely to generate major earthquakes.

在内蒙古河套断陷带中—西段,1979年8月和1996年5月分别发生了五原6.0级地震和包头6.4级地震.针对这2次地震的发震构造,在前人研究工作的基础上,进一步集成活动构造、石油物探、重新定位的地震分布、烈度分布以及震源机制解等信息进行分析,构建通过这2次地震震源区的地震构造剖面,并重新确定了相应的发震断层.结果表明:1979年五原6.0级地震更可能是走向近EW、倾向S的色尔腾山山前主断层发生正断层作用的结果;而1996年包头6.4级地震更可能是乌拉山凸起之下的一条走向NWW、倾向SSW的无名隐伏断层发生斜滑正断层作用的结果.对这2次地震发震构造的新认识,能够最大程度符合并解释各自震源区的地表活动构造、余震分布、主震的烈度分布与震源机制解以及主震时的地面宏观破坏等现象.惟一不能完全解释的是包头地震的极震区(Ⅷ度区)面积约有2/5位于所判定的发震断层北侧(下盘).另外,包头地震发震构造的例子显示出在大型活动断陷带内部的相对凸起区之下,也可能存在具有发生强震能力的活动性正断层或斜滑正断层.

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.