Strong Earthquake Activity and Its Relation to Regional Neotectonic Movement in Sichuan-Yunnan Region

Based on analyzing space inhomogeneous image of strong earthquake activity, the image of source rupture and the mechanical property of the source fault in Sichuan-Yunnan region, the relations among the strong earthquake activity, active fault, modern movement status of active blocks and structural background of the deep media have been discussed, and the characteristics of strong earthquake activity and possible mechanism have been also discussed.

Magnitude and distance distribution of strong aftershocks in Sichuan-Yunnan region

研究了川滇地区1966年以来12次主震震级Ms≥6.5的地震序列中，主震与强余震（本文定义为所有Ms≥5的余震）震级差分布特征和强余震与主震距离的分布特征。结果表明，强余震与主震震级差服从截断的指数分布，据此推导出了强余震与主震震级差的概率密度函数；强余震距离分布的优势范围是距主震10-39km，且强余震与主震震中的距离服从正态分布。

The Two-dimensional Time Coordinate System and Time Prediction Research of M≥6.7 Strong Earthquakes in the Sichuan-Yunnan Region

Since the 20 thcentury,the time intervals of M ≥6.7 strong earthquakes in the SichuanYunnan region show obvious regularity.Using the years of the strong events,a twodimensional time coordinate system is generated,based on which,the time prediction model is constructed for strong earthquakes in the Sichuan-Yunnan region.Prediction analysis shows that there is risk of generating four earthquakes with M ≥ 6.7 in the Sichuan-Yunnan region in the future 16 years,and there are strong signals for M ≥6.7earthquakes for periods 2012-2021 and 2025-2029.The strong earthquakes may occur around 2014-2015,2019 and 2027.

Study on the Relationship Between Recent Crustal Deformation and the Temporal-Spatial Distribution of Strong Earthquakes (M_s≥6.0) in the Sichuan-Yunnan Region

Analysis of deformation data measured across the faults, regional vertical deformation data and GPS measurements in the Sichuan-Yunnan region made since the 1980s permitted us to conclude that the crustal deformation in the region during this period of time was relatively weak and caused the occurrence of earthquakes (M S≥6.0), which were not distributed along the major boundary active faults in the region after the 1981 Dawu M S 6.9 earthquake and that the seismic activity is characterized by quasi-clockwise migration. Thus, it follows that earthquake prediction research should be focused on the central part of the Sichuan-Yunnan region in the coming years. Finally, a concept of temporal division of the region into active blocks is suggested and the preliminary result of the division is given in the paper.

SLC Analysis on Earthquake Activity in the Regions of Sichuan and Yunnan of China

Using the method of Single-link Cluster(SLC),analyzing the pattern of time sliding of SLCparameters,the earthquake activity in the western Sichuan-Yunnan of China is studiedcombining with the regional earthquake catalogue since 1970 and the tectonic background.Comparing with the high level of earthquake activity in 1970’s and the low in 1980’s,theearthquake activity is in general at the middle level in 1990’s.This paper suggests that SLCmethod,which considers the temporal and spatial relationship among earthquake events,could be adopted to analyze the trend of regional earthquake activity,it is very useful forstudying seismic activity.

Dynamic variation of ambient shear stress field before and after three strong earthquakes in Yunnan

A method calculating relative shear stress values in seismic source areas using data of seismic wave is used in thispaper which is introduced by PEI-SHAN CHEN, based on a rupture model studying seismic rupture process fromtheory of fracture mechanics. The shear stress values τ0 in mid-small earthquakes occurred in 1986~June of 1997in Yunnan and its adjacent areas are used to trace the variation process of ambient shear stress field before andafter three strong earthquakes in Yunnan and China-Myanmar border areas. The results show that there exists abackground of high values in ambient shear stress field. In the development process of a strong earthquake, theambient shear stress field in and nearby its potential source area increases obviously, for a long time, experiences aprocess of multiple down - up - down - up. The pattern of dynamic variation of ambient shear stress field hasreflected the multi-source fields of seismogenesis in and nearby Yunnan. There exist multi-increased areas of ambient shear slress fields synchronously in southwest, northwest. west and east of Yunnan, and three strong eathquakes of magnitude about 7 occurred in the most obviously increased areas of ambient shear stress fields.

Study on Integrated Recurrence Behaviors of Strong Earthquakes Along Entire Active Fault Zones in the Sichuan-Yunnan Region, China

Based on historical earthquake data, we use statistical methods to study integrated recurrence behaviors of strong earthquakes along 7 selected active fault zones in the Sichuan-Yunnan region. The results show that recurrences of strong earthquakes in the 7 fault zones display near-random, random and clustering behaviors. The recurrence processes are never quasi-periodic, and are neither strength-time nor time-strength dependent. The more independent segments for strong earthquake rupturing a fault zone has, the more complicated the corresponding recurrence process is. And relatively active periods and quiescent periods for earthquake activity occur alternatively. Within the active periods, the distribution of recurrence time intervals between earthquakes has relatively large discretion, and can be modelled well by a Weibull distribution. The time distribution of the quiescent periods has relatively small discretion, and can be approximately described by some distributions as the normal. Both the durations of the active periods and the numbers of strong earthquakes within the active periods vary obviously cycle by cycle, leading to the relatively active periods having never repeated quasi-periodically. Therefore, the probabilistic assessment for middle- and long-term seismic hazard for entireties of active fault zones based on data of historical strong earthquakes on the fault zones still faces difficulty.

Basic Characteristics of Active Structures in Western Sichuan and Its Vicinity and Strong Earthquake Recurrence Model

Western Sichuan and its vicinity are located in the juncture of three big active blocks, namely, the Chuandian (Sichuan and Yunnan), the Bayan Har and the South China blocks, on the eastern margin of the Qinghai-Xizang(Tibet) Plateau. Many groups of active faults that are capable of generating earthquakes are developed there. Because there exist lateral secondary active faults, the Chuandian block can be further divided into the central Yunnan and northwestern Sichuan sub_blocks; while the Longmenshan sub_block can be divided on the east end of the Bayan Har block. Joint exploration of deep crustal structure shows that there exist low-velocity and high-conductivity layers in the crust of the Chuandian and Bayan Har blocks, which are one of the important factors that make the upper crust prone to earthquake. The results of geological study and modern GPS observation show that blocks of different orders all have SE-or SSE-trending sliding, clockwise rotation and upwelling movement; but there are some differences in amplitude. This paper has also given the geological or GPS slip rates of main active fault zones and discussed the main scientific problems still existing now.

Influence of the Kunlun Mountain M_S8.1 Earthquake on Horizontal Crustal Deformation in the Sichuan and Yunnan Area

In order to track the space-time variation of regional strain field holistically(in a large scale) and to describe the regional movement field more objectively,the paper uses a nonlinear continuous strain model focused on extracting medium-low frequency strain information on the basis of a region with no rotation.According to the repeated measurements(1999～2001～2004) from GPS monitoring stations in the Sichuan and Yunnan area obtained by the Project of "China Crust Movement Measuring Network",and with the movement of 1999～2001(stage deformation background) as the basic reference,we separated the main influencing factors of the Kunlun Mountain M-S8.1 earthquake in 2001 from the data of 2001 and 2004,and the results indicate:(1) the Kunlun Mountain M-S8.1 earthquake has a discriminating effect on the Sichuan and Yunnan area,moreover,the deformation mode and background had not only certain similitude but also some diversity;(2) The movement field before the earthquake was very ordinal,while after the earthquake,order and disorder existed simultaneously in the displacement field;The displacement quantities of GPS monitoring stations were generally several millimeters;(3) The principal strain field before earthquake was basically tensile in an approximate EW direction and compressive in the SN direction,and tension was predominant.After the earthquake,the principal strain field in the Sichuan area was compressive in the EW direction and tensile in the SN direction,and the compression was predominant.In the Yunnan area,it was tensional in the NE direction and compressive in the NW direction,and tension was predominant;(4) The surficial strain before the earthquake was dominated by superficial expansion,the contractive area being located basically in the east boundary of Sichuan and Yunnan block and its neighborhood.After the earthquake,the Sichuan area was surface contractive(the further north,the greater it was),and south of it was an area of superficial expansion.Generally speaking,the Kunlun Mountain M-S8.1 earthquake played an active role in the accumulation of energy in the Sichuan and Yunnan area.Special attention shall be focused on the segment of Xichang-Dongchuan and its neighborhood.

Deep medium environment of strong earthquakes occurrence in Yunnan region

The deep structure background of earth medium for strong earthquakes ccurrence in Yunnan area is discussed inthis paper, by using the results on the study of the velocity structure, elect fieal conductivity stricture, geothermalstructure in the crust and upper mantle in Yunnan area. The results show that the occurrence of strong earthquakes in Yunnan region is obviously related to the deep medium and tectonic environment such as the existenceof the high velocity zone in the upper crust, the low velocity zone or high electrical conductivity layer in themiddle crust, local uplift in the upper mantle, high geothermal activity and deep and large fault, etc. The large earthquakes could not take place at anywhere, they often occur at some regions which have a certainbackground in the deep medium structure. The activity of the earthquakes with magnitude of 5 or less is quite random,the occurrence of them have not the obvious background of the deep medium strUcture.

Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults＇ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.

Study of the Seismicity of Strong Earthquakes in the Yunnan Area

Yunnan is located in the east margin of the collision zone between the India Plate and the Eurasian Plate on the Chinese Continent, where crustal movement is violent and moderate-strong earthquakes are frequent. In addition, the area features marked active block movement. Therefore, Yunnan is a perfect place for research on strong earthquake activity. Through the study on the temporal and spatial distribution of the M≥6.7 earthquakes and the related earthquake dynamics in Yunnan in the last century, we conclude that the four seismically active periods, which are characterized by alternative activity in the east and the west part of Yunnan, possibly result from a combination of active and quiescent periods in each of the east and west part. And for every 100 years, there may be a period in which strong earthquakes occur in the east and west parts simultaneously. In addition, the seismicity of strong earthquakes in Yunnan corresponds well to that in the peripheral region. The seismicity of the great earthquakes in the Andaman-Myanmar Tectonic Arc belt indicates, to some extent, the beginning of a seismically active period in Yunnan. The seismicity of strong earthquakes in east Yunnan is closely related to that in Sichuan. Strong earthquakes in Sichuan often occur later than those in Yunnan. Furthermore, in the east part of Yunnan, the three procedures including continuous occurrence of moderate-strong earthquake, quiescent period, and the occurrence of the first strong earthquake may be the style of the beginning of the earthquake active period. The above cognition is helpful to the study of earthquake prediction, seismogenic mechanism, and the dynamics of the plate margin in Yunnan.

Types of focal mechanism solutions and parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces

Based on P- and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes （1994-2005） in four sub-blocks of Sichuan and Yunnan Provinces, namely Sichuan-Qinghai, Yajiang, Central Sichuan and Central Yunnan blocks. Combining these calculation results with those of the focal mechanism solutions of moderately strong earthquakes, we analyzed the stress field characteristics and dislocation types of seismogenic faults that are distributed in the four sub-blocks. The orientation of principal compressive stress for each block is： EW in Sichuan-Qinghai, ESE or SE in Yajiang, Central Sichuan and Central Yunnan blocks. Based on a great deal of focal mechanism data, we designed a program and calculated the directions of the principal stress tensors, σ1, σ2 and σ3, for the four blocks. Meanwhile, we estimated the difference （also referred to as consistency parameter θ^- ） between the force axis direction of focal mechanism solution and the direction of the mean stress tensor of each block. Then we further analyzed the variation of θ^- versus time and the dislocation types of seismogenic faults. Through determination of focal mechanism solutions for each block, we present information on the variation in θ^- value and dislocation types of seismogenic faults.

Study on Postseismic Impact of Wenchuan Earthquake on the Sichuan-Yunnan Region Based on Three-Dimensional Viscoelastic Finite Element Method

Based on the lateral segmentation and vertical stratification characteristics of the crustal medium in Sichuan-Yunnan region,and the asymmetry of the static dislocations on the coseismic fault plane of the Wenchuan M_S8.0 earthquake,we built a three-dimensional viscoelastic finite element model of the crust in the Sichuan-Yunnan region.The postseismic impact of the Wenchuan M_S8.0 earthquake on the Sichuan-Yunnan region was studied.The results show that:①The far-field horizontal deformation caused by the viscoelastic relaxation of the medium in the 10 years after the earthquake is about 0-20 mm within the Sichuan-Yunnan diamond-shaped block,which has a greater influence on north side and smaller on south side.②In the 10 years after the earthquake,the far-field vertical deformation caused by the viscoelastic relaxation effect of the medium is small,and it shows an increase of about 0-4 mm in most areas of the Sichuan-Yunnan diamond-shaped block.③The Xianshuihe fault and the eastern segment of the East Kunlun fault,which are close to the seismogenic fault,show a high gradient on deformation fields after the earthquake.④In order to compare with the strong earthquake activity in the SichuanYunnan region after the Wenchuan earthquake,the horizontal stress state and the Coulomb failure stress change of the active block boundary are also calculated.From the spatial distribution of the coseismic and postseismic displacement field,the fault activity characteristics reflected by the stress state and the stress loading of the fault layer reflected by the Coulomb failure stress change,there is a certain correlation with the spatial distribution of strong earthquake activity in this region.

Subarea characteristics of earthquake types in Yunnan

Studies on the earthquake sequences and the source mechanisms of the strong earthquakes show that Yurman has more obvious subarea characteristics of earthquake type. Strike-slip seismic fault and mainshock-aftershock earthquake sequences are dominant in whole Yunnan area. Considering the ratio of non strike-slip faults and non mainshock-aftershock, Yurman area can be divided into four subareas with different characteristics, which are strike-slip mainshock-aftershock in central Yunnan （A1）, incline-slip swarm in northwestern Yunnan （A2）, strike-slip double shocks in western Yunnan （B1） and quasi-strike-slip mainshock-aftershock in southwestern Yunnan （BE）, respectively.

Temporal variation of gravity field prior to the Ludian Ms6.5 and Kangding Ms6.3 earthquakes

Using mobile gravity data from the central area of Sichuan and Yunnan Provinces, the relationship between gravity variation and earthquakes was studied based on the Ludian Ms6.5 earthquake that occurred on August 3rd, 2014, and the Kangding Ms6.3 earthquake that occurred on November 22 nd, 2014; the mechanism of gravity variation was also explored. The results are as follows：（1） Prior to both earthquakes, gravity variation exhibited similar characteristics as those observed before both the Tangshan and Wenchuan earthquakes, in which typical precursor anomalies were positive gravity variation near the epicenter and the occurrence of a high-gravity-gradient zone across the epicenter prior to the earthquake.（2） A relatively accurate prediction of the occurrence locations of the two earthquakes was made by the Gravity Network Center of China（GNCC） based on these precursor anomalies. In the gravity study report on the 2014 earthquake trends submitted at the end of 2013, the Daofu-Shimian section at the junction of the Xianshuihe and Longmenshan fault zones was noted as an earthquake-risk region with a predicted magnitude of 6.5, which covered the epicenter of the Kangding Ms6.3 earthquake. In another report on earthquake trends in southwestern China submitted in mid-2014, the Lianfeng, Zhaotong fault zone was also classified as an earthquake-risk region with a magnitude of 6.0, and the central area of this region basically overlapped with the epicenter of the Ludian Ms6.5 earthquake.（3） The gravity variation characteristics are reasonably consistent with crustal movements, and deep material migration is likely the primary cause of gravity variation.

Characteristics of isostatic gravity anomaly in Sichuan-Yunnan region,China

Sichuan-Yunnan region in China, a tectonic transition belt where earthquakes occurred frequently and intensely, has a distinct variation characteristic of gradient zone of Bouguer gravity anomaly （BGA）. Many deep faults and epicenters of severe earthquake scatter along the BGA gradient zones. Here we apply two forward models （Airy model and Vening Meinesz model） of isostatic gravity mechanisms （local versus regional） in this region to calculated the isostatic gravity anomaly （IGA）. Afterwards, the relationship between IGA and distribution of faults as well as seismicity is also illustrated. The IGA results show that the two models are similar and most parts of the study area are in an isostatic state. Most featured faults are distributed along the steep anomaly gradient zones; earthquakes tend to occur in the non-isostatic area and steep gradient belt of IGA. The distribution of root thickness based on regional mechanism can be associated with the main trend of BGA variation. The regional mechanism is more plausible and closer to the reality because of its relatively further consideration of the horizontal forces derived from adjacent particles in the crust. Then we analyze the effect of isostasy on the tectonic movements and find that the isostatic adjustment is not the main cause of the continuous uplift process of Longmenshan Mountain fault zone. which is due to the Indian-Eurasian continental collision.

The Recent Crustal Deformation on the Sichuan-Yunnan Rhombic Block Boundaries

Based on data from an across-fault survey along the Sichuan-Yunnan rhombic block boundaries, the recent deformation characteristics on each fault have been analyzed. It was found that the rate of crustal deformation is slowing down along the northern segment and increasing along the southern segment. Each fault has different features of deformation. The horizontal deformation is mainly characterized by left-lateral strike-slip. The rate of vertical deformation is less than that of the horizontal deformation. The faults have the feature of upper wall uplifting alternated with descending. The anomaly changes of crustal deformation at some sites are closely related to the seismicities near the sites.

云南地区强震的超长时间平静异常及未来地震趋势研究

从1996年丽江M_(S)7.0地震发生后至2023年8月,云南地区M≥6.7和M≥7.0地震出现了长达27.55 a的超长时间平静现象。本文从更大时空尺度上对该平静异常现象及其与青藏高原和川滇地块的强震活动之间的关系进行了分析和总结,并讨论了云南地区未来地震趋势。结果表明,1887年以来,云南地区5次M≥7.0地震平静期内其周边地震活动环境有很大差异性:第Ⅰ,第Ⅱ和第Ⅴ平静期出现在青藏高原M≥7.0地震活跃背景下(即云南地区平静,但青藏高原活跃),第Ⅲ和第Ⅳ平静期出现在青藏高原M≥7.0地震平静背景下(即云南地区平静,青藏高原也平静)。当前云南地区处于27.55 a超长时间平静期(即第Ⅴ平静期),其所处的周边地震活动环境与第Ⅰ和第Ⅱ平静期相似,据此推断,其后续地震趋势可能也与第Ⅰ和第Ⅱ活跃期相似,处于相对弱的活跃期。同时,未来较长时间青藏高原可能仍将处于M≥7.0地震活跃时段;2022年泸定M_(S)6.8地震后,川滇地块可能会进入M≥6.7地震活跃时段。

川滇菱形块体东边界震源机制与应力场特征

文中利用四川、云南、重庆、青海、甘肃地震台网以及西昌密集台阵和巧家密集台阵的数字地震波形资料,采用CAP全波形反演方法及HASH初动极性和振幅比方法,获得了川滇菱形块体东边界区域3 951组M L≥1.0地震的震源机制。进而基于以上震源机制,采用阻尼区域应力场反演算法(MSTASI)和Vavrycuk的迭代联合反演方法获得了研究区的构造应力场分布特征、主要活动断裂的应力性质和摩擦系数。结果显示,研究区震源机制P轴、T轴以及最大主应力轴σ1和最小主应力轴σ3总体上倾角较小,揭示了近水平的挤压或剪切应力环境。σ1以NW-SE和NWW-SEE向为主,从北到南有顺时针旋转的趋势,应力性质以走滑型为主,局部兼有逆冲型和拉张型,整体分布特征与区内走滑型边界断裂活动性质一致。R值具有明显的空间差异,鲜水河断裂-龙门山断裂-安宁河断裂交会地区R值相对较高,有明显的挤压特征;鲜水河断裂带、安宁河断裂带北段和小江断裂带的R值均在0.25~0.5之间,表现为NE-SW向挤压和NW-SE向拉张,拉张应力可能远小于挤压应力;大凉山断裂带北段和则木河断裂带的R值均在0.5~1之间,表现为NW-SE向压缩和NE-SW向拉张,且挤压应力大于拉张应力。研究区域主要断裂的摩擦系数也有差异:安宁河断裂带和大凉山断裂带北段的摩擦系数相对较低,在0.75以下,鲜水河断裂带、则木河断裂带及大凉山断裂带南段的摩擦系数偏高,在0.80以上。川滇菱形块体东边界活动断裂带上的构造应力相对较高,尤其是鲜水河断裂带和小江断裂带的应力更高,需要关注其地震危险性。