Seismicity anomalies before the great earthquake of M_S=8.1 in the Kunlun Pass and its significance to earthquake prediction

A great earthquake of MS=8.1 took place in the west of Kunlun Pass on November 14, 2001. The epicenter is lo-cated at 36.2N and 90.9E. The analysis shows that some main precursory seismic patterns appear before the great earthquake, e.g., seismic gap, seismic band, increased activity, seismicity quiet and swarm activity. The evolution of the seismic patterns before the earthquake of MS=8.1 exhibits a course very similar to that found for earthquake cases with MS7. The difference is that anomalous seismicity before the earthquake of MS=8.1 involves in the lar-ger area coverage and higher seismic magnitude. This provides an evidence for recognizing precursor and fore-casting of very large earthquake. Finally, we review the rough prediction of the great earthquake and discuss some problems related to the prediction of great earthquakes.

Late Cenozoic high-resolution magnetostratigraphy in the Kunlun Pass Basin and its implications for the uplift of the northern Tibetan Plateau

The Kunlun Pass Basin, located in the middle of the eastern Kunlun Mountains, received relatively continuous late Cenozoic sediments from the surrounding mountains, archiving great information to understand the deformation and uplift histories of the northern Tibetan Plateau. The Kunlun-Yellow River Movement, identified from the tectonomorphologic and sedimentary evolution of the Kunlun Pass Basin by Cui Zhijiu et al. (1997, 1998), is roughly coincident with many important global and Plateau climatic and environmental events, becoming a crucial time interval to understand tectonic-climatic interactions. However, the ages used to constrict the events remain great uncertainty. Here, we present the results of detailed magnetostratigraphy of the late Cenozoic sediments in the Kunlun Pass Basin, which show the basin sediments were formed between about 3.6 Ma and 0.5 Ma and the Kunlun-Yellow River Movement occurred at 1.2 to ～0.78 Ma. The lithology, sedimentary facies and lithofacies associations divide the basin into five stages of tectonosedimentary evolution, indicating the northern Tibetan Plateau having experienced five episodes of tectonic uplifts at ～3.6, 2.69－2.58, 1.77, 1.2, 0.87 and ～0.78 Ma since the Pliocene.

Pliocene cyprinids (Cypriniformes,Teleostei) from Kunlun Pass Basin,northeastern Tibetan Plateau and their bearings on development of water system and uplift of the area

Here described are the cyprinid fossils from the Pliocene Lower Member of Qiangtang Formation of the Kunlun Pass Basin,northeastern Tibetan Plateau,collected at a locality 4769 m above the sea level(asl).The materials consist of numerous disarticulated and incomplete bones as well as thousands of pharyngeal teeth,fin rays,and vertebrae.The fossils were referred to the genus Gymnocypris,lineage Schizothoracini,family Cyprinidae;the lineage Schizothoracini;and the family Cyprinidae respectively.The Schizothoracini is a freshwater fish group endemic to the Tibetan Plateau and its surrounding area.Previous workers on living schizothoracins regarded that Gymnocypris belongs to the highly specialized grade of the group,colonizing higher altitudes than other members of the group.Two species are so far unequivocally assigned to the genus,i.e.,G.przewalskii and G.eckloni,and they are inhabiting Qinghai Lake and the waters on both north(the Golmud River) and south(upper reach of the Yellow River) sides of the East Kunlun Mountain,respectively.The abundant fossil schizothoracins occur in the Kunlun Pass Basin on the southern slope of the East Kunlun Mountain(at 4769 m asl),close to the present Golmud River,indicating comparatively rich waters in the area and possible connections between the water systems on north and south sides of the East Kunlun Mountain during the Pliocene.This also suggests a more humid climate in the area during the Pliocene than it is today.The presence of the highly specialized schizothoracin Gymnocypris may also imply less amplitude of uplift(approximately 1000 m) in the area since the Pliocene than previously proposed.

Analysis of Remote Sensing Images of Ground Ruptures Resulting from the Kunlun Mountain Pass Earthquake in 2001

On November 14, 2001, an earthquake measuring a magnitude of 8.1 occurred to the west of the Kunlun Mountain Pass which is near the border between Xinjiang and Qinghai of China. Since its epicenter is located in an area at an elevation of 4900 m where the environment is extremely adverse, field investigation to this event seems very difficult. We have performed interpretation and analysis of the satellite images of ETM, SPOT, Ikonos, and ERS-1/2SAR to reveal the spatial distribution and deformation features of surface ruptures caused by this large earthquake. Our results show that the rupture zone on the ground is 426 km long, and strikes N90-110°E with evident left-lateral thrusting. In spatial extension, it has two distinct sections. One extends from the Bukadaban peak to the Kunlun Mountain Pass, with a total length of 350 km, and trending N95-110°E. Its fracture plane is almost vertical, with clear linear rupture traces and a single structure, and the maximum left-lateral offset is 7.8 m. This section is the main rupture zone caused by the earthquake, which is a re-fracturing along an old fault. The other is the section from Kushuihuan to the Taiyang Lake. It is 26 km long, trending N90-105°E, with the maximum strike-slip displacement being 3 m, and is a newly-generated seismic rupture. In a 50 km-long section between the Taiyang Lake and the Bukadaban peak, no rupture is found on the ground. The eastern and western rupture zones may have resulted from two earthquakes. The macroscopic epicenter is situated at 65 km east of the Hoh Sai Lake. The largest coseismic horizontal offset in the macroscopic epicenter ranges from 7 m to 8 m. Based on the dislocation partition of the whole rupture zone, it is suggested that this rupture zone has experienced a process of many times of intensification and fluctuation, exhibiting a remarkable feature of segmentation.

WHEN THE KUNLUN MTS.PASS AREA UPLIFTED TO PRESENT ELEVATION

Introduction There are big disputes on the uplift of the Tibetan Plateau. Some scientists believe the plateau uplifted to its highest elevation by 14Ma BP,and it decreased in elevation afterward. The second idea about the uplift of the plateau was similar to the first one in tectonic theory, but they consider the time of C3 plant change to C4 in the South Asia, as the time when the plateau reached its highest elevation. This time is 8Ma BP. The third idea, the most Chinese scientists believed, the nearest uplift of the plateau took place since 3 4Ma BP. The occurrence of fault depression and gravel deposit with large thickness in the inner and the marge of the plateau represented the speed uplift. After the collision of the India plate and the Eurasia plateau by the 36Ma, the Tibetean area uplifted several times, but it had been planed to lower area. The plateau with 4500m elevation formed in the Quaternary. While our recent research in the Kunlun Pass area suggest , the nearest uplift occurred at the border of the Early and Middle Pleistocene, and before that time, the elevation of this area was no more than 1500 m.

A discussion on Corioli force effect and aftershock activity tendency of the M=8.1 Kunlun Mountain Pass earthquake on Nov. 14, 2001

Following the theory and definition of the Corioli force in physics, the Corioli force at the site of the M=8.1 Kunlun Mountain Pass earthquake on November 14, 2001, is examined in this paper on the basis of a statistical research on relationship between the Corioli force effect and the maximum aftershock magnitude of 20 earthquakes with M7.5 in Chinese mainland, and then the variation tendency of aftershock activity of the M=8.1 earthquake is discussed. The result shows: a) Analyzing the Corioli force effect is an effective method to predict maximum aftershock magnitude of large earthquakes in Chinese mainland. For the sinistral slip fault and the reverse fault with its hanging wall moving toward the right side of the cross-focus meridian plane, their Corioli force pulls the two fault walls apart, decreasing frictional resistance on fault plane during the fault movement and releasing elastic energy of the mainshock fully, so the maximum magnitude of aftershocks would be low. For the dextral slip fault, its Corioli force presses the two walls against each other and increases the frictional resistance on fault plane, prohibiting energy release of the mainshock, so the maximum magnitude of aftershocks would be high. b) The fault of the M=8.1 Kunlun Mountain earthquake on Nov. 14, 2001 is essentially a sinistral strike-slip fault, and the Corioli force pulled the two fault walls apart. Magnitude of the induced stress is about 0.06 MPa. After a comparison analysis, we suggest that the aftershock activity level will not be high in the late period of this earthquake sequence, and the maximum magnitude of the whole aftershocks sequence is estimated to be about 6.0.

Estimation of the stress levels in the focal region before and after the 2001 M=8.1 Western Kunlun Mountain Pass earth-quake

A method estimating the stress level in the focal region of an earthquake is proposed here. Taking the 2001 M=8.1 Western Kunlun Mountain Pass earthquake as an example, we estimate its stress level in the focal region before and after it by this method. The results show that the stress level in the focal region just prior to the initiation of this event is approximately 6.3-8 MPa, and about 5-6.7 MPa remained in the focal region after its occurrence. The stress in the focal region decreased by roughly twenty percent after this event.

Characteristics of Far-field Precursory Anomalies Before the M_S8.1 Earthquake in the West of Kunlun Mountains Pass

In this study, a number of typical precursory anomalies recorded by stations in Qinghai, Gansu, Sichuan, Xinjiang, Ningxia, Hebei and Shaanxi provinces and autonomous regions before the M_S8.1 earthquake in the west of Kunlun Mountains Pass are collected and checked. According to the standards of earthquake cases in China, the criteria of the precursory anomalies are determined, and 53 distinguished. The characteristics of these anomalies before the M_S8.1 earthquake are analyzed, with results showing a very large earthquake affected area. The precursory anomalies recorded by instruments were 2900 km away from the epicenter, and according to the study in this paper, reached 2100 km away. The results also show that the anomalies present characteristics of long duration, multi-measurement items and large-amplitude variation. The authors believe that in large earthquake monitoring, attention should be paid to the variation of data over a large area, ranging up to thousands kilometers, with much denser earthquake observation networks.

Characteristics of Collapses Caused by the M8.1 Earthquake West of the Kunlun Mountains Pass

An M 8.1 earthquake that occurred west of the Kunlun Mountains Pass has caused more than 20 collapse bodies or zones, which are mainly distributed near the surface seismic rupture zone, west of Hoh Sai Lake. The collapses are of four types, bedrock, soil mass and ice mass collapses and avalanches. The spatial distribution and the characteristics of development of the collapses are analyzed in the paper. Comparised with those caused by other earthquakes, the collapses are smaller in scale. In addition to the lithological characteristics of the crustal media, topographic, geomorphic and climatic factors, weaker seismic ground motion is an important cause for formation of the smaller-scale collapses. The long surface rupture zone and weaker ground motion are important features of the seismic rupture, which may be related to the structure of the preexisting fault.

The Frozen Soils and Devastating Characteristics of West Kunlun Mountains Pass M_S 8.1 Earthquake Area in 2001

The investigation on damages to frozen soil sites during the West Kunlun Mountains Pass earthquake with M S 8.1 in 2001 shows that the frozen soil in the seismic area is composed mainly of moraine, alluvial deposit, diluvial deposit and lacustrine deposit with the depth varying greatly along the earthquake rupture zone. The deformation and rupture of frozen soil sites are mainly in the form of coseismic fracture zones caused by tectonic motion and fissures, liquefaction, seismic subsidence and collapse resulting from ground motion. The earthquake fracture zones on the surface are main brittle deformations, which, under the effect of sinistral strike-slip movement, are represented by shear fissures, tensional cracks and compressive bulges. The distribution and configuration patterns of deformation and rupture such as fissures, liquefaction, seismic subsidence and landslides are all related to the ambient rock and soil conditions of the earthquake area. The distribution of earthquake damage is characterized by large-scale rupture zones, rapid intensity attenuation along the Qinghai-Xizang (Tibet) Highway, where buildings distribute and predominant effect of rock and soil conditions.

New Insight into the Surface Rupture Parameters of the Kunlun Mountains Pass M_S8.1 Earthquake

Field observation shows that the surface rupture of the Kunlun Mountains Pass M_S 8.1 earthquake is about 426km long, and the maximum sinistral displacement is about 6m. Distribution of horizontal displacement along the surface ruptures is markedly controlled by fault structure. The rupture length of this earthquake is significantly longer than statistic value. In this paper, using the method of “ultimate linear strain", we discussed the independency and integrality of the whole rupture zone and rupture segments of the Kunlun Mountains Pass earthquake by comparing with some large earthquakes on strike-slip faults on the Chinese continent. The conclusion is that the Kunlun Mountains Pass earthquake consists of successively triggered multiple earthquake events, other than a single earthquake event.

青海省玛多县“5·22 M s7.4地震”地表破裂与次生灾害发育特征

2021年5月22日2时04分在青海省果洛藏族自治州玛多县发生7.4级地震,为了查明此次地震产生的地表破裂和伴生次生灾害发育特征,调查组采用遥感解译和实地调查方式对地震影响区进行了调查。调查结果显示,在玛多-甘德断裂和昆仑山口-江错断裂之间形成了一条宽约75 km、长约230 km的活动断裂带,地表新发现地裂缝653条、砂土液化和喷砂冒水点1237个、地震鼓包97个、塌陷坑2个。地表破裂总体呈北西—南东向展布,自西向东出现马尾状分支的现象,东段与玛多-甘德断裂带以一定角度斜交,地裂缝整体走向与昆仑山口-江错断裂走向高度一致。地震地表破裂发育特征显示,此次玛多M s7.4级地震的发震断裂为昆仑山口-江错断裂,断面整体南倾,性质为左行走滑。此次大地震的发生是在印度板块向青藏高原挤压背景下,巴颜喀拉地块强烈向东挤出构造作用导致其北部走滑断裂发生左旋运动的结果。调查结果为地方政府关于灾区地震灾害损失评估及灾后重建提供了第一手基础资料。

基于SBAS-InSAR技术的西大滩—昆仑垭口地区多年冻土形变分析

多年冻土活动层内部冰-水相变会导致多年冻土地表出现季节性的冻胀和融沉,而其上限处地下冰融化将引起地表的长期沉降,因此揭示地表形变的季节和长期变化规律可为多年冻土变化研究提供新的视角和方法。本文以青藏高原多年冻土区北界(西大滩—昆仑垭口)为研究区,利用C波段降轨Sentinel-1数据,采用SBAS-InSAR技术获取该地区多年冻土2014—2020年的地表形变时间序列结果,并基于长期形变速率和季节性形变量探讨了该地区的多年冻土形变规律。结果表明:在多年冻土北界西大滩沟谷地区,不连续多年冻土区形变空间差异较大,多年冻土区的长期沉降速率和季节性的形变量高于季节冻土区。此外,高温多年冻土地表沉降比低温多年冻土更为显著,形变空间分布特征与地貌单元紧密联系。与西大滩谷地相比,昆仑山垭口地区和楚玛尔河高平原区域的长期形变速率与季节性形变量都明显增大。同时热融湖塘的形成过程与地表形变有着直接的关联,在热融湖塘发展早期,地下冰融化使得区域季节性形变量增大,随着热融湖塘扩张,区域长期沉降速率加剧,热融湖塘进一步发展后,区域季节性形变量可能降低。

昆仑山垭口地区第四纪环境演变

昆仑山垭口地区有５个在时代上可以衔接的天然剖面，即垭口剖面（包括惊仙谷组、羌塘组、平台组和望昆冰碛层）、纳赤台沟组剖面、三岔河组剖面以及小南川和热水剖面，分别分布在垭口盆地、纳赤台后沟、野牛沟口、青藏公路原５９道班和东大滩热水地区。通过野外考察和室内分析，本区经历了以下演变阶段：３．４～２．５ＭａＢ．Ｐ．为温暖环境；２．５～１．１ＭａＢ．Ｐ．气候凉湿，高原海拔在１５００ｍ以内；１．１～０．６ＭａＢ．Ｐ．的昆仑－黄河运动将本区抬升至４５００ｍ以上，其中０．７～０．６ＭａＢ．Ｐ．期间经历了本区最大一次冰期——望昆冰期；０．６～０．３ＭａＢ．Ｐ．期间为间冰期，本区接受沉积；０．３～０．２ＭａＢ．Ｐ．发生垭口冰期；２００～１１ｋａＢ．Ｐ．为末次间冰期与末次冰期；

2001年昆仑山口西8.1级地震地表破裂带

20 0 1年 1 1月 1 4日昆仑山口西 8.1级地震是近 5 0年来在我国大陆发生的震级最大、地表破裂最长的地震事件。地震地表破裂带全长 42 6km ,宽数米至数百米 ,总体走向 90°～1 1 0°,具有明显的破裂分段特征 ,自西向东由 5条次级破裂段组成。各破裂段又由若干更次级左阶或右阶斜列的破裂组成 ,具有自相似的分形结构特征。地震破裂带以左旋走滑为主 ,倾滑量很小。宏观震中区位于库赛湖东北 93.0°～ 93.5°E一带的昆仑山南麓断层谷地内。最大地表同震左旋水平位移 6.4m ,最大垂直位移为 4m。地表水平位移沿地震破裂带走向出现 6个峰值 ,各峰值之间存在相对独立的衰减序列 ,这表明此地震具有多点破裂特征。

2001年昆仑山口西M_S8.1地震地表同震位移分布特征

沿长约 4 2 6km的 2 0 0 1年昆仑山口西MS8 1地震地表破裂带共获得 2 91个点的地表同震水平左旋位移数据 ,并在其中 1 1 1个点获得了垂直位移数据。该地震总体以左旋水平位移为主 ,兼具一定的垂直位移。最大地表左旋水平位移值可达 6 4m ,平均水平位移约为 2 7m ,绝大多数测点的垂直位移均 <1m。地表水平位移沿主破裂带走向位移梯度变化于 1 0 - 1～ 1 0 - 4之间 ,这一起伏变化可能起因于野外测量误差、沿主破裂带岩性或松散沉积物厚度的变化、地表破裂带几何结构的不均匀性、地表破裂走向的变化、不同破裂段在昆仑山口西 8 1级地震之前的地震中滑动量的起伏变化 ,以及大量非脆性变形、次级破裂的存在等。水平位移沿主破裂带的长波长 (数十公里至数百公里 )起伏变化较有规律 ,在布喀达坂峰以东表现为分别以 5个水平位移峰值为中心而有规律地起伏变化。这5个位移峰值分别对应于不同的次级地震地表破裂段。各破裂段水平位移峰值均向阶区或拐点逐渐衰减 ,不同地表破裂段位移峰值向两侧衰减的速率是不同的 ,这种位移梯度的不对称分布可能指示了地震破裂的扩展方向。上述位移分布特征真实地反映了地表可见脆性破裂带上的同震位移状况 ,可能只是该断错事件的最小滑动量。在活动构造研究中 。

昆仑山垭口地区“望昆冰期”冰碛宇宙成因核素^(10)Be测年

昆仑山垭口地区是东昆仑山现代冰川作用中心之一,在第四纪冰期间冰期旋回中保留了多套较为完整的冰川沉积.位于垭口盆地西侧山脊上的望昆冰碛,是本区已知最老的冰川沉积.应用宇宙成因核素10Be暴露年龄方法对望昆冰碛进行年代测定,5个样品中有4个介于(56.9±5.6)～(38.2±3.5)ka BP之间,相当于MIS3阶段,明显年轻于前人所测ESR和古地磁年代,且与地貌系列不符.这一结果不能代表望昆冰碛的实际年代,而应为所测漂砾后期暴露的年代,反映了MIS3阶段昆仑山垭口地区曾遭受过强烈地表剥蚀过程.望昆冰碛10Be暴露年龄显著地年轻,表明应用宇宙核素暴露年代测定冰碛物形成时代具有复杂性,应充分考虑后期剥蚀等地表过程,并应使用其他测年方法进行相互验证.

GPS初步结果揭示的中国大陆水平应变场与构造变形

根据中国大陆不同来源的多个GPS区域监测网 1991～ 1999年间的观测资料和“中国地壳运动观测网络”基本网 1998～ 2 0 0 0年的观测资料 ,联合处理得到中国大陆地壳水平运动速度场结果 ,通过最小二乘配置法建立中国大陆水平运动速度场模型 ,获得了基于连续介质假设的中国大陆水平应变场 (或称为视应变场 )初步结果 .分析了水平运动、应变场空间分布特征及其与强震的关系 ,并简要分析了 2 0 0 1年 11月 14日昆仑山口西 8.1级大地震的区域构造变形背景 .结果表明 :中国大陆中西部构造变形强烈 ,应变速率值高 ,又以青藏块体及其边缘和新疆西部最为显著 .除川滇、新疆西部外 ,大部分地区的近东西向断裂存在左旋剪切变形 ,近南北向的断裂存在右旋剪切变形 .而东部地区构造变形相对较弱 .强震通常发生在剪切应变率的高值区及其边缘 ,尤其是与构造变形背景相一致的剪应变率高值区 .昆仑山口西 8.1级地震发生在最显著的东西向左旋剪切应变率高值区 ,从该区域的应变状态分析 ,具备近东西向断裂产生巨型走滑破裂错动的构造变形背景 .

根据GPS和InSAR数据反演2001年昆仑山口西地震同震破裂分布

2001年昆仑山口西地震经历了一个相当复杂的破裂过程,迄今为止用不同资料、不同方法和模型得到的同震破裂发布具有很大差异.我们采用地震前后GPS和InSAR观测数据得到的同震位移反演该地震的同震破裂分布。检验各种可能的模型参数,得到在数据与平滑优化约束下尽可能详尽的结果.建模过程经历三个步骤:(1)采用直立断层模型反演.根据解的分辨率和拟合差的折中曲线得到最优平滑约束;(2)改变断层倾角,找到使得观测数据和正演计算拟合最好的断层倾角;(3)根据前面两步得到的最优平滑约束和断层倾角求得地震同震破裂分布.比起前人的研究结果,我们得到的地表走滑分量随断层分布与地质考察数据符合得更好.我们还发现形变沿断层两盘并不对称.断层南盘的位移比北盘大10%～20%.这种位移场的不对称性可以由倾角约为80°～81°的南倾断层所解释.我们首次用大地测量数据揭示了太阳湖断层东端和东昆仑主断层西端～50 km的左阶断层上吸收了0.1～0.2 m的正断层分量,昆仑山口断层段吸收了～0.8 m的逆冲分量.地震释放的总地震矩为9.3×10^(20)N·m,对应于M_w8.0的地震.

2001年11月14日昆仑山口西8.1级地震前地震活动性异常及其思考

2001年11月14日在昆仑山口西发生了8.1级地震,震中位置为90.9°E、36.2°N。分析表明,8.1级地震前一些主要地震活动性异常都出现了,例如空区、条带、增强、平静和震群等,且与7级大震比较,这些前兆图象的演变具有类似的过程,所不同的是8.1级地震的前兆地震活动图象涉及的区域范围更大、地震震级更高,这为特大地震的前兆识别和预报提供了依据。最后回顾了对这次大震的粗略预测,并讨论了有关大震预测的某些问题。