以震源机制类型划分汶川、玉树地震构造块体归属

2001年11月14日昆仑山口7.8级地震、2008年3月21日于田7.1级和5月12日的汶川7.9级地震,全部发生在青藏高原中部,构成新的地震活动组.昆仑山口和汶川地震分别位于巴颜喀拉活动地块的北部和东部边界,于田地震发生在该地块的西端.GPS的观测资料分析表明该块体整体向东运动.发生在块体不同部位的昆仑山口、于田和汶川3次大地震震源机制类型体现了巴颜喀拉块体活动力学的一致性.汶川主震和强余震发生之前,出现于田序列余震的强度和频度显著增高,进一步证实同一活动地块地震之间的内在联系.2010年4月14日玉树发生6.9 级地震,连同1996年11月19日的喀喇昆仑山口6.9 级地震和1997年11月18日玛尼7.5级地震,都发生在巴颜喀拉活动地块的南部边界上,震源机制表现为一致的左旋走向滑动,证实它们属于羌塘块体向东运动的结果.事实表明活动块体具有整体运动的性质,而整体运动也是两组各自3次大地震成组活动的原因.     

巴颜喀拉块体强震活动与2010年玉树7.1级地震的关系

系统地总结分析了巴颜喀拉地块北、东、南边界带的构造特征、各个边界带上的强震活动、部分强震的震源机制解和区域地震活动,从而探讨了2010年玉树7.1级地震发生前巴颜喀拉地块地震活动特征。从2008年玉树地区地震活动变化、巴颜喀拉块体北边界和南边界强震呼应及块体动力学过程进行了地震趋势预测的思考。     

巴颜喀拉块体边界断裂的跨断层形变与地震活动

<正>1997年至今围绕巴颜喀拉块体发生了一系列强震,为块体边界跨断层形变过程与强震活动研究提供了一个前所未有的范例。鲜水河、龙门山断裂带分别属于巴颜喀拉块体的西南边界和东边界,跨断层形变观测有几十年的记录。本文基于这些数据,结合GPS观测、震源机制解、地震活动性资料,着重关注这2条断裂带跨断层形变的时空演化过程和巴颜喀拉块体的强震活动,以及它们共同反映的巴颜喀拉块体的运动。跨断层形变随时间的演变和在空间上的差异,是断层活动在时、空两个维度中的表现,     

巴颜喀拉块体北东地区现今水平运动与变形

本文利用GPS数据研究了巴颜喀拉块体北东地区现今水平运动与变形特征。 在球坐标系中解算了各应变分量, 分析了应变率场的空间分布特征, 并与地球物理学和地震地质学研究结果进行了综合对比分析。 最新的GPS速度场结果表明, 巴颜喀拉块体北东地区与高原整体运动性质一样具有顺时针向南东方向旋转的特征, 自西向东和北东方向测站水平运动速度呈现明显的衰减特征。 应变场结果显示, 研究区以北东向的主压应变为主, 伴随着近北西向的张性应变。 应变较强的区域主要分布在活动块体的边界断裂东昆仑断裂带的东段塔藏段和龙门山断裂带上。 东昆仑断裂带东段塔藏段的主压应变明显, 结合地震地质和活动构造资料, 认为东昆仑断裂带东段塔藏段的运动性质自西向东发生了改变, 水平滑动速率逐渐减小, 垂向运动逐渐增强。 研究区GPS速度场和应变场的这一变形特征表明, 青藏高原内部的块体运动特征较为明显, 变形主要集中在作为活动块体边界的活动断裂带上, 边界断裂带的运动特征在调节活动块体间的相互运动中起着重要作用。     

巴颜喀拉块体东南缘应力演化及鲁甸地震自发破裂模拟研究

巴颜喀拉块体活动导致的昆仑—汶川地震系列,是目前中国大陆唯一一组7级以上地震序列,未来几年该块体仍是发生7级地震的主体地区。巴颜喀拉块体东缘从走滑运动向逆冲运动转换的构造动力学性质使得活动块体两个相邻边界带存在大地震序列的关联性。研究表明近期逐渐加速的块体东边界强震序列是北边界强震活动的响应,巴颜喀拉块体南边界的鲜水河断裂带同样以左旋走滑的方式向东边界挤压,但巴颜喀拉块体东南缘地震呈现以下两个特点,一是强震大都发生在巴颜喀拉块体东边界断裂带,东边界强震序列呈现加速状态;二是南边界鲜水河断裂带在1973年炉霍7.6级地震后,已有40多年没有发生7级以上地震,鲜有6.5级以上强震,在此次地震序列中表现出一种大震缺失的状态。因此,巴颜喀拉块体东南缘主要断裂带的地震危险性值得关注和研究。同震库仑破裂应力及震后粘弹性松弛在解释余震序列分布,分析强震序列相互作用关系及未来地震危险性分析等方面发挥了重要的作用。比如汶川地震之后,一些研究者利用弹性位错模型计算了汶川地震的同震库仑破裂应力变化,并对龙门山区域未来的地震危险性进行了估计。一些研究者分别计算了汶川地震和芦山地震后周边断层的同震库仑应力变化,以及汶川地震和芦山地震引起的同震和震后粘弹性松弛应力场变化,得到了汶川地震可能有效地促进了芦山地震的发生的结论。在应力演化过程中,震间构造应力加载对强震危险性的作用也非常重要,目前比较流行的计算这部分应力积累的做法是采用"负位错"的理论进行计算,这一方法虽然简洁,但假设了断层是完全闭锁的,可能会造成一定误差。本文拟以巴颜喀拉块体区域构造活动作为三维有限元模型的边界约束,综合研究区域介质速度、密度结构最新反演结果计算介质的弹性材料参数,从初始应力场出发模拟在构造应力和重力作用下的现今背景应力场,计算强震序列引起的区域内断裂带震前、震间、震后的应力演化,获得更为准确可靠的应力场结果。利用速率-状态关系计算库仑应力变化对研究区域背景地震发生率的影响,利用泊松概率过程计算研究区域在汶川地震和芦山地震震后未来10年6级以上强震的发震概率及研究区域主要断裂带的6级以上强震发震概率。除以上述准动力学的方法研究巴颜喀拉块体东南缘应力演化及地震危险性,本文还以鲁甸地震为例,研究鲁甸地震从成核区开始破裂,然后扩展直至终止的自发破裂动力学过程,探索影响地震自发破裂位错的主要因素。研究内容:(1)建立巴颜喀拉块体东南缘三维粘弹性有限元模型。(2)模拟巴颜喀拉块体东南缘主要断裂带上1900年以来7级以上强震序列的应力演化过程,并对强震序列对芦山地震的触发作用进行了研究,对比库仑应力场及等效应力场演化结果与背景地震活动的关系。(3)应用速率-状态关系研究库仑破裂应力对汶川地震、芦山地震震后研究区域的地震发生率的影响,应用泊松过程给出研究区域主要断层未来的强震发生概率。(4)应用可以描述断层摩擦机制的FAULTS有限元软件对芦山地震断层错动方式,初始破裂点进行了模拟,研究了汶川地震后,断层强度的变化对芦山地震的影响。(5)应用曲线有限差分方法建立鲁甸地震非平面断层自发破裂模型,模拟鲁甸地震在包谷垴—小河断裂上的自发破裂过程,分析影响鲁甸地震自发破裂过程的主要因素。本文的创新点:(1)以三维速度、密度反演结果建立能反映更真实地下介质的三维粘弹性有限元模型;(2)从初始应力场出发,进行数十万年的应力场演化模拟直至获得与现今构造应力场更为吻合的背景应力场用于地震序列模拟;(3)采用泊松概率模型计算地震发生概率时直接基于三维粘弹性有限元模型得到的库仑应力结果,充分考虑了同震、震间和震后效应的影响。同时,在考虑背景地震发生率时采用了更全面完整的地震目录,这可能使得对未来地震危险性的估计更准;(4)对鲁甸地震自发破裂过程进行了动力学模拟,将震间应力积累过程与同震地震破裂过程的模拟结合起来。     

汉中与安康盆地地震活动特征

根据块体内地震受块体边缘地震影响的观点,对汉中与安康盆地的历史及现代地震活动特点、规律进行了讨论和分析。得到这样结果：(1)块体内断陷盆地的地震活动水平将小于块体边界上的地震活动水平;(2)块体内盆地的地震活动,主要受块体边缘较大地震活动的影响,反映块体边界较大地震发生前后其附近构造的调整运动;(3)当与块体内盆地较近的块体交界带上发生大地震时,应注意盆地的地震危险。     

巴颜喀拉块体东部活动块体的划分、形变特征及构造意义

基于活动块体的基本概念,综合对研究区内活动断裂带空间展布、地震活动性等资料的分析将巴颜喀拉块体东部及邻区划分为巴颜喀拉块体（I）、华南块体（Ⅱ）、川滇块体（Ⅲ）和西秦岭块体（IV）等4个一级块体.利用GPS形变场、地球物理场等资料结合F检验法,将巴颜喀拉块体划分为阿坝（I₁）、马尔康（I₂）和龙门山（I₃）3个次级块体,将西秦岭块体划分为岷县（IV₁）和礼县（IV₂） 2个次级块体.利用分布在各个块体内部的GPS测站,计算各活动块体及块体边界断裂带的运动变形特征.结果表明：各活动块体的整体运动包括平移和旋转运动;东昆仑断裂带、甘孜—玉树断裂带和鲜水河断裂带的滑动速率明显高于龙门山断裂带的滑动速率;巴颜喀拉块体东部走向北西或北西西的边界断裂表现出左旋拉张的特性;走向北东的边界断裂带,除成县—太白断裂带外,均表现出右旋走滑兼挤压的活动特征.巴颜喀拉块体的东向运动存在自西向东的速度衰减,衰减主要被龙日坝断裂带和岷江断裂带分解吸收,其中龙日坝断裂带的水平右旋分解非常明显,约为~4.8±1.6 mm/a,岷江断裂带的水平分解较弱.龙门山断裂带被马尔康、龙门山和岷县等次级块体分成南、中、北三段,龙门山断裂带中段上的主压应变率要明显小于龙门山断裂带南段上的应变率,其北西侧变形幅度从远离断裂带较大到靠近断裂带逐渐减小,表明其在震前已经积累了较高的应变能,有利于发生破裂滑动.汶川地震后,地表破裂带和余震分布揭示的断裂带运动性质自南西向北东由以逆冲运动为主,逐渐转为逆冲兼走滑的特征可能与龙门山断裂带中段所受主压应力方向自南西向北东的变化有关.马尔康、龙门山和岷县3个次级块体与华南块体之间较低的相对运动速度以及龙门山断裂带低应变率、强闭锁的特征都决定了汶川地震前龙门山断裂带低滑动速率的运动特征.     

基于重复微震研究小江断裂带的深部构造变形

小江断裂带是我国大陆地震活动最为强烈的地区之一。古地震和历史地震资料、综合断裂活动背景和地震活动性分析表明,位于川滇块体东边界的小江断裂带存在地震空区与潜在的强震危险。巴颜喀拉地块东边界发生2008年汶川8．0级巨震不到两年,其南边界的甘孜一玉树断裂带在2010年发生了玉树7．1级大地震,使与该地块相连的小江断裂带的强震危险性更受关注。断层滑动速率是评估未来地震危险性的重要依据,但获取孕震深处的滑动速率具有相当大的难度。     

巴颜喀拉块体东部及邻区块体边界断裂变形特征及其强震危险性分析

在对巴颜喀拉块体东部及其邻区活动块体划分的基础上,基于刚性块体运动模型,利用1999—2007年GPS数据反演得到巴颜喀拉块体东部及邻区活动块体边界断裂带的长期滑动速率,并且利用一维弹性位错模型反演了各活动块体边界断裂的闭锁深度。根据反演得到的滑动速率和闭锁深度,结合GIS平台上获取的各边界断裂的几何长度得到各边界断裂的主要地震矩累积,并通过对比各断裂带上历史强震目录估算的地震矩释放分析各边界断裂带上的地震矩亏损量,进而分析各活动块体边界断裂的强震背景,以期为区域强震的中长期地震危险性分析和防震减灾策略的制定提供资料。     

九寨沟地震发震断层属性及青藏高原东南缘现今应变状态讨论

九寨沟地震(M_s7.0或M_w6.5)震中位于青藏高原巴颜喀拉块体东缘东昆仑断裂带东端塔藏断裂、岷江断裂和虎牙断裂交汇部位,中国地震局相关科研机构的研究人员曾将该震中区判定为玛沁—玛曲高震级地震危险区.地震应急科学考察期间没有发现地震地表破裂带,但地震烈度等震线长轴方位、极震区基岩崩塌和滑坡集中带、重新定位余震空间展布和震源机制解等显示出发震断层为NNW向虎牙断裂北段,左旋走滑性质,属东昆仑断裂带东端分支断层之一.此外,汶川地震后,在青藏高原东缘和东南缘次级活动断层上发生了包括2017年九寨沟地震(Mw6.5)、2014年鲁甸(M_w6.2)、景谷(M_w6.2)、康定(M_w6.0)等多次中强地震,显示出青藏高原东缘至东南缘各块体主干边界活动断层现今处于中等偏高的应变积累状态,即在巴颜喀拉、川滇等块体主干边界活动断层上具备了发生高震级(M_w≥7.0)地震的构造应力-应变条件,未来发生高震级地震的危险性不容忽视.     

巴颜喀拉块体东缘形变及九寨沟地震孕震环境数值分析

2017年8月8日四川发生九寨沟M7.0地震,是继2008年汶川M8.0地震后发生在巴颜喀拉块体东部的又一强震.现今GPS速度观测数据显示,2008年汶川地震前后的1999-2007年和2011-2016年两个时间段内巴颜喀拉块体东部地表速度场存在明显的差异.本文以实际GPS速度观测资料为约束,构建三维有限元地球动力学模型,分别计算分析了两个时段内震源区及周边现今地壳形变、弹性应变能和应力积累特征,进一步探讨汶川地震的发生对九寨沟地区变形及应力的影响.数值模拟结果显示,汶川地震之后(2011-2016年)巴颜喀拉块体东部的地壳形变、弹性应变能积累及应力积累速率均明显大于震前,增加量值达1.5-3倍;九寨沟地震发震断裂上库仑应力增长率在1999-2007年约为0.7 kPa·a~(-1),2011-2016年间增至1.2 kPa·a~(-1).上述结果表明,现今巴颜喀拉块体东部地壳应力积累过程有利于左旋走滑型九寨沟地震的发生,汶川地震的发生调整了区域应力状态,加速了九寨沟地震的孕育过程.     

基于库仑应力改变和地震活动性研究巴颜喀拉块体周缘强震序列的触发关系及其构造意义

自本世纪以来,青藏高原巴颜喀拉块体周缘密集地发生了7次M_W≥6.5强震事件,包括伤亡惨重的2008年汶川M_W7.9地震和2017年九寨沟M_W6.5地震。本文根据这7次强震事件中先前地震对后续地震的库仑应力改变（dCFS）的计算结果,结合基于ETAS模型得到的背景地震活动性的变化结果,研究了强震间的触发关系,以试图解释发生在环巴颜喀拉块体的几次强震的发震机理并探讨其构造意义。结果表明,各个强震的库仑应力变化与其造成的背景地震活动性变化呈正相关,并且在这7次强震中,汶川地震对芦山地震有显著的触发作用,并造成芦山地区背景地震活动性的显著提高。同时汶川地震对九寨沟地震的发生具有一定的延迟作用,造成九寨沟地区背景地震活动性降低。除此之外,其它地震之间均无明显的触发/延迟作用或显著的背景地震活动性变化。这表明该强震序列的孕震机制主要是巴颜喀拉块体东南向持续挤压的构造运动,推断巴颜喀拉块体目前仍处于构造运动活跃期,因此包括巴颜喀拉块体周缘在内的我国西南地区未来的强震危险性值得持续关注。      

EARTHQUAKE FOCAL MECANISMS IN THE DALIANGSHAN SUB-BLOCK AND ADJACENT AREAS AND CHARACTERISTICS OF THE REGIONAL STRESS FIELD

The Daliangshan sub-block is a boundary region among the Bayan Har block, the Sichuan-Yunnan block and the South China block. It hosts four major fault systems:The southwest to south trending Xianshuihe-Zemuhe Fault zone in the west, the Longmenshan fault zone is the northern boundary, the Zhaotong-Lianfeng fault zone in the south, and the NS-trending Mabian-Yanjin fault zone in the east. This study focused on focal mechanisms and the regional stress field of the Daliangshan sub-block to help understand the earthquake preparation process, tectonic deformation and seismic stress interaction in this area. We collected broadband waveform records from the Sichuan Seismic Network and used multiple 1-D velocity models to determine the focal mechanisms of moderate and large earthquakes(M_L ≥ 3.5)in the Daliangshan sub-block by using the CAP method. Results for 276 earthquakes from Jan 2010 to Aug 2016 show that the earthquakes are dominated by strike-slip and trust faulting, very few events have normal faulting and the mixed type. We then derived the regional distribution of the stress field through a damp linear inversion(DRSSI)using the focal mechanisms obtained in this study. Inversion results for the spatial pattern of the stress field in the block suggest that the entire region is predominantly under strike-slip and trust faulting regimes, largely consistent with the focal mechanisms. The direction of maximum compression axes is NW-NWW, and part of the area is slightly rotated, which is consistent with the GPS velocity field. Combining geodynamic background, this work suggests that because the Sichuan-Yunnan block is moving to SE and the Tibetan plateau to SE-E along major strike-slip faults, the stress field of the Daliangshan sub-block and its adjacent regions is controlled jointly by the Bayan Har block, the Sichuan-Yunnan block and the South China block.     

Relation between the characteristics of strong earthquake activities in Chinese mainland and the Wenchuan earthquake

This paper studies the relations between the great Wenchuan earthquake and the active-quiet periodic characteristics of strong earthquakes, the rhythmic feature of great earthquakes, and the grouped spatial distribution of M_S8.0 earthquakes in Chinese mainland. We also studied the relation between the Wenchuan earthquake and the stepwise migration characteristics of M_S?≥7.0 earthquakes on the North-South seismic belt, the features of the energy releasing acceleration in the active crustal blocks related to the Wenchuan earthquake and the relation between the Wenchuan earthquake and the so called second-arc fault zone. The results can be summarized as follows: ① the occurrence of the Wenchuan earthquake was consistent with the activequiet periodic characteristics of strong earthquakes; ② its occurrence is consistent with the features of grouped occurrence of M_S8.0 earthquakes and follows the 25 years rhythm (each circulation experiences the same time) of great earthquakes; ③ the Wenchuan M_S8.0 earthquake follows the well known stepwise migration feature of strong earthquakes on the North-South seismic belt; ④ the location where the Wenchuan M_S8.0 earthquake took place has an obvious consistency with the temporal and spatial characteristic of grouped activity of M_S≥?7.0 strong earthquakes on the second-arc fault zone; ⑤ the second-arc fault zone is not only the lower boundary for earthquakes with more than 30 km focal depth, but also looks like a lower boundary for deep substance movement; and ⑥ there are obvious seismic accelerations nearby the Qaidam and Qiangtang active crustal blocks (the northern and southern neighbors of the Bayan Har active block, respectively), which agrees with the GPS observation data.     

2008年汶川8.0级地震前横跨龙门山断裂带的震间形变

利用区域GPS和水准测量资料,结合地震构造背景的分析,本文研究2008年汶川8.0级地震前横跨龙门山断裂带地区的震间地壳形变,探讨引起这种形变的活动构造与动力学模式,并由此认识汶川地震的孕育与成因机制.主要结果表明：1997～2007年期间,自龙门山断裂带中段朝北西约230 km的地带内存在垂直于断裂的水平缩短变形、以及平行于断裂的水平右旋剪切变形,缩短率为1.3×10^-8/a （即：0.013 mm/km/a）,角变形速率为2.6×10^-8/a;同一地带在1975～1997年期间还表现出垂直上隆变形,上隆速率在龙门山前山断裂与中央断裂之间仅0.6 mm/a,而至龙门山后山断裂及其以西达2～3 mm/a.这些反映了在汶川地震之前至少10～30余年,龙门山断裂带中段的前山与中央断裂业已闭锁、并伴有应变积累.造成这种形变的主要原因是：以壳内的低速层为“解耦”带,巴颜喀拉地块上地壳朝南东的水平运动在四川盆地西缘受到华南地块的阻挡、转换成龙门山断裂带中段的逆冲运动;由于该断裂段的震间闭锁,致使西侧的巴颜喀拉地块的上地壳发生横向缩短以及平行断裂的右旋剪切变形.然而,龙门山断裂带北段在1997～2007年期间除了有大约0.9 mm/a的右旋剪切变形外,横向的缩短变形极微弱,这可能与该断裂段西侧的岷江、虎牙、龙日坝等断裂带吸收了巴颜喀拉地块朝东水平运动的大部分有关.另外,汶川地震前,横跨龙门山断裂带中段与北段的地壳形变特征的差异,与汶川地震时能量释放的空间分布吻合.     

Coseismic deformation of the 2021 MW7.4 Maduo earthquake from joint inversion of InSAR,GPS, and teleseismic data

The M_W7.4 Maduo earthquake occurred on 22 May 2021 at 02:04 CST with a large-expansion surface rupture. This earthquake was located in the Bayan Har block at the eastern Tibetan Plateau, where eight earthquakes of M_S >7.0 have occurred in the past 25 years. Here, we combined interferometric synthetic aperture radar, GPS, and teleseismic data to study the coseismic slip distribution, fault geometry, and dynamic source rupture process of the Maduo earthquake. We found that the overall coseismic deformation field of the Maduo earthquake is distributed in the NWW-SEE direction along 285°. There was slight bending at the western end and two branches at the eastern end. The maximum slip is located near the eastern bending area on the northern branch of the fault system. The rupture nucleated on the Jiangcuo fault and propagated approximately 160 km along-strike in both the NWW and SEE directions. The characteristic source rupture process of the Maduo earthquake is similar to that of the 2010 M_W6.8 Yushu earthquake, indicating that similar earthquakes with large-expansion surface ruptures and small shallow slip deficits can occur on both the internal fault and boundary fault of the Bayan Har block.     

基于库仑应力变化分析巴颜喀拉地块东端的强震相互关系

本研究基于分层黏弹介质模型,考虑同震位错效应和震后黏滞松弛效应,分析巴颜喀拉地块东端1976年松潘地震序列、2008年汶川8.0级地震、2013年芦山7.0级地震和2017年九寨沟7.0级地震等多次大地震的可能存在的触发关系,计算大地震引起的周边各主要断裂的库仑应力变化.结果显示,1976年松潘地震序列各次地震间关系密切,存在明显的相继触发作用;综合考虑同震和震后效应,汶川8.0级地震对同属于龙门山断裂带的芦山7.0级地震有触发作用,且震后效应影响不可忽略;1976年地震序列,特别是1976年8月16日7.2级地震促进了2017年8月8日九寨沟7.0级地震的发生;汶川地震对九寨沟地震的影响研究中,采用不同的汶川地震同震位错模型,计算结果有差异.综合考虑多次大地震对周边断裂带的影响,龙门山断裂带南段、鲜水河断裂带中南段、平武—青川断裂北段、灌县—安县断裂北段、文县断裂的累积库仑应力增加显著,巴颜喀拉地块东端的东昆仑断裂带东段、迭部—白龙江断裂带西段以及金沙江断裂带库仑应力亦有所增加.综合考虑各重要断裂带已有的大地震危险性分析结果和库仑应力变化计算结果,龙门山断裂带南段、鲜水河断裂带中南段、东昆仑断裂带玛沁—玛曲段和金沙江断裂带的发震紧迫性有所增强,需引起关注.     

FEATURES OF EARTHQUAKE CLUSTERING FROM CALCULATION OF COULOMB STRESS AROUND THE BAYAN HAR BLOCK,TIBETAN PLATEAU

Since 1997, several major earthquakes occurred around the Bayan Har block in the Tibetan plateau, providing an opportunity to further understanding the mechanism of intraplate earthquakes. What is the effect of interactions among these events on the earthquake occurrence pattern is an issue to be addressed. In this article, we use the visco-elastic Coulomb stress changes model to calculate the stress interactions among the historical events close to or large than M_S7.0 since 1893 in the Bayan Har block. We apply the relationships between the slip rate and stress accumulation rate to transform the Coulomb stress changes into the influenced time. Then we remove such influence time from the occurrence years, and analyze the effects of the earthquake interactions on the clustering patterns of the historical earthquakes in the Bayan Har block. The results show that the major earthquakes in the Bayan Har block are characterized by a quasi-period of about 16 years from 1893 to 1973 and a clustering occurrence time period from 1997 to present following a relatively long quiescence period. The Bayan Har block is still in the active period with high probabilities of major quakes. We calculate the conditional probabilities of the rupture segments that did not rupture since 1893 of the boundary faults of the Bayan Har block in the next 30 years. The following faults or fault sections seem to be of major risk:The Maqin segment and the Maqu fault of the East Kunlun fault zone, the Awanang fault, the Luocha segment of the Tazhong fault, the Moxi segment of the Xianshuihe fault, and the Dangjiang fault. Other Fault segments in the Bayan Har block without seismic events since 1893 probably also have hazard of M_S7 earthquakes in the future.     

FOCAL FAULTS AND STRESS FIELD CHARACTERISTICS OF M7.0 JIUZHAIGOU EARTHQUAKE SEQUENCE IN 2017

On August 8, 2017, Beijing time, an earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, Sichuan Province, with the epicenter located at 33.20°N 103.82°E. The earthquake caused 25 people dead, 525 people injured, 6 people missing and 170000 people affected. Many houses were damaged to various degrees. Up to October 15, 2017, a total of 7679 aftershocks were recorded, including 2099 earthquakes of M ≥ 1.0. The M7.0 Jiuzhaigou earthquake occurred in the northeastern boundary belt of the Bayan Har block on the Qinghai-Tibet Plateau, where many active faults are developed, including the Tazhong Fault(the eastern segment of the East Kunlun Fault), the Minjiang fault zone, the Xueshan fault zone, the Huya fault zone, the Wenxian fault zone, the Guanggaishan-Daishan Fault, the Bailongjiang Fault, the Longriuba Fault and the Longmenshan Fault. As one of the important passages for the eastward extrusion movement of the Qinghai-Tibet Plateau(Tapponnier et al., 2001), the East Kunlun fault zone has a crucial influence on the tectonic activities of the northeastern boundary belt of Bayan Kala. Meanwhile, the Coulomb stress, fault strain and other research results show that the eastern boundary of the Bayan Har block still has a high risk of strong earthquakes in the future. So the study of the M7.0 Jiuzhaigou earthquake' seismogenic faults and stress fields is of great significance for scientific understanding of the seismogenic environment and geodynamics of the eastern boundary of Bayan Har block. In this paper, the epicenter of the main shock and its aftershocks were relocated by the double-difference relocation method and the spatial distribution of the aftershock sequence was obtained. Then we determined the focal mechanism solutions of 24 aftershocks(M ≥ 3.0)by using the CAP algorithm with the waveform records of China Digital Seismic Network. After that, we applied the sliding fitting algorithm to invert the stress field of the earthquake area based on the previous results of the mechanism solutions. Combining with the previous research results of seismogeology in this area, we discussed the seismogenic fault structure and dynamic characteristics of the M7.0 Jiuzhaigou earthquake. Our research results indicated that:1)The epicenters of the M7.0 Jiuzhaigou earthquake sequence distribute along NW-SE in a stripe pattern with a long axis of about 35km and a short axis of about 8km, and with high inclination and dipping to the southwest, the focal depths are mainly concentrated in the range of 2~25km, gradually deepening from northwest to southeast along the fault, but the dip angle does not change remarkably on the whole fault. 2)The focal mechanism solution of the M7.0 Jiuzhaigou earthquake is:strike 151°, dip 69° and rake 12° for nodal plane Ⅰ, and 245°, 78° and -158° for nodal plane Ⅱ, the main shock type is pure strike-slip and the centroid depth of the earthquake is about 5km. Most of the focal mechanism of the aftershock sequence is strike-slip type, which is consistent with the main shock's focal mechanism solution; 3)In the earthquake source area, the principal compressive stress and the principal tensile stress are both near horizontal, and the principal compressive stress is near east-west direction, while the principal tensile stress is near north-south direction. The Jiuzhaigou earthquake is a strike-slip event that occurs under the horizontal compressive stress.