Upper crustal velocity and seismogenic environment of the M7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8,2017,a magnitude 7.0 earthquake occurred in Jiuzhaigou County,Sichuan Province,China.The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad.The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area;the complex tectonic background and the reason(s)for the frequent seismicity in the Jiuzhaigou earthquake region are unclear.In order to reveal the characteristics of the deep medium and the seismogenic environment of the M7.0 Jiuzhaigou earthquake region,and to interpret the tectonic background and genesis of the seismicity comprehensively,in this paper,we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale,dense mobile seismic array(China Array)for the northern section of the North-South Seismic Belt around Jiuzhaigou earthquake region.Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region,we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region,its deep tectonic environment,and the ongoing seismic risk in this region.We report that:the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity;the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology;the M7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust;the mainshock of the M7.0 earthquake occurred in the upper crust;the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence.The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB(Songpan-GarzêBlock)stretches into MSM(Minshan Mountain),and migrates to the northeast,but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block.Our results reveal an uneven distribution of high-and low-velocity structures around the Tazang segment of the East Kunlun fault zone.Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault,it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu-Maqên segment of the East Kunlun fault zone.

Near-field surface deformation during the April 20,2013,Ms7.0 Lushan earthquake measured by 1-Hz GNSS

The April 20, 2013, Ms7.0 Lushan earthquake was successfully recorded by closely spaced Continuous Global Positioning System （CGPS） stations owned by the Crustal Movement Observation Network of Chi- na （CMONC）. The 1-Hz GNSS data from eight CGPS stations, which are located between 30 km and 200 km from the hypocenter, were processed within quasi-real-time. The near-field surface deformation indicated the following characteristics ： the near-field movements were limited to several centimeters ; the peak of the deformation wave was significantly larger than the static permanent offset; at the beginning of the event, the north wall of the fault moved to the southeast as the south wall moved to the southwest ; station SCTQ, which was the closest station to the hypocenter at 30 km, had the largest static permanent displacement of 2 cm; the peaks of the deformation waves were 1.5 cm, 5 cm and 3 cm, to the east, the south and vertically upward, respectively ; and the peaks of velocity and acceleration, derived from the deformation, were 3.4 cm/s and 5.3 cm/s^2,respectively.

Directional seismic response to the complex topography:A case study of 2013 Lushan Ms 7.0 earthquake

Azimuthal variations in site response can provide a good insight into the site amplification and seismic conditions of geohazard occurrences.In this study,multiple directional site response methods,including D-Arias(Directional-Arias),D-SER(Directional-Shaking energy ratio),D-HVSR(Directional-Horizontal to vertical spectral ratio)and D-SSR(Directional–Standard spectral ratio),are adopted to analyse seismic data of the 2013 Lushan Ms 7.0 earthquake captured by the self-established Lengzhuguan(LZG)station which consists of the complex topography of isolated ridge,large mountain and some typical micro-reliefs.The results show that the isolated ridge could cause stronger site responses than the large mountain,and whose pronounced response direction is roughly perpendicular to its ridgeline.With the growth of elevation,the siteresonant frequency decreases.The different microreliefs on the mountain cause different site responses,which present as protruding slope>linear slope.The site response mainly exists on the surficial layer of the mountain and shows that with the increase of the distance to mountain surface,the site response gets weaker,the site resonant frequency gets higher,and the pronounced response direction is perpendicular to its ridgeline.

Coseismic displacements and inospheric changes of the 2013 Ms7. 0 Lushan earthquake from GPS measurements

By inverting GPS data recorded at stations of the Crustal Movement Observation Network of China （CMONOC） near the 2013 Lushan Ms7.0 earthquake, we found a horizontal displacement of 22 mm at a site about 32 kin SW of the epicenter and vertical displacements of as much as 12.4 mm at several sites. The vertical displacements were generally uplift on the west side of the nearby Longmenshan fault zone and subsidence on the east side. We also found coseismic ionospheric disturbances about 0.5 to 0.9 TECU in amplitude that lasted for about one hour.

A rapid stability assessment of China's IGS sites after the Ms7.0 Lushan earthquake

A rapid and accurate assessment of the stability of surveying and mapping reference points is important for post - disaster rescue, disaster relief and reconstruction activities. Using Precise Point Positioning （PPP） technology, a rapid assessment of the stability of the IGS sites in China was performed after the Ms7.0 Lushan earthquake using rapid precise ephemeris and rapid precise satellite clock products. The results show that the earthquake had a very small impact and did not cause significant permanent deformation at the IGS sites. Most of the sites were unaffected and remained stable after the earthquake.

2022年四川芦山M_(S)6.1地震前应力状态研究

为研究2022年6月1日四川芦山M_(S)6.1地震的孕育和发生过程,采用CAP方法反演了2013年芦山M_(S)7.0主震及M_(S)≥5.0余震的震源机制解,并基于应力张量方差与b值时空分布特征,探讨了芦山M_(S)6.1地震的力学机制和震源区的应力状态。结果表明:2022年芦山M_(S)6.1地震震源机制表现出与2013年芦山M_(S)7.0主震和5级余震相似的逆冲型破裂特征,压应力轴方位与龙门山断裂带南段区域应力场一致。2013年芦山M_(S)7.0地震后震中及附近的应力张量方差和b值长期处于低值状态,2022年芦山M_(S)6.1地震前震中及附近出现了应力张量方差和b值的低值异常,表明芦山余震区处于较高的应力水平。分析认为:巴颜喀拉块体持续东向运动受到华南块体的阻挡,震中所在区域长期受挤压逆冲作用,从而使芦山余震区长期处于应力积累的状态,芦山M_(S)6.1地震也是在这种动力学背景下发生的。

四川芦山地区农村典型民居LiDAR点云背景值分析

使用地基三维激光扫描仪,采集了2013年四川芦山7.0级地震灾区农村典型民居基本完好砌体建筑物140个独立墙面的高精度LiDAR点云数据。根据标准差原理,采用统计回归分析方法,对LiDAR数据进行了处理,分析和验证LiDAR点云数据背景值区间。结果表明:芦山地区农村典型民居砌体结构建筑物墙面的LiDAR点云数据背景值区间为0~0.45 cm,并证实其合理性。

华北平原中南部聊城-兰考断裂的第四纪晚期活动性探测——兼论1937年菏泽7.0级地震发震机制

华北平原是中国人口最多、经济最为发达的地区之一,也是受地震灾害影响最为严重的地区之一。对于该地区断裂活动性和大地震发震机制的研究有利于探索板内地震的发震规律、减轻地震灾害所造成的损失。聊城-兰考断裂是华北平原中南部一条重要的隐伏深大断裂。结合浅层地震勘探、钻孔勘探和第四纪测年方法,对聊城-兰考断裂的活动性进行了精细的研究。坝城寺钻孔揭示聊城-兰考断裂南段错断了全新统底界,为全新世早期活动断裂,揭露出该断裂晚更新世以来造成了4次古地震事件,单次事件的垂直位错为1.2±0.2~3.7±0.2 m。根据钻孔揭示的地层落差计算出该断裂晚更新世早期的平均垂直滑动速率约为0.1±0.05 mm/a,晚更新世晚期—全新世中期的平均滑动速率为0.35±0.04 mm/a。根据1937年菏泽7.0级和6(3/4)级地震的等震线和地表破裂分布特征认为,小留-解元集断裂和东明-成武断裂为该地震的发震断裂;聊城-兰考断裂对于该地区应力的积累、地震的发生具有很好的控制和约束作用,为区域控震构造。

丽江与芦山两次M_(S)7.0地震震例回溯研究及亚失稳特征探讨

基于川滇地区的跨断层观测资料,采用多种形变分析方法,针对川滇地区监测状况较好的1996年丽江、2013年芦山两次M_(S)7.0地震开展震例回溯研究,并以亚失稳理论及实验室结果为指导,归纳总结跨断层观测获得的断层运动在亚失稳阶段的时空变化特征及可能的表现形式,寻找区域范围内断层运动协同化现象,探索地震进入亚失稳状态的跨断层识别标志。结果表明:两次地震前,震源区在震前数年主要表现为弱变形的状态,可能是孕震晚期发震断层闭锁、断层近场应力应变积累趋于极限的表现。震源区边缘附近300 km范围内在震前数月至3年出现显著的断层形变异常,断层活动具有协同化现象,但仅个别跨断层场地观测到了与实验室岩石破裂失稳过程类似的图像,大多数场地仅仅观测到了稳态(线性阶段)、亚稳态(偏离线性阶段)特征,亚失稳态特征并不清晰,分析可能与跨断层场地所处的构造位置及观测周期有一定关系。

Emergency Response to the MS7.0 Jiuzhaigou,Sichuan Earthquake,and Characteristics of Seismic Disasters in the Stricken Area in Gansu Province

This paper introduces the response process of the Gansu Earthquake Agency during the Jiuzhaigou M_S7.0 earthquake in Sichuan Province,including earthquake emergency disposal procedures,information reports,disaster investigation and intensity assessment,seismic monitoring and trend determination,and emergency dissemination. This paper reveals the characteristics of earthquake damage in the quake-hit areas of Gansu Province,draws some corresponding conclusions and summarizes the countermeasures for recovery and reconstruction in the quake-hit areas of Gansu Province.

Characteristics and Analysis of the Seismic Damage from the MS7.0 Jiuzhaigou Earthquake in the Area between Songpan and Jiuzhaigou

This paper expounds the features of the buildings and analyzes the seismic disaster characteristics of the Jiuzhaigou M_S7. 0 earthquake in the area between Songpan and Jiuzhaigou. New buildings (especially the frame structure) had good anti-seismic performance,but brick-wood structures and brick-concrete structures sustained large amounts of damage in the earthquake. By computing the seismic damage index,we found that the seismic damage index of the frame structure was far less than that of civil structures and brick-wood structures. The seismic damage index of frame structures were all zero in the Ⅵ area,and increased rapidly with the increase of intensity,but the increasing range was reduced. We also discussed how to evaluate the intensity in areas where there was a lack of buildings or there was only one structure type,which can be referenced in future field work.

2013年四川芦山M_(S)7.0地震仪器烈度与调查烈度对比分析

基于2013年四川芦山M_(S)7.0地震宏观调查点、调查烈度和强震仪记录数据,采用四川省地震局强震动监测技术组计算得到的各台仪器烈度值,绘制了芦山M_(S)7.0地震宏观调查点与调查烈度等震线和仪器烈度等值线分布图,分析了芦山M_(S)7.0地震的仪器烈度与调查烈度的对应关系。结果表明:仪器烈度与调查烈度有较好的对应关系,仪器烈度Ⅵ度点在调查烈度Ⅵ度以上区域总体占比为88.24%,仪器烈度Ⅶ度点在调查烈度Ⅶ度以上区域内总体占比为81.82%;仪器烈度的等值线高值范围可较好展示出地震灾害的程度;仪器烈度区的空间分布特征可表征芦山M_(S)7.0的发震断裂的控制作用,仪器烈度的等值线区总体呈现沿龙门山断裂带方向相对于垂直方向衰减慢;仪器烈度的等值线区发震断裂上盘影响范围相对发震断裂下盘影响范围大;仪器烈度分布与调查烈度分布既有对应关系又存在差异,这在震后短时间内灾区的震灾信息尚不清楚的情况下,能够快速对可能的震害涉及范围、人员伤亡分布、经济损失和生命线工程等损失作出预估,仪器烈度分布可以为应急救援决策、救灾方案制定和救灾力量部署等工作提供重要依据。

2013年四川省芦山“4.20”7.0级强烈地震触发滑坡

2013年4月20日,四川省芦山县发生了M S7.0地震。文中简要介绍了芦山地震的基本情况与芦山地震区历史地震及其相关地震滑坡情况。依据2008年汶川地震滑坡与地震动峰值加速度(PGA)的空间关系,对芦山地震滑坡大体分布范围进行了推测。根据地震滑坡分类学,将芦山地震滑坡分为破坏型滑坡、连贯型滑坡、流滑型滑坡3大类。其中,破坏型滑坡包括岩质崩塌、岩质滑动、岩质崩滑、土质崩塌、土质滑动等5类;连贯型滑坡包括土质坍塌与慢土流2类;流滑型滑坡为快速流滑。破坏型滑坡如岩质崩塌、岩石滑动、土质崩塌这3类是芦山地震滑坡中最常见的类型。基于震后可利用的高分辨率航片,初步解译得到3 883处滑坡位置点数据。最后,从余震对滑坡的影响,芦山地震滑坡与邻区地震滑坡对比分析,对后续基于高分辨率遥感影像的滑坡精细解译的启示等3个方面开展了分析与讨论。

四川省芦山M_S7.0地震发震构造分析

2013年4月20日的芦山"4·20"M S7.0地震发生在龙门山断裂带西南段,震中地区分布多条NE向断裂,构造较为复杂。这次地震震源机制解显示为逆冲型地震,破裂面为NE走向,与龙门山断裂带的运动性质和走向一致。地表调查只在大川-双石断裂(前山断裂)和新开店断裂(大邑断裂南段)发现局部分布的NE向地表裂缝、沿地表裂缝分布的喷砂冒水和砂土液化,不规则的边坡开裂等地表变形,以及断裂沿线较严重的滑坡崩塌和房屋破坏。野外调查没有发现明显的地震地表破裂。GPS测量结果显示,此次地震的发震断裂位于芦山县城附近或其以东,而芦山西侧的断裂也可能参与了部分活动。根据野外地质调查、GPS观测、震源机制解、震源深度、余震分布等结果综合判定,芦山7.0级地震的主要发震构造是芦山之下、大川-双石断裂和新开店断裂之间的龙门山前缘滑脱带。此滑脱带在该段的运动导致了这次地震的发生,并可能带动了它上面的大川-双石和新开店等断裂的活动。

九寨沟7.0级地震中典型非结构构件震害特征

九寨沟7.0地震给九寨沟地区建筑结构及非结构构件造成了严重破坏。本文主要涉及调查区域包括漳扎镇、甲藩古城、若尔盖县阿西茸乡、求吉乡及包座乡等。主要调查框架结构、砖混结构、大跨空间结构以及木结构的非结构构件震害。此次调查的结构多数是2000年以后建造,8度设防,在2008年汶川地震中主体结构和非结构构件均未受到影响。在此次地震中,非结构构件大量损坏,破坏主要以框架、砌体填充墙开裂或变形或局部倒塌、吊顶脱落、地板隆起、吊顶设备掉落、玻璃震碎等震害为主。调查结果表明,与以往地震不同,Ⅷ度区内仅有的三个大跨度结构均发生不同程度的破坏,其中一个结构出现节点破坏,此次地震中大跨空间结构非结构构件震损尤为明显,非结构构件设备损失最为严重。另一个不同于以往地震震害的是位于Ⅵ度区山坡上的木结构非结构震害明显比坡下要严重一些。

芦山县龙门乡芦山“4·20”7.0级强烈地震地表破裂迹象与讨论

在芦山县龙门乡发现芦山"4·20"7.0级强烈地震地表破裂的迹象,这些迹象点呈NE-SW向线状分布,总体走向N40°～50°E,长2～3km。根据水泥路面变形推断水平缩短量为8cm,垂直抬升量1～2cm。地表未见走滑分量,运动学特征表现为由NW向SE的推挤作用。在地震地表破裂的力学性质方面,有斜向剪切裂缝,也有挤压对冲逆断层性质,但更多地表现为张性裂缝,这与拱曲顶部的局部张性应力场有关。虽然这些地表破裂组合特征不同,性质也有差异,但均反映了龙门乡一带受到NW-SE向挤压作用以及逆断层发震构造沿线近地表常见的拱曲作用。与大川-双石断裂(前山断裂)、大邑-名山断裂(山前断裂)相比,芦山-龙门隐伏推测断裂更有可能是此次地震的发震断裂,这一推论也与此次地震序列的精定位结果以及地震烈度分布特征相符。有关芦山"4·20"7.0级强烈地震发震断裂的认识,对研究此次地震的发震构造特征以及评价未来山前地带地震危险性具有重要意义。

关于芦山7.0级地震在龙门一带是否存在同震地表破裂的讨论

4.20芦山地震后,有学者在芦山县龙门乡发现一系列的线性裂缝和砖块的旋转变形等"地震地表破裂迹象",由此推测芦山—龙门一线存在隐伏逆断裂,并认为该断裂有可能是此次地震的发震断裂。因此,进一步探讨芦山—龙门一线是否存在潜在的发震断裂,无论是对研究芦山7.0级地震的发震断裂,还是对灾区的重建指导都十分重要。在龙门乡开展了地质灾害调查、跨谷地的地质剖面实测,槽探和人工地震勘探等工作。结果显示:至少在800m深度范围内,不存在芦山-龙门隐伏断裂。此带上的地裂缝等现象不是由断层位错引起,而更可能是地震动在阶地陡坎附近造成的地基或边坡效应所致。

川西温泉水温观测及其在芦山M_S7.0地震前的异常现象

在2013年芦山MS7.0地震震中附近流体观测点进行现场考察的基础上,本文选择川西地区观测环境较好、干扰较少的温泉水温观测资料进行回溯性分析.结果表明,康定龙头沟和二道桥温泉、道孚龙普沟温泉、理塘毛垭温泉、泸定共和温泉水温在芦山地震前均出现不同程度的异常变化.其主要表现为,距离震中较远的理塘毛垭温泉和道孚龙普沟温泉水温在震前2年左右出现中期尺度异常,距离震中较近的康定龙头沟和二道桥温泉水温在震前3个月内出现短期异常.通过温泉水化学及δD-δ18 O稳定同位素特征分析,认为这些温泉水具有深循环特征,能灵敏地反映地壳深部地热及构造变化信息,是芦山地震前观测到异常的可能原因.加强对这些温泉点的观测,对区域地震预测研究具有重要的科学意义和现实意义.

丽江地震序列的震源机制、发震应力场和破裂特征

丽江７０级地震震区位于我国西南地区现代构造应力场空间分布的复杂地区，区域应力场主压应力优势方位为南南东。震区位于可能受到多种构造动力源作用的特定构造运动环境中。获得了主震和２２个ＭＬ≥４０级余震的震源机制Ｐ波初动解，分析表明，主震发震应力场为北３°东，与震区区域应力场主压应力优势方位有一个小角度的偏差。主震的发震应力不仅有水平应力的作用，同时还有显著的垂直应力的作用。在余震序列发展中震区呈现出应力场分布的复杂图象。地质资料表明，震区处在北西、北东和北北西———北北东向三条较大规模构造断裂所围限的三角形断块内。推测主震发生在北西向边界断裂与断块内近南北向断裂的交汇部位，并且自交汇点分别向此两断裂两侧破裂，形成震源的两次破裂过程。主震的破裂错动以正倾滑动为主，兼具左旋剪切错动分量。余震主要发生在三角形断块内。

汶川8.0级地震和芦山7.0级地震对周边断层的影响

采用分层粘弹性介质模型,研究龙门山断裂带发生的两次大地震(2008年5月12日汶川8.0级和2013年4月20日芦山7.0级地震)的同震位错和震后粘滞松弛效应引起的周边断裂带上的库仑应力变化。结果表明,汶川8.0级地震同震和震后效应均引起了芦山7.0级地震震中位置的库仑应力增加,有利于芦山地震的发生;汶川和芦山地震的同震和震后效应引起的周边各断层上的累积库仑应力变化分布显示,这两次大地震引起了其间的破裂空段的库仑应力显著增加;此外,累积库仑应力显著升高的断裂还有:鲜水河断裂带中段、灌县—安县断裂北段、文县断裂的一部分、平武—青川断裂北段、略阳—勉县断裂。累积库仑应力显著降低的断裂有:马尔康断裂、岷江断裂南端、虎牙断裂、龙泉山断裂北端、北川—映秀断裂北段。