Analysis of debris flow control effect and hazard assessment in Xinqiao Gully,Wenchuan M_(s)8.0 earthquake area based on numerical simulation

Xinqiao Gully is located in the area of the 2008 Wenchuan M_(s)8.0 earthquake in Sichuan province,China.Based on the investigation of the 2023"6-26"Xinqiao Gully debris flow event,this study assessed the effectiveness of the debris flow control project and evaluated the debris flow hazards.Through field investigation and numerical simulation methods,the indicators of flow intensity reduction rate and storage capacity fullness were proposed to quantify the effectiveness of the engineering measures in the debris flow event.The simulation results show that the debris flow control project reduced the flow intensity by41.05%to 64.61%.The storage capacity of the dam decreases gradually from upstream to the mouth of the gully,thus effectively intercepting and controlling the debris flow.By evaluating the debris flow of different recurrence intervals,further measures are recommended for managing debris flow events.

Process Analysis of In-situ Strain during the M_s8.0 Wenchuan Earthquake—Data from the Stress Monitoring Station at Shandan

There were huge life and property losses during the Ms8.0 Wenchuan earthquake on May 12, 2008. Strain fluctuation curves were completely recorded at stress observatory stations in the Qinghai-Tibet plateau and its surroundings in the process of the earthquake. This paper introduces the geological background of the Wenchuan earthquake and the profile of in-situ stress monitoring stations. In particular, data of 174 earthquakes （Ms4.0-Ms8.5） were processed and analyzed with various methods, which were recorded at the Shandan station from August 2007 to December 2008. The results were compared with other seismic data, and further analyses were done for the recoded strain seismic waves, co-seismic strain stepovers, pre-earthquake strain valleys, Earth＇s free oscillations before and after the earthquake and their physical implications. During the Wenchuan earthquake, the strainmeter recorded a huge extensional strain of 70 seconds, which shows that the Wenchuan earthquake is a rupture process predominated by thrusting. Significant precursory strain anomalies were detected 48 hours, 30 hours, 8 hours and 37 minutes before the earthquake. The anomalies are very high and their forms are very similar to that of the main shock. Similar anomalies can also be found in strain curves of other shocks greater than Ms7.0, indicating that such anomalies are prevalent before a great earthquake. In this paper, it is shown that medium aftershocks （Ms5.5- 6.0） can also cause Earth＇s free oscillations. Study of free oscillations is of great significance to understand the internal structure of the Earth and focal mechanisms of earthquakes and to recognize slow shocks, thus providing a scientific basis for the prevention and treatment of geological disasters and the prediction of future earthquakes.

Effect of Poisson’s ratio on stress state in the Wenchuan M_S8.0 earthquake fault

The Wenchuan Ms8.0 earthquake occurred on the Longmenshan fault which inclines at a dip angle exceeding 60 degrees. Since most thrust earthquakes occur on faults with dip angles of about 30 degrees, it is enigmatic why the Wenchuan earthquake occurred on such a steep fault. In this study we use a simple finite element model to investigate how the stress state in the fault changes with the variation of Poisson＇s ratio. The results show that, with the Poisson＇s ratio in the fault increasing, the magnitudes of the principal stresses increase and the maximum Shear stress decrease, and, especially, the angle between the maximum principal stress and the fault plane decreases, which will enhance the driving force to overcome the frictional resistance on the fault. The increase of Poisson＇s ratio in the fault may be an important factor to affect the occurrence of the fault earthquakes with large angles between maximum principal stress and fault plane.

Temporal and Spatial Evolution of Precursory Anomalies of the Wenchuan MS8.0 Earthquake and Their Mechanical Analysis

In order to study the spatiotemporal evolution of the precursory anomalies 10 years before the Wenchuan M_S8. 0 earthquake in 2008, the epicentral distance of the precursory anomalies is calculated by using the geometric center of the rupture region and the elliptical centerline of the aftershock region. The result shows, precursor anomalies gradually increased about 2 years before the Wenchuan earthquake. The ratio of abnormal items is greater than 25% in the near source area (about twice the source scale) and 17%-24% in the remote area (about 3-5 times the source scale). There are three different stages of spatiotemporal evolution of precursory anomalies. During the α stage (including α_1 and α_2,between 700 to 3000 days before the main earthquake),the anomalies are mainly distributed in the southwest and northwest area of the Wenchuan aftershocks area. It is shown that the precursors of the far source region and the near source area have the characteristics of outward expansion. During the β stage (between 300 to 700 days before the main earthquake), the anomalies are distributed in the southwest and northern region of the aftershock region, showing a large range of anomalies. During the γ stage (including γ_1 and γ_2, 300 days before the main earthquake),the range of anomaly distribution is wide,and the anomalies are distributed in the southwest and northeast of the aftershock area. The anomalies converged to epicenter (γ_1) in the far source region and expand outwards (γ_2) in the near source region. Results of the experimental study and mechanical analysis of earthquake preparation process indicate that the three-stage characteristics of precursory anomalies in the process of earthquake preparation may be controlled by the seismogenic body,which is a form of expression in the process of earthquake preparation and a universal featureduring the earthquake preparation process,which has a certain guiding role in earthquake prediction.

Recent crustal movement and great earthquakes in Qinghai-Tibet sub-plate

Crustal movement and incremental-movement data observed repeatedly at GPS stations during 1999 -2009 were analyzed to study the effect of two earthquakes of Ms8.1 and Ms8.0 that occurred in 2001 and 2008, respectively, in Qinghai-Tibet sub-plate and its eastern margin. The result revealed certain anoma- lous pre-earthquake deformation and some large co-seismic changes. Prior to the 2008 Wcnchuan Ms8.0 earthquake, the seismogenic Kunlunshan fault zone became a geographic boundary between different regional movements. At the time of the earthquake, there was an average cross-fault crustal shortening of - 1.04 m and an average right-lateral strike slip of 0.76 m along the ruptured segment, as well as a strain-energy release of -62.66 ×10.7.

2008年汶川M_(S)8.0地震地表破裂带沙坝村吴家院一带几何结构变化特征研究

以往学者通常侧重于大尺度(数公里级别)的地表形态对大地震同震地表破裂的影响研究,而鲜有涉及短距离尺度(10~100 m级)的相关报道。本文基于2008年汶川M_(S)8.0地震地表破裂带沙坝村吴家院一带的几何展布样式与微地貌测绘,为回答短距离尺度是否存在显著的几何结构变化和探究其影响因素提供一个典型案例。具体通过无人机摄影测量技术及激光雷达技术精确的获取了该段落断层展布特征及近断层地形地貌数据,发现吴家院一带地表破裂带在西南侧地形相对稳定无起伏时,破裂带集中为一支断层,而往北东向延伸遇到地形起伏如负地形时,断层分叉为三支分支断层。上述研究结果表明局部范围内地形起伏可能会导致同震地表破裂发生分叉现象,单一断层可能弥散为多支断层。

龙门山构造带及汶川震源区的S波速度结构

利用四川地震台网的观测资料和体波地震层析成像方法反演了龙门山地区的S波速度结构,据此分析了龙门山断裂带的地壳结构和汶川震源区的深部构造特征.反演结果表明,地震破裂与龙门山断裂及其两侧的地壳结构差异存在明显的对应关系,汶川以北的龙门山上地壳具备较高的强度且明显抬升,灌县至江油是龙门山西侧应力积累的主要地区,汶川8.0级地震位于其南部边缘;四川盆地的刚性地壳向西俯冲于龙门山之下,其凸出部与造山带古老基底在坟川附近发生碰撞是汶川成为8.0级地震破裂起始点的主要原因.汶川以南的龙门山地区地壳上层具有较大的韧性,岩石强度相对减弱,与龙门山北部相比不易于应力积累和产生破裂,因而汶川以南的龙门山断裂缺少余震活动.龙门山地区地壳厚度明显增加,其原因与中下地壳具备较大的柔韧性有关.由于青藏东部向东挤出时受到四川盆地刚性岩石层的阻挡,龙门山中下地壳的塑性变形和垂向物质的增加导致地壳厚度加大和莫霍面下沉,以此方式吸收了龙门山地区的大部分地壳缩短量,地表则强烈褶皱抬升形成数千米的龙门山脉.

汶川Ms8．0地震重灾区次生地质灾害遥感精细解译

汶川Ms8.0地震诱发了大量次生地质灾害,主要包括崩塌、滑坡与泥石流等。在获取到全面的研究区震后多源遥感影像后,采用人工目视解译的方法,对14个重灾县(市)进行次生地质灾害解译工作,共解译出约46560处崩塌、滑坡、泥石流等地震次生地质灾害,灾害总面积约687.1km2。基于GIS平台获取到它们的位置、平面面积等基本信息,绘制了研究区的地震次生地质灾害分布图与点密度图。结果表明,本次地震次生地质灾害与龙门山3条主断裂有较好的对应关系,且主要分布在4个集中区域。最后,分析了地震次生地质灾害分布与高程、坡度、地震烈度之间的关系,高程范围为1000—2000m的灾害面积占灾害总面积的54.00%;坡度范围为30°—50°的灾害面积占灾害总面积的62.42%;地震烈度范围为Ⅺ—Ⅷ的次生地质灾害面积占灾害总面积的90.94%。为进一步研究汶川地震次生地质灾害的发育规律、发生机理、预测理论、灾区恢复重建与选址等问题提供了重要的次生地质灾害基础信息。

S变换在汶川地震前后应变变化分析中的应用

引入时频分析中的S变换方法,对姑咱台四分量钻孔应变观测曲线中出现的"压性脉冲"和"潮汐畸变"异常信号进行分析。结果表明,在背景信号的基础上,周期约10～60分钟的信号在汶川地震前开始大量出现,震后逐渐衰减;能量最大的信号仍然是背景信号,其能量幅度随地震发生时间的临近而增大,震后逐渐减弱。

2008年汶川M_(S)8.0地震前后地电阻率时空变化

采用归一化速率变化方法对2008年汶川M_(S)8.0地震震中周边19个定点台站观测到的地电阻率的时空变化过程进行了计算分析。时序分析结果显示:在汶川地震前,震中周边多个地电阻率台站准同步地出现了中短期前兆异常;空间分布过程显示,地震孕震过程中地电阻率在震中区域有负异常丛集和由远及近的迁移现象,负异常丛集呈象限分布,长轴方向与震源机制解主压力方向基本吻合,且受孕震断裂、台站位置、观测装置布设方向等的影响。

基于GRACE卫星和地表流动重力的汶川M_(S)8.0地震前重力时空变化

利用GRACE卫星重力场模型和地表流动重力观测资料,计算2008年汶川MS 8.0地震发生前6年的重力变化,对卫星和流动重力段差结果与卫星重力反映的重力场动态变化特征进行研究,结果表明:①GRACE卫星重力段差受滤波半径影响显著,与地表流动重力观测结果相比,在重力变化数值上差异较大,在变化率上较为一致;②在汶川地震孕育阶段,川滇地区重力等值线呈“增大—减速增大—减小”的特征,震前2年形成近似垂直于龙门山断裂带的重力变化梯度带。

汶川M_(S) 8.0、芦山M_(S) 7.0地震晚期显著余震的“窗口”效应

1研究背景地震窗是一些频度较高、地震丛集的小区域,如一些大震的余震区、经常发生小震群的区域等。这些区域对构造应力场的变化比较敏感,其地震频度或应变能等的异常变化有可能反映周边区域的应力场变化,从而可用以提取周围区域发生中强地震的前兆信息(中国地震局监测预报司,2020)。早期研究发现华北邢台、海城、唐山3次强震的余震序列可作为监视华北地区中强以上地震孕育的“窗口”(王泽皋,1979;姜秀娥等,1982)。

汶川M_(S)8.0地震前后南北地震带北段GPS垂向形变特征

2008年汶川M_(S)8.0地震对南北地震带北段垂向变形产生了影响,对1999-2016年多期GPS观测数据进行精密解算,采用多面函数方法分别对研究区垂向运动进行周年运动剔除和空间滤波,获取了汶川地震前后垂向形变特征。结果表明:①以巴颜喀拉块体与华南块体交界带西侧为界,其西部自东向西呈指数函数衰减;东部自西向东呈对数函数形式衰减,且东部衰减程度大于西部,两块体交界南北段垂向形变特征存在差异性,北段隆升速度和变化梯度均弱于南段;②地震前后南北地震带北段垂向形变特征具有明显差异性,相对震前,震后大部分区域隆升速度加快,平均约为2 mm/a,局部区域加快程度较大,最大值达7 mm/a;③垂向速度的差异性变化对地壳的垂向应变积累具有影响,最大正影响区域为巴颜喀拉块体与华南块体交界及其西侧区域,增大了约10×10^(-8)/a,约为震前的2倍,最大负影响区域为川滇块体北部局部区域,减小了约7×10^(-8)/a。建议对巴颜喀拉块体与华南块体交界及其西侧区域和川滇块体北部局部区域强化跟踪分析。

汶川M_(S) 8.0地震前小震潮汐应力触发特征研究

1研究背景地震是地球内部运动的一种力学过程,其动力来源于地球内部。但当震源区处于临界状态时,外界因素如固体潮等的影响在一定条件下可能会引起系统的突变而发生地震(Cochran et al,2004;Tanaka et al,2004;Cadicheanu et al,2007;Tanaka,2010)。针对潮汐触发地震的问题,国内外学者做了大量研究工作,主要集中在以下3个方面:一是对部分地区地震活动和月相的相关关系研究,结果显示,地震在某些月相附近发生的几率相对较高;二是基于潮汐应力分量的地震调制研究,通常是将潮汐应力分解为南北、东西、垂直3个方向,其中水平分量与地震的关系往往被作为研究的重点;三是利用一定的统计方法分析潮汐相位角,通过严格定量的大样本检验,其中Schuster检验应用较多,通过p值变化来反映潮汐与地震的关系。

Focal depths for moderate-sized aftershocks of the Wenchuan M_S8.0 earthquake and their implications

Sliding-window cross-correlation method is firstly adopted to identify sPn phase, and to constrain focal depth from regional seismograms, by measuring the time separation between sPn and Pn phases. We present the focal depths of the 17 moderate-sized aftershocks (MS≥5.0) of the Wenchuan MS8.0 earth-quake, using the data recorded by the regional seismic broadband networks of Shaanxi, Qinghai, Gansu, Yunnan and Sichuan. Our results show focal depths of aftershocks range from 8 to 20 km, and tend to cluster at two average depths, separate at 32.5°N, i.e., 11 km to the south and 17 km to the north, indicating that these aftershocks are origin of upper-to-middle crust. Combined with other results, we suggest that the Longmenshan fault is not a through-going crustal fault and the Pingwu-Qingchuan fault may be not the northward extension of the Longmenshan thrust fault.

The formation of the Wulipo landslide and the resulting debris flow in Dujiangyan City, China

The Wulipo landslide, triggered by heavy rainfall on July 10, 2013, transformed into debris flow,resulted in the destruction of 12 houses, 44 deaths, and 117 missing. Our systematic investigation has led to the following results and to a new understanding about the formation and evolution process of this hazard. The fundamental factors of the formation of the landslide are a high-steep free surface at the front of the slide mass and the sandstone-mudstone mixed stratum structure of the slope. The inducing factor of the landslide is hydrostatic and hydrodynamic pressure change caused by heavy continuous rainfall. The geological mechanical model of the landslide can be summarized as "instability-translational slide-tension fracture-collapse" and the formation mechanism as "translational landslide induced by heavy rainfall". The total volume of the landslide is 124.6×104 m3, and 16.3% of the sliding mass was dropped down from the cliff and transformed into debris flow during the sliding process, which enlarged 46.7% of the original sliding deposit area. The final accumulation area is found to be 9.2×104 m2. The hazard is a typical example of a disaster chain involving landslide and its induced debris flow. The concealment and disaster chain effect is the main reason for the heavy damage. In future risk assessment, it is suggested to enhance the research onpotential landslide identification for weakly intercalated slopes. By considering the influence of the behaviors of landslide-induced debris flow, the disaster area could be determined more reasonably.

b值下降幅度与汶川M_(S)8.0地震孕震区的关系

b值常用于评估区域应力水平。基于b值随应力升高而减小的理论基础,文中试图找出b值的下降幅度与汶川地震孕震区的关系。以汶川地震的震中为中心,选取10°×10°范围内2000年1月—2008年4月的地震目录,通过格点搜索法搜索出汶川地震前b值具有明显下降变化的异常格点,分析异常格点所在区域内b值的时序变化。结果显示,异常格点的分布随着b值下降幅度的增大逐渐收敛至龙门山断裂带的都江堰—北川段,与汶川地震破裂区的南段重合,汶川地震起始破裂点位于异常区的东南缘。异常区内b值随时间呈现“趋势下降—平稳变化—临震前急剧下降”的变化形态,分别持续3.5a、2a和4个月。文中关注b值的下降变化过程,能在较大范围内搜索出b值呈下降变化形态的区域,而这些区域对应于应力增强区,这可为未来强震的震中判定提供参考依据。

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。

汶川地震医学救援反思与区域性灾难医学救援体系建设

目的通过全面系统地总结和反思汶川地震医学救援中四川省应急救援的经验和启示,分析并探讨区域性灾难医学救援体系建设。方法收集、整理、分析各类统计数据、信息、专报资料和相关评估报告,采用描述性方法进行分析。结果①汶川地震灾区医疗卫生机构受到重创;②迅速紧急响应,各级、各类医疗救援队伍集结灾区;③震后72 h内为救治最高峰;④各级医院救治91 177例伤病员,跨省大规模伤员转移救治,强化危重伤病员集中收治策略。结论建立区域性灾难医学紧急救援基地和应急医学救援常态化运行管理机制;完善信息指挥调度平台;科学合理的配置医疗资源;建立不同层次、不同类型的应急医疗队伍和响应机制;参与其他区域灾难医学专业紧急医学救援。

汶川县渔子溪地震地质灾害特征及灾害链成生分析

渔子溪下游耿达乡-映秀镇河段是汶川地震触发震害最为严重、灾害链效应最为显著的河段之一。本文通过详细的实地调查和遥感影像分析,力求揭示该河段地质灾害的特征及灾害链的成生过程、成生条件。依据震害特征,将地质灾害划分为斜坡中上部强风化岩土体失稳坠落、块状岩质边坡滑移式垮塌及局地暴雨启动型泥石流3类,并分析震害发育规律。调查表明,灾害点的空间展布受控于发震断裂,且北岸发育密度更大,茂汶断裂两侧差异显著。对51个崩塌点及17条泥石流研究发现,地震崩塌灾害主要发生在40°以上斜坡,主要分布在斜坡中上部(0.4倍坡高以上)及地貌突出部位,且大纵比降的壮年期沟谷易发泥石流。同时,诱发因素的转变致使地质灾害向降雨主导的小规模单体崩塌、泥石流方向发展。区内主要存在2种地质灾害链:(1)内动力地质灾害链"地震→崩塌→压迫河道、毁路或形成堰塞湖",其成生过程经历高速启动、滑移运动、堵河3个阶段;(2)内外动力耦合作用地质灾害链"地震→崩塌、震裂山体→暴雨→泥石流→压迫河道、毁路或形成堰塞湖",成生过程可划分为启动、堵塞(沟谷后)溃决、铺床、堵河4个阶段。灾害链的成生条件概括为:脆弱的地质环境;强烈地震动震垮、震裂高陡斜坡(>50m,>40°);强降雨及适宜的堵河条件。