Preliminary Results of In‐situ Stress Measurements along the Longmenshan Fault Zone after the Wenchuan Ms 8.0 Earthquake

Abstract: Four months after the Wenchuan Ms 8 earthquake in western Sichuan, China, in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake hazard assessment. In-situ stresses were measured in three new boreholes by using overcoring with the piezomagnetic stress gauges for shallow depths and hydraulic fracturing for lower depths. The maximum horizontal stress in shallow depths (~20 m) is about 4.3 MPa, oriented N19°E, in the epicenter area at Yingxiu Town, about 9.7 MPa, oriented N51°W, at Baoxing County in the southwestern Longmenshan range, and about 2.6 MPa, oriented N39°E, near Kangding in the southernmost zone of the Longmenshan range. Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth. A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan range, and is still moderate in the epicenter zone. These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.     

汶川地震后沿龙门山裂断带原地应力测量初步结果

2008年5月12日在中国四川省西部汶川发生Ms8.0地震,震中位于青藏东缘龙门山断裂带。地震发生后的4个月,沿龙门山断裂带中南段开展了原地应力测量,获得了3个测点的应力大小和方向。在3个测孔中浅部采用压磁应力解除法,深部采用水压致裂法。浅部测量结果显示,位于震中区映秀测点,水平最大主应力值为4.3MPa,最大主应力方向为N19°E;宝兴测点位于震中区西南的龙门山断裂带南段,汶川地震没有导致该段地表破裂,该点获得的水平最大主应力值为9.8MPa,最大主应力方向为N51°W;位于龙门山断裂带最西南端的康定测点,水平最大主应力值为2.6MPa,最大主应力方向为N39°E。利用水压致裂法对各钻孔100～400m深度进行了应力测量,获得了应力随深度变化趋势和应力状态。与震前其它应力测量结果和中国其它地区表层地应力测量结果比较,龙门山断裂带西南段处于相对高应力水平,震中区仍处于中等应力水平。这项研究成果将为评价龙门山断裂带余震和今后强震发展趋势提供关键构造物理参数。     

Tectonic Stress State Changes Before and After the Wenchuan Ms 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

Crustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan Ms 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.     

汶川Ms8级地震形成的擦痕形迹及其意义探讨

2008年5月12日14时28分(北京时间)中国四川省汶川县境内发生里氏80级地震。北川-映秀断裂是汶川Ms8级地震的发震断裂之一,以逆(右行)走滑活动为主,延伸长约220 km,活动强度大于其他两条断裂。主震在北川-映秀地表破裂带局部地点形成清晰的擦痕,它的位置和规模可能是深部破裂滑动量峰值在地表的响应。根据断层擦痕反映的应力场及位移与破裂长度关系等特征,将北川-映秀破裂带分为虹口段、北川段、南坝段。虹口段长度约22~45 km,同震位移量约45 m,σ1呈NWW-NW向,至少经历了三期活动,第三期逆冲活动强度最大,改造前两期逆(右行)走滑活动,它们的形成可能均与本次主震相关;北川段长约90~100 km,同震位移量约35 m,σ1呈NWW向近水平,活动强度弱于虹口第三期;南坝段长约35~50 km,同震位移量约25 m,以NWW向近水平挤压为特征,σ1方位角与中、南段相差约180°,活动强度与南段前两期相近。     

龙门山地震断裂带地应力分布及其对井壁稳定的 影响———以WFSD-2 井为例

[摘 要]龙门山地震断裂带是我国最为强烈的地震带之一,地层破碎、地应力异常,钻孔缩径造成孔内事故频发,井壁稳定问题十分突出。本文介绍了汶川地震断裂带科学钻探施工中所发生的孔内事故情况,并通过对地层应力数据的统计分析,得出龙门山地震断裂带最大水平应力和最小水平应力随深度变化的回归曲线,最大水平应力梯度为4. 52MPa/100 m,最小水平应力梯度为2. 51 MPa/100 m。以WFSD-2 井为例,分析了龙门山地震断裂带地层应力、尤其是断层泥应力对井壁稳定的影响;针对膨胀性地层,介绍了一种通过泥浆密度微调现场测定地层坍塌压力和破裂压力的简易方法。     

Dextral‐Slip Thrust Faulting and Seismic Events of the Ms 8.0 Wenchuan Earthquake,Longmenshan Mountains,Eastern Margin of the Tibetan Plateau

Abstract: Dextral-slip thrust movement of the Songpan-Garzê terrain over the Sichuan block caused the Ms 8.0 Wenchuan earthquake of May 12, 2008 and offset the Central Longmenshan Fault (CLF) along a distance of ~250 km. Displacement along the CLF changes from Yingxiu to Qingchuan. The total oblique slip of up to 7.6 m in Yingxiu near the epicenter of the earthquake, decreases northeastward to 5.3 m, 6.6 m, 4.4 m, 2.5 m and 1.1 m in Hongkou, Beichuan, Pingtong, Nanba and Qingchuan, respectively. This offset apparently occurred during a sequence of four reported seismic events, EQ1–EQ4, which were identified by seismic inversion of the source mechanism. These events occurred in rapid succession as the fault break propagated northeastward during the earthquake. Variations in the plunge of slickensides along the CLF appear to match these events. The Mw 7.5 EQ1 event occurred during the first 0–10 s along the Yingxiu-Hongkou section of the CLF and is characterized by 1.7 m vertical slip and vertical slickensides. The Mw 8.0 EQ2 event, which occurred during the next 10–42 s along the Yingxiu-Yanziyan section of the CLF, is marked by major dextral-slip with minor thrust and slickensides plunging 25°–35° southwestward. The Mw 7.5 EQ3 event occurred during the following 42–60 s and resulted in dextral-slip and slickensides plunging 10° southwestward in Beichuan and plunging 73° southwestward in Hongkou. The Mw 7.7 EQ4 event, which occurred during the final 60–95 s along the Beichuan-Qingchuan section of the CLF, is characterized by nearly equal values of dextral and vertical slips with slickensides plunging 45°–50° southwestward. These seismic events match and evidently controlled the concentrations of landslide dams caused by the Wenchuan earthquake in Longmenshan Mountains.     

汶川地震后龙门山断裂带活动特征

利用地震后2009~2011年GPS监测数据,获得了龙门山断裂带所在地区2009~2010年、2010~2011年以及2009~2011年GPS测站运动速度场,分析了区域地壳运动总体趋势及形变特征;通过分析龙门山断裂带北段、中段、南段横切剖面的测站运动速度变化,探讨了汶川地震后龙门山断裂带运动特征。分析表明:汶川地震前后,地壳运动总体趋势未变,作顺时旋转;断裂带西侧GPS测站运动速度变大,东侧运动速度变小;龙门山断裂带的断裂性质地震前后都为右旋走滑挤压,断裂带运动速率受汶川地震影响较大,震后运动速率较震前有显著的增加。龙门山断裂带震后各段次级断裂活动不同,中南段以前山断裂运动为主,其它各段以后山断裂运动为主。地震后龙门山断裂带表现出的运动特征主要与地震活动有关。受汶川地震的影响,区域动力学、运动学平衡被打破,龙门山断裂带东侧震后初期弹性回返,表现为低速反向运动。龙门山断裂带西侧震后松弛为拉张区,运动速度加大。地震对断裂带的影响不同,导致断裂带各段及次级断裂表现出不同的运动特征。     

Piezomagnetic In-situ Stress Monitoring and its Application in the Longmenshan Fault Zone

The relative change of in-situ stress is an inevitable outcome of differential movement among the crust plates. Conversely, changes of in-situ stress can also lead to deformation and instability of crustal rock mass, trigger activity of faults, and induce earthquakes. Hence, monitoring real-time change of in-situ stress is of great significance. Piezomagnetic in-situ stress monitoring has good and longtime applications in large engineering constructions and geoscience study fields in China. In this paper, the new piezomagnetic in-situ stress monitoring system is introduced and it not only has overall improvements in measuring cell＇s structure and property, stressing and orienting way, but also enhances integration and intelligence of control and data transmission system, in general, which greatly promotes installing efficiency of measuring probe and quality of monitoring data. This paper also discusses the responses of new piezomagnetic system in large earthquake events of in-situ stress monitoring station at Qiaoqi of Baoxing and Wenxian of Gansu. The monitoring data reflect adjustments and changes of tectonic stress field at the southwestern segment of and the northern area near the Longmenshan fault zone, which shows that the new system has a good performance and application prospect in the geoscience field. Data of the Qiaoqi stress-monitoring station manifest that the Lushan Earthquake did not release stress of the southwestern segment of the Longmenshan fault zone adequately and there still probably exists seismic risk in this region in the future. Combined with absolute in-situ stress measurement, carrying out long-term in-situ stress monitoring in typical tectonic position of important regions is of great importance for researchers to assess and study regional crust stability.     

Analysis of the Wenchuan Ms8.0 Earthquake’s Co-seismic Stress and Displacement Change by Using the Finite Element Method

The variation of in situ stress before and after earthquakes is an issue studied by geologists. In this paper, on the basis of the fault slip dislocation model of Wenchuan Ms8.0 earthquake, the changes of co-seismic displacement and the distribution functions of stress tensor around the Longmen Shan fault zone are calculated. The results show that the co-seismic maximum surface displacement is 4.9 m in the horizontal direction and 6.5 m in the vertical direction, which is almost consistent with the on-site survey and GPS observations. The co-seismic maximum horizontal stress in the hanging wall and footwall decreased sharply as the distance from the Longmen Shan fault zone increased. However, the vertical stress and minimum horizontal stress increased in the footwall and in some areas of the hanging wall. The study of the co-seismic displacement and stress was mainly focused on the long and narrow region along the Longmen Shan fault zone, which coincides with the distribution of the earthquake aftershocks. Therefore, the co-seismic stress only affects the aftershocks, and does not affect distant faults and seismic activities. The results are almost consistent with in situ stress measurements at the two sites before and after Wenchuan Ms8.0 earthquake. Along the fault plane, the co-seismic shear stress in the dip direction is larger than that in the strike direction, which indicates that the faulting mechanism of the Longmen Shan fault zone is a dominant thrust with minor strike-slipping. The results can be used as a reference value for future studies of earthquake mechanisms.     

汶川Ms8.0地震强震动基线改正及其在位错反演中的初步应用

通过研究近场强震动记录, 发现汶川M_s8.0地震近场峰值加速度在空间上存在较明显的上盘效应和方向性效应, 与汶川引起的地质灾害空间分布具有较好的一致性.但在所有强震仪所记录的汶川M_s8.0地震同震加速度记录积分所得地壳同震速度中, 有的台站数据存在典型的线性偏移, 有的台站数据除线性偏移外还存在明显的非线性偏移.采用非线性基线改正方法处理汶川M_s8.0强震同震记录, 改正后所得同震位移明显要比线性基线改正更合乎实际情况.以强震动、GPS和InSAR同震位移处理结果做约束, 反演了汶川M_s8.0地震同震位错分布, 对于汶川M_s8.0地震主要同震破裂断裂(北川-映秀断裂), 强震动反演结果不仅较好地刻画了汶川M_s8.0地震同震主断裂上地表破裂空间分布详细变化特征, 同时也较好地反映北端破裂衰减情况, 该结果表明: 强震动资料可以为强震后的救援和灾害评估等工作提供具有参考价值的研究结果; 另一方面, 受数据数量的制约, 用强震动改正后位移反演所得位错分布中仅汉旺断裂南段存在较为明显位错, 强震仪布设时应更多地考虑是否相对均匀地分布在具有发震潜势的断裂周缘, 以期更好地在震后应急救灾中发挥更好的作用.      

Dextral-Slip Thrust Faulting and Seismic Events of the Ms 8.0 Wenchuan Earthquake,Longmenshan Mountains,Eastern Margin of the Tibetan Plateau

Dextral-slip thrust movement of the Songpan-Garze terrain over the Sichuan block caused the Ms 8.0 Wenchuan earthquake of May 12, 2008 and offset the Central Longmenshan Fault (CLF) along a distance of-250 km. Displacement along the CLF changes from Yingxiu to Qingchuan. The total oblique slip of up to 7.6 m in Yingxiu near the epicenter of the earthquake, decreases northeastward to 5.3 m, 6.6 m, 4.4 m, 2.5 m and 1.1 m in Hongkou, Beichuan, Pingtong, Nanba and Qingchuan, respectively. This offset apparently occurred during a sequence of four reported seismic events, EQ1-EQ4, which were identified by seismic inversion of the source mechanism. These events occurred in rapid succession as the fault break propagated northeastward during the earthquake. Variations in the plunge of slickensides along the CLF appear to match these events. The Mw 7.5 EQ1 event occurred during the first 0-10 s along the Yingxiu-Hongkou section of the CLF and is characterized by 1.7 m vertical slip and vertical slickensides. The Mw 8.0 EQ2 event, which occurred during the next 10-42 s along the Yingxiu-Yanziyan section of the CLF, is marked by major dextral-slip with minor thrust and slickensides plunging 25°-35° southwestward. The Mw 7.5 EQ3 event occurred during the following 42-60 s and resulted in dextral-slip and slickensides plunging 10° southwestward in Beichuan and plunging 73° southwestward in Hongkou. The Mw 7.7 EQ4 event, which occurred during the final 60-95 s along the Beichuan-Qingchuan section of the CLF, is characterized by nearly equal values of dextral and vertical slips with slickensides plunging 45°-50° southwestward. These seismic events match and evidently controlled the concentrations of landslide dams caused by the Wenchuan earthquake in Longmenshan Mountains.     

用地震资料估计的龙门山断裂深部形变及其对于汶川地震成因的意义

2008年5月12日汶川8.0级地震前龙门山断裂被“忽视”的原因之一是地质学证据和GPS测量证据均显示龙门山断裂长期以来形变速率很低。问题是,构造地质的结果是对一个较长的时间尺度的,而GPS结果反映的是较短时间尺度上的、地表上的、水平方向的形变,因此有理由怀疑由此得到的结论能否反映现今龙门山断裂的深部形变的全貌。我们采用类似于Kostrov方法的思路,利用最近30 a的地震资料,试图研究龙门山断裂的深部形变,并与其相邻的断裂进行比较。利用ML2.5以上的微震资料,给出了沿龙门山断裂带的累积Benioff应变,并根据震级频度关系计算了a值的空间分布。作为对照,同时计算了龙门山断裂邻区沿鲜水河、安宁河、则木河断裂各区域内的累积Benioff应变和a值。结果表明,在与地质学证据不同的时间尺度上,在与GPS证据不同的时间尺度、不同的深度上,并且不仅考虑到水平形变,与周边的断裂带相比,龙门山断裂带其实并不是一个“安静的”断裂带。在更短的时间尺度上,可以回溯性地观察到微震活动的一些异常变化,但这些变化似乎很难用于该地震的预测。我们讨论了相关的观测资料分析对于汶川地震的成因的意义。     

Process Analysis of In-situ Strain during the Ms8.0 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.     

汶川Ms 8.0级地震强震记录所揭示的地震断层特征分析

基于2008年5月12日汶川Ms8.0级地震的强震台站记录,对比发震断层两侧的峰值加速度与地表破裂带上同震位移的分布特点,探讨了地震动强度分布特征与地表破裂位移分布之间的相关关系。分析近断层典型台站的强震动记录时程特征,获得了强地震动记录中所包含的断层破裂过程和破裂习性信息,从强震观测记录的角度进一步证实了汶川地震主震的多次破裂特征。结果表明,汶川地震主震至少包含了4次地震破裂事件,最主要的前两次破裂事件分别对应映秀—北川断裂段和北川—南坝断裂段的破裂过程,后两次破裂事件释放的能量相对较小,应该是第二次破裂过程触发局部次级破裂所引起的。此外,垂直于断层的峰值加速度剖面揭示的发震断层的高倾角逆冲特性,与地震地质调查和小震精定位等确定的相应结果是一致的。     

汶川8.0级特大地震震源断裂特征及其动力学分析

笔者根据地壳表层地质构造和地震地质研究与地震测深和大地电磁测深成果,运用现代构造解析理论与方法,论证了汶川8.0级特大地震的深部构造环境,探讨了汶川8.0级特大地震震源区的地震断裂和震源断裂基本特征与相互关系,及其形成的地球动力学问题。笔者认为龙门山碰撞造山带深处隐伏壳幔韧性剪切带可能是汶川8.0级特大地震主震区的震源断裂,而地壳表层发育的映秀断裂带、北川断裂带和彭县—灌县断裂带等可能是汶川8.0级特大地震主震区的地震断裂,该区震源断裂与地震断裂既有显著区别,又有密切联系。研究表明,在印度板块与太平洋板块和菲律宾海板块对欧亚板块俯冲碰撞的动力学作用下,形成上扬子地块向青藏高原东缘碰撞—楔入以及青藏高原东缘向东仰冲,深部向东俯冲的动力学态势,造成龙门山碰撞造山带切割莫霍界面的壳幔韧性剪切带向中上地壳扩展,应力高度集中与能量快速释放破裂,从而引起汶川8.0级特大地震的发生,以及大型同震地表破裂带的形成。探索震源断裂与地震断裂区别与联系,对进一步研究地震机制与发震动力学以及防震减灾有重要意义。     

汶川Ms 8.0级地震地表破裂带近断层水平缩短量研究

2008年5月12日四川汶川发生Ms8.0特大地震后,地表同震位移量已有了大量的详细调查和研究,然而对于近断层同震水平缩短量的研究却相对较少。笔者在中央断裂、前山断裂以及北西向分支断裂上选择合适地点进行大量探槽开挖,获得了近断层同震水平缩短量的分布情况为:中央断裂清平镇(2.8 m)、擂鼓镇(3.2 m)、平通镇(1.3 m),前山断裂白鹿镇(2.5 m)、九龙镇(1.4 m)、汉旺镇(0.6 m)。文中进一步在中央断裂和小鱼洞分支断裂地表破裂带上开挖数个探槽研究其近断层水平缩短量的问题,得到结果如下:龙门山中央断裂带映秀镇、擂鼓镇、平通镇探槽近断层水平缩短量分别约为(2.6±0.1)m、(2.6±0.2)m、(1.8±0.1)m;小鱼洞分支断层近断层水平缩短量约为(2±0.1)m。汶川5.12地震中央断裂地表破裂近断层较大水平缩短量出现在深溪沟和擂鼓一带,分别约为3.4 m、3.2 m;前山断裂地表破裂带近断层水平缩短量最大值出现在白鹿一带,约为2.5 m,白鹿以北,近断层水平缩短量逐渐减小。中央断裂和前山断裂联合破裂段水平缩短量值之和大于中央断裂带其两侧段落,最大水平缩短量总和可能约为5.3 m,地表破裂带近断层水平缩短量为整个地壳缩短量的主体部分。     

四川汶川5.12大地震同震滑动断层泥的发现及意义

2008年汶川8.0级地震沿龙门山断裂带内的映秀—北川断裂和灌县—安县断裂产生了近300 km的同震地表破裂带。震后地质科学考察发现地表变形以逆冲为主,并伴有右旋走滑。地震地表破裂带大多沿古生代碳质泥岩、页岩和三叠系煤系地层内的滑动面出露地表,这些软弱地层为地震破裂带冲到地表提供了超低摩擦滑动带。我们发现在同震垂直和水平位错达6m左右的地表破裂带,地震的同震滑动发生在厚度约0.5~2cm 的狭窄滑动带内,以发育新鲜的灰色断层泥为特征,这些断层泥是地震断层快速滑动过程中岩石—流体相互作用的结果。     

四川汶川5.12大地震同震滑动断层泥的发现及构造意义

2008年汶川8．0级地震沿龙门山断裂带内的映秀-北川断裂和灌县-安县断裂产生了近300km的同震地表破裂带。震后地质科学考察发现地表变形以逆冲为主,并伴有右旋走滑。地震地表破裂带大多沿古生代碳质泥岩、页岩和三叠系煤系地层内的滑动面出露地表,这些软弱地层为地震破裂带冲到地表提供了超低摩擦滑动带。我们发现在同震垂直和水平位错达6m左右的地袁破裂带,地震的同震滑动发生在厚度约O．5—2cm的狭窄滑动带内,以发育新鲜的灰色断层泥为特征,这些断层泥是地震断层快速滑动过程中岩石-流体相互作用的结果。     

Surface Rupture and Co-seismic Displacement Produced by the Ms 8.0 Wenchuan Earthquake of May 12th, 2008, Sichuan, China： Eastwards Growth of the Qinghai-Tibet Plateau

An earthquake of Ms 8 struck Wenchuan County,western Sichuan,China,on May 12~(th), 2008 and resulted in long surface ruptures (>300 km).The first-hand observations about the surface ruptures produced by the earthquake in the worst-hit areas of Yingxiu,Beichuan and Qingchuan, ascertained that the causative structure of the earthquake was in the central fault zones of the Longmenshan tectonic belt.Average co-seismic vertical displacements along the individual fault of the Yingxiu-Beichuan rupture zone reach 2.5-4m and the cumulative vertical displacements across the central and frontal Longmenshan fault belt is about 5-6 m.The surface rupture strength was reduced from north of Beichuan to Qingchuan County and shows 2-3 m dextral strike-slip component.The Wenchuan thrust-faulting earthquake is a manifestation of eastward growth of the Tibetan Plateau under the action of continuous convergence of the Indian and Eurasian continents.