采自汶川地震断层带样品渗透性的初步实验研究

断层带被认为是深部流体的主要通道和流体活动最强的区段之一.近年来,地震断层带中流体活动及其效应倍受关注,其最主要原因是地震断层流体活动与地震的孕育和发生似乎存在某种关联,流体被认为是理解地震成核、发生和断层扩展等过程的重要因素.     

断层岩气体渗透率及Klinkenberg效应

在围压2~40MPa变化范围内,以恒流法多次变化上游压力测量了以断层岩为主的样品的气体(N2)渗透率,将实验结果进行了Klinkenberg效应校正。对实验数据的拟合分析表明,滑脱因子b值与绝对渗透率kl存在b=λkl-d形式的幂律关系,断层岩符合b=0.004 6kl-0.476的变化关系。与沉积岩相比,断层岩的气体滑脱效应更强,采用气体测量渗透率时,其滑脱效应不能忽略。断层岩气体渗透率和绝对渗透率与测量所用的孔隙压力间的关系为kg/kl=1+(0.009 2kl-0.476)/(Pu+Pd)。结果表明样品渗透率越低,滑脱效应越强,提高孔隙压力,滑脱效应逐渐减小;对于高渗(10-15m2~10-18m2)的样品,高孔隙压力下(4MPa以上)的气体渗透率与绝对渗透率基本一致,对于超低渗(10-22m2~10-20m2)的样品,即使提高孔隙压力亦很难避免滑脱效应。在40MPa有效压力下断层泥样品的绝对渗透率为4.54×10-19m2~2.43×10-17m2,角砾岩的绝对渗透率较断层泥高出1~2个数量级,为2.25×10-17m2~7.94×10-16m2,表明汶川地震断层带具有核部低、破碎带高的渗透结构,断层带核部具备热压作用发生所要求的低渗条件。     

龙门山断层脆-塑性转化带流变结构与汶川地震孕震机制

汶川地震发震断层为高角度逆断层,这种断层滑动和发生强震需要断层深部具备特殊的力学条件。发震断层地区地表出露若干韧性剪切带,其中不同类型石英变形具有不同的变形温度。细粒糜棱岩中的石英表现为高温位错蠕变,变形温度为500~700℃;含残斑初糜棱岩中的石英表现为中温位错蠕变,其变形温度为400~500℃;早期石英脉中的石英表现为低温位错蠕变,变形温度为280~400℃;晚期石英脉以碎裂变形为主,其变形温度为150~250℃。石英的这些变形特征显示出断层带经历了多期脆-塑性转化。根据糜棱岩中的重结晶石英的粒度估计的断层塑性流动应力为15~80MPa。石英和长石内的微量水以晶体缺陷水、颗粒边界水和流体包裹体水的形式存在,水含量随岩石的应变增加而升高,变化范围为0.01~0.15wt%。断层脆-塑性转化带内石英含有大量与裂隙愈合相关的次生流体包裹体,其捕获温度为330~350℃,流体压力为70~405MPa,估计的流体压力系数为0.16~0.9,代表强震发生后,断层带内产生的大量微裂隙逐渐愈合过程中的流体特征。在考虑断层带流体压力和应变速率变化条件下,利用石英流变参数建立了从间震期到地震成核阶段断层脆-塑性转化带流变结构和震后快速蠕滑阶段断层脆-塑性转化带流变结构。结果表明,在间震期、地震成核阶段、震后快速滑动阶段,断层强度和脆-塑性转化深度随应变速率和流体压力变化而变化,且脆-塑性转化特征与石英的变形机制、断层速度弱化和强化转化深度、汶川地震震源深度等吻合,显示映秀-北川断层具备摩擦滑动速度弱化和地震成核的基础,而断层带内存在高压流体可能是触发高角度逆断层滑动和汶川地震发生的主要机制。     

汶川地震断层岩气体和液体渗透率实验研究

以N2和水为孔隙流体在20-180 MPa围压范围内详细测量了汶川地震断裂带断层岩的渗透率,并同时测量了作为参照的砂岩的渗透率.实验结果表明,气体和液体渗透率均随围压增加而幂次衰减.在相同围压和孔隙压条件下,N2渗透率高于水渗透率接近1个数量级.将Klinkenberg效应校正后的气体渗透率（绝对渗透率）与水渗透率进行了比较.对比结果显示,砂岩的绝对渗透率与水渗透率基本一致;但断层岩绝对渗透率显著高于水渗透率,表明断层岩气体渗透率与水渗透率之差别不能完全由Klinkenberg效应解释.对实验数据的拟合分析表明,滑脱因子b值与绝对渗透率kl存在幂律关系（b=λkld）.断层岩符合b=0.2×10–3kl–0.557关系（R2=0.998）,显示绝对渗透率在10^–16-10^–20 m2范围内,控制断层岩渗透率的因素是一致的.另一方面,断层岩的d值明显小于砂岩的d值,暗示二者的滑脱效应存在差异.分析表明,断层岩中的粘土矿物颗粒表面吸附水及粘土矿物吸水膨胀导致有效孔隙尺寸减小是造成断层岩渗透率显著低于绝对渗透率的主要原因.研究结果显示,对于富含粘土矿物的断层岩,经Klinkenberg校正后的气体渗透率与水渗透率并不一致,因此采用相应的液体作为测量介质才能够更为准确和真实地揭示地下流体的渗流状况.     

汶川M_W7.9地震起始破裂断层几何结构

本文收集使用紫坪铺水库台网记录到的汶川地震主震P波波形资料,利用P波反投影叠加法获取了2008年5月12日汶川M_W7.9地震起始破裂的时空演化过程.通过分析本次大地震起始破裂阶段(0～1s)破裂点在三维空间内的分布特征,确定了本次大地震起始破裂位置及起始破裂断层几何结构模型.得到以下结果:汶川地震起始破裂点位于31.013±0.002°N、103.392±0.002°E,震源深度为8.2±0.4km,发震时刻为2008年5月12日14∶27∶58.80±0.4.汶川地震起始破裂的最佳断层面走向为NE48°,倾向NW35°,起始阶段破裂的深度范围为地下7.5～9km.     

由构造应力场研究汶川地震断层的分段性

采用构造应力场均匀性对断层或板块边界进行分段的方法, 利用ldquo;裁剪-粘贴rdquo;法给出的余震震源机制解资料, 进一步从应力场角度确定汶川地震发震断层南、 北段分界点位置及南、北段震后应力场. 研究结果表明, 发震断层南、北段分界点位于北川附近, 与先前的研究结果较为一致. 南、 北段震后应力场反演结果显示, 南、北段的最大主应力轴方位均呈北东东向, 且近水平. 南、北段应力场反演的平均拟合残差和置信区间均较大, 主要原因可能是因为南、北段内应力场不均匀性造成的. 因为本文仅由大余震给出了震源断层的一级分段, 南、北段内应做进一步的细分.      

汶川地震区断层围陷波探测

对汶川地震区开展震后科考和断层围陷波探测,本文主要介绍平通镇断层围陷波探测及初步结果。平通镇处在汶川8．0级地震断层带正上方,地表破坏十分严重。该断层围陷波测线横跨断层,以地震探槽为大致中点,沿NW—SE方向两端延伸,测线长约400m。这次观测结果表明,在汶川8．0级地震的新破裂带中可以观测到断层围陷波,反映了断层带内外的介质在物理性质上有较大的差异。该测线记录的断层围陷波优势频率大约为3—4Hz。探槽附近的台站断层围陷波较强,初步推测,该地段地壳内断层的宽度大约有200m。     

芦山地震断层的滑动分布与汶川地震断层的关系

本文利用2013年芦山M_S7.0级地震同震GPS数据反演了芦山断层几何与断层滑动分布,结果表明：芦山地震发震断层具有南陡北缓、上陡下缓的特征,低倾角的区域位于发震断层北段且靠近映秀断层的一侧;滑动分布模型的最大滑动量为0.82 m,其深度为13.67 km与小震发生集中平均深度12.5 km接近.我们选取1998-2014年龙门山断裂带区域地壳形变观测数据,拟合获得了龙门山断裂带走向方向上的速度分量,发现在汶川M_S8.0地震与芦山M_S7.0地震之间宽度约30 km破裂空区,龙门山断裂带西南段与东北段的形变分量以破裂空区为界方向相反.断裂带东北段（汶川地震主要发震断层）的形变分量方向与断层右旋走滑运动方向一致,而在断裂带西南段（芦山地震发震断层）的形变分量方向与断层左旋走滑运动方向一致.芦山地震走滑方向与汶川地震走滑方向相反是因为该断裂带构造运动在特有几何构造下受青藏高原东南向挤压,遇龙门山中段岩石圈楔状构造的阻挡,在汶川M_S8.0地震与芦山M_S7.0地震间的地震空区,形成了构造运动向其两侧分流的结果.

     

汶川8.0级地震发震断层的累积地震位错研究

2008年5月12日,四川省汶川县内发生MS8.0地震。此次地震沿龙门山中央断裂产生1条长达200km的同震地表破裂带。文中选择位于地震地表破裂带北段的南坝镇、凤凰村以及南段的映秀镇这3个地点,以被断层错断的河流阶地为研究对象,对多级阶地面上的地震地表破裂及断层陡坎地貌进行了野外实测工作。经过测量数据的计算和分析,得到了各级阶地上断层陡坎的高度,该值即为该阶地记录的地震断层的累积垂直位错量。若以本次地震的垂直位错量作为古地震位错量的均值,则可计算得到每级阶地累积的地震次数。研究结果表明,各点T1阶地形成以来仅经历过1次事件,即本次地震事件;T2阶地形成以来约经历了5次事件;T3阶地形成以来约经历了9～11次事件;T4阶地形成以来约经历了20次事件。在本文研究的基础上,结合前人的阶地测年数据,则可获得古地震复发间隔的可靠数据     

汶川MS 8.0地震断层与地震灾害初步分析

对于特大地震近断层地震场的空间分布的复杂性学术界一直很关注,由于样本地震数量和资料的不足,至今还没有非常清晰的结论。通过参加汶川MS8.0地震的应急科学考察,对震中区和高烈度区断层破裂带附近的地震灾害情况进行了现场调查。文中以典型事例为主线介绍了现场考察的结果,结合既往的研究成果和汶川地震的震源特性,分析讨论了地表破裂带、地震动以及建筑物震害之间的关系。结果表明:1)地震断层发生强变形和地表破裂对建筑物的损害现象非常明显,对具有大震级发震危险的断裂带,今后应该考虑进行一定宽度的破裂避让或采取针对性的必要措施。2)初步探讨了紧邻断裂带的建筑物没有倒塌的可能机理,第一,出现地表破裂的大部分区域为基岩或坚硬的场地,场地条件相对较好;第二,存在导致地表破裂的浅部有效应力降和破裂速率相对较低,导致了1s附近的地震动相对低下的可能性     

Wenchuan earthquake fault and seismic disaster

Major cases of the MS8.0 Wenchuan earthquake are obtained through field investigations of the epicenter and high-intensity areas, and the relationships among earthquake faults, ground motion and earthquake disasters near fault zones are analyzed.Both strong deformation and ground rupture lead to significant damages of the buildings, indicating that it is necessary to keep safe distance away from active faults and to take other necessary measures.There are two reasons for that the buildings near the surface ...     

龙门山断裂带地壳精细结构与汶川地震发震机理

利用2001年1月至2008年6月四川固定地震台网和临时地震台站记录到的大量P波到时资料,反演了龙门山断裂带及周边地区的地壳精细三维P波速度模型. 结果表明,汶川主震以北和以南地区的结构存在较大差异,以北地区的龙门山断裂带具有很强地壳不均匀性,这与该区发生了大量汶川地震的余震相一致. 这些结果有意义地改进了前人对龙门山断裂带仅为不同块体过渡带的认识. 汶川主震震源区下方存在有明显低波速异常体,表明流体可能存在于龙门山断裂带内. 这些流体可能直接影响汶川大震的形成. 本文的成像结果为下地壳流沿龙门山断裂带上浸提供了可靠的地震学证据.     

Internal structure of Longmenshan fault zone at Hongkou outcrop,Sichuan,China,that caused the 2008 Wenchuan earthquake

This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake,at an outcrop in Hongkou,Sichuan province,China.Present work is a part of comprehensive project of Institute of Geology,China Earthquake Administration,trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties.Outcrop studies could be integrate...     

Internal structure of Longmenshan fault zone at Hongkou outcrop,Sichuan, China,that caused the 2008 Wenchuan earthquake

This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30–40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. Fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. Fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drilled very close to the Hongkou outcrop. The presence of carbon and graphite, observed next to the slipping-zone, may affect the mechanical properties of the fault and also provide useful information about coseismic chemical changes.     

Internal structures and high-velocity frictional properties of Longmenshan fault zone at Shenxigou activated during the 2008 Wenchuan earthquake

This paper reports internal structures of a wide fault zone at Shenxigou, Dujiangyan, Sichuan province, China, and high-velocity frictional properties of the fault gouge collected near the coseismic slip zone during the 2008 Wenchuan earthquake. Vertical offset and horizontal displacement at the trench site were 2.8 m (NW side up) and 4.8 m (right-lateral), respectively. The fault zone formed in Triassic sandstone, siltstone, and shale about 500 m away from the Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system. A trench survey across the coseismic fault, and observations of outcrops and drill cores down to a depth of 57 m revealed that the fault zone consists of fault gouge and fault breccia of about 0.5 and 250–300 m in widths, respectively, and that the fault strikes N62°E and dips 68° to NW. Quaternary conglomerates were recovered beneath the fault in the drilling, so that the fault moved at least 55 m along the coseismic slip zone, experiencing about 18 events of similar sizes. The fault core is composed of grayish gouge (GG) and blackish gouge (BG) with very complex slip-zone structures. BG contains low-crystalline graphite of about 30 %. High-velocity friction experiments were conducted at normal stresses of 0.6–2.1 MPa and slip rates of 0.1–2.1 m/s. Both GG and BG exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient μ _p to steady-state friction coefficient μ _ss over a slip-weakening distance D _c. Deformation of GG and BG is characterized by overlapped slip-zone structures and development of sharp slickenside surfaces, respectively. Comparison of our data with those reported for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault zones are quite uniform and the high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.     

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaojiaqiao outcrop that was moved by about 0.5 m during the 2008 M_W7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243° or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coefficient of around 0.6 to very low steady-state friction coefficient of 0.1-0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.     

断层横向构造在逆冲型地震破裂中的作用——以汶川地震小鱼洞断层为例

大地震破裂大多由横向构造(如阶区、弯曲和分叉)所分割的多个段落组成.2008年5·12汶川地震破裂沿北东走向上穿过了多个横向构造部位,特别在震中北东45 km的位置,小鱼洞断层、北川断层和彭灌断层三者之间呈现复杂的断裂切割相交关系.复杂断层几何结构对破裂的扩展是有抑制还是促进的作用？在相交的断裂段之间是否存在最优的破裂顺序？本文以库仑应力分析为手段,探讨在汶川同震破裂初始30 s内,破裂在多分支断裂中选择扩展路径时的可能应力相互作用.库仑应力分析显示:如果北川断层先发生破裂,其滑动对小鱼洞断层和彭灌断层均产生强烈负应力的抑制作用,而彭灌断层的滑动却反而对小鱼洞断层和北川断层浅部有强烈正应力的促进作用.因此,从准静态应力分析角度,彭灌断层先于北川断层发生破裂的可能性较大,这一破裂顺序与小鱼洞断层参与同震破裂过程的事实相符.此外,小鱼洞断层在链接北川和彭灌断层的同震位移中可能起到桥梁作用,但非静态应力的影响.横向构造在逆冲型地震破裂扩展过程中起到的牵引作用使得逆冲型地震破裂能够比走滑型地震跨越更宽的阶区.横向构造是逆冲断裂带内广泛发育的构成单元,因此在地震危险性分析的最大潜在震级测算中应该考虑其作用.     

汶川地震区的流变结构与发震高角度逆断层滑动的力学条件

本文利用龙门山地区的地质、地球物理剖面、弹性波速和流变实验数据等,建立了汶川地震相关构造单元的地壳流变结构.川西高原和龙门山构造带的地壳流变结构中存在多个塑性流变层,而四川盆地地壳基本没有出现塑性流变层,这种复杂的流变结构是汶川地震孕育和发生的基础.岩石破裂-黏滑-摩擦实验表明,以二长花岗岩为代表的震源区岩石具有很高的破裂强度和摩擦强度,能够承受极大的差应力和积累巨大的能量,这是高角度逆断层能够滑动和汶川地震强度大的原因之一.高流体压力是高角度逆断层滑动和触发汶川地震的另一个必要条件,而龙门山断层带内可能存在这种比较高的流体压力.     

Experimental studies on gas and water permeability of fault rocks from the rupture of the 2008 Wenchuan earthquake,China

The permeabilities of fault rocks from the rupture of Wenchuan earthquake were measured by using nitrogen gas and distilled water as pore fluids under the confining pressure ranging from 20 to 180 MPa at room temperature. Experimental results indicate that both gas and water permeabilities decrease with increasing confining pressure, described by power law relationship, i.e., b = 0.2×10–3kl–0.557. The water permeability is about one order less than gas permeability and also half order smaller than the permeability corrected by the Klinkenberg effect, so-called intrinsic permeability. The differences in the permeabilies imply that the reduction of effective pore size caused by the adhesion of water molecules to clay particle surface and water-swelling of expandable clay minerals contributes to lessening the water permeability besides the Klinkenberg effect. Hence, the liquid permeability of fault rocks cannot be deduced by gas permeability by the Klinkenberg correction reliably and accurately, and it is necessary to use liquid as pore media to measure their transport property directly.