【作者】 姚路；

【导师】 马胜利； 嶋本利彦；

【作者基本信息】 中国地震局地质研究所 ， 固体地球物理学， 2013， 博士

【摘要】 为了认识龙门山断裂带在汶川地震中的同震滑动力学性质，我们对龙门山断裂带地表断层带露头上的断层泥及少量WFSD-1断层泥开展了中速-高速摩擦实验研究。主要关注的问题包括：龙门山断裂带高速摩擦性质及其不均匀性、震后断层强度恢复问题、高速滑动可能的主导弱化机制问题和宽速度域内摩擦滑动速度依赖性问题。基于对断层带结构和断层岩样品的精细分析，我们从矿坪子和八角庙断层带露头采集了四种断层泥样品。对这些样品及另外三种WFSD-1样品开展的高速摩擦实验揭示除了矿坪子露头黑色断层泥，其他几种断层泥的高速摩擦性状均表现为负指数衰减型滑动弱化。矿坪子露头黑色断层泥具有非常独特的力学性状：初始阶段快速的滑移弱化，接着出现微弱的滑移强化至第二个峰值，之后逐渐弱化至稳态。微结构观察和对比实验表明初始快速弱化和之后的微弱强化主要是黑色断层泥特殊的微结构演化造成的。基于经验公式和进一步的推导，我们选取了六个能够描述稳态摩擦系数、滑动弱化距离和比破裂能对速率和正应力依赖性的本构参数。我们对本论文和前人研究过的四个露头共计六种断层泥的高速摩擦性质进行了比较，从这上述六个本构参数的定量化比较来看，龙门山断裂带高速摩擦性质的不均匀性是存在的。不过，如果仅仅是考察摩擦强度，高速滑移下的稳态摩擦系数在整个断层带上的均匀性还是相当高的；如干燥条件下速率等于1.4m/s时几乎均小于0.2，在含水条件下还将更低。这些断层泥在高速滑移下显著的滑移弱化必定在汶川地震中极大地促进了破裂的传播，也能够解释震后钻探观测到的温度异常较小的现象。高速滑移下的slide-hold-slide实验揭示断层在经历高速滑移显著滑移弱化之后，能够在5~10s内快速恢复大部分的强度(摩擦系数增加约0.4)，愈合率(摩擦系数增量与保持时间对数的比值)达0.154~0.188；之后是跟时间对数成正比的缓慢愈合，愈合率约0.015~0.016。温度计算表明初始快速的强度恢复和温度的快速降低有关。5~10s的强度恢复时间尺度小于一般大地震的上升时间，从而支持自愈合滑动脉冲式的地震破裂模式。同时，断层高速滑移之后快速的强度恢复对稳定断层的摩擦滑动、减少余震也可能起到了重要作用。对极薄(0.1~0.14mm)和常规厚度断层泥(1.0~1.4mm)开展的中速-高速摩擦对比实验揭示薄层断层泥的剪切滑移能够更快地弱化断层，而且断层显著滑动弱化的特征滑动速率也低得多；微结构观察表明这可能是凹凸体急剧加热弱化机制在薄层断层泥实验中更加显著造成的；极薄和较厚断层泥高速摩擦性状的差异可能暗示着野外断层上相对更薄的断层泥层能够更快地在同震滑移中弱化断层、也能更大程度地促进地震破裂的传播。利用辉长岩和黄铜作围岩开展的高速摩擦对比实验揭示黄铜作围岩的实验中仅观察到微弱的滑动弱化，温度计算表明这与黄铜的高热导抑制了断层带的急剧升温有关，这暗示了凹凸体急剧加热弱化和热压作用这两种跟摩擦生热相关的机制对断层弱化的重要性。利用黄铜作围岩对纳米氧化镁开展的高速摩擦实验表明，在升温受抑制的情况下，即使是纯纳米颗粒物质也不能有效地显著弱化断层；这一结果表明纳米颗粒的滚动润滑机制不是断层高速滑动弱化的主要机制。对断层泥从低速到高速开展的摩擦实验研究表明，中等速率域内显著的速度强化是有可能存在的。但是实验同时揭示数百μm/s以下的摩擦滑动在大剪切应变下(如,>1000)存在显著的应变硬化(滑移强化)，这可能与断层泥在大应变下发生的物质和结构上的改变有关，今后对宽速度域内摩擦滑动速度依赖性进行研究时，需要充分考虑应变硬化带来的影响。更多还原

【Abstract】 In order to understand frictional properties of the Longmenshan fault zone in thecoseismic sliding during the2008Wenchuan earthquake, we conducted intermediate-to high-velocity frictional experiments on fault gouges collected from theLongmenshan fault zone at surface outcrops and from the WFSD-1borehole (a smallamount). The mainly concerned issues in this thesis include the high-velocityfrictional properties and their heterogeneities of the Longmenshan fault zone, thepost-seismic strength recovery of the seismic fault, possible dominant slip-weakeningmechanisms during the high-velocity sliding of the seismic fault and thevelocity-dependence of fault friction over a wide velocity regime.Based on detailed analysis of the fault-zone structures and fault rocks, weselected four kinds of fault gouges from the fault zone at the Kuangpingzi outcrop andthe Bajiaomiao outcrop. High-velocity experiments conducted on these four gougesand other three WFSD-1gouges revealed typical slip-weakening behaviors of anexponential-decay type, except that a kind of black gouge from the PingxiKuangpingzi outcrop showed peculiar frictional behavior which was characterized bya sharp initial weakening, subsequent slight strengthening to the second peak friction,and slip weakening towards steady-state friction. Microstructure observations andresults of some comparison experiments suggested that the special microstructureevolutions were the causes for the rapid initial weakening and subsequentstrengthening of the black gouge. Based on several empirical formulas and furtherderivation, we selected six constitutive parameters that can quantitatively describe thedependences of steady-state friction coefficient, slip-weakening distance and specificfracture energy on the slip rate and normal stress. We compared the high-velocityfrictional properties of six kinds of gouges from four surface outcrops studied in thiswork and former published work. The results show that the heterogeneity do exist forthose six constitutive parameters. But the Longmenshan fault zone is quite uniform interms of steady-state friction coefficient at high slip rates, e.g. less than0.2almost for all gouges at a slip rate of1.4m/s under dry conditions and much smaller under wetconditions. The observed dramatic high-velocity weakening must have promoteddynamic rupture propagation during the Wenchuan earthquake, and also can explainthe very small temperature anomalies measured in the drilling borehole.Slide-hold-slide (SHS) tests conducted on fault gouges at high velocity revealedvery rapid strength recovery by more than0.4in friction coefficient in less than5~10sfrom the former high-velocity slip weakening, and the healing rate (increase in frictioncoefficient divided by the logarithm of hold time) was as high as0.154-0.188duringthis rapid recovery stage. Then the fault strength recovered slowly in proportion to thelogarithm of time with a healing rate of0.015-0.016. Temperature calculationindicated that initial rapid strength recovery was related to rapid temperature drop.The time scale of the initial rapid strength recovery is about5-10s, which is less thanthe common rise time of large earthquakes; thus the observed results supportself-healing slip pulses in earthquake rupture. Also, rapid strength recovery of thefault after high-velocity sliding may play an essential role in stabilizing the fault andreducing aftershocks.Comparison experiments conducted on very thin (0.1-0.14mm) andcommon-width gouge layers (1.0-1.4mm) at intermediate to high velocitydemonstrated that shearing thin gouge layers could weaken faults more rapidly, andthe characteristic weakening velocity was also much lower for the thin gouge layercase. Microstructure observations suggested this was probably due to much notableflash heating in thin gouge experiments. The difference of high-velocity frictionalbehaviors between the fairly thin and relatively thick gouge tests suggest thatrelatively thin gouge layers in a natural fault zone may weaken the fault more rapidlyduring the seismic slip and can probably promote the propagation of a seismic rupturemore effectively. Comparison experiments using gabbro and brass host blocksrevealed that only slight slip weakening was observed in the latter tests. Thetemperature calculation showed that this might result from the restrained temperaturerise due to high thermal conductivity of brass host blocks. This phenomenon alsohighlights the importance of the thermally-related mechanisms, flash heating and thermal pressurization, on fault weakening. High-velocity experiments conducted onMgO nano-particles using brass host blocks revealed that even the pure nano-particlescould not weaken the fault effectively when the temperature rise was restricted. Theseresults suggest that rolling lubrication of nano-particles may not be one of the mainhigh-velocity weakening mechanism.Low-to high-velocity friction experiments on fault gouges from theLongmenshan fault zone revealed possible strong velocity strengthening in theintermediate velocity regime. But we also observed notable strain hardening (slipstrengthening) at slip rates less than hundreds of μm/s under large shearstrain(e.g.>1000), which may be related to material or structural change of the testedgouges under large shear strain. We will fully consider this effect in the study ofvelocity-dependence of friction over wide velocity range in the future.更多还原