【作者】 刘照星；

【导师】 周永胜；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2011， 硕士

【摘要】 汶川地震发震断层为高角度逆断层,断层倾角在50°-80°范围。高角度逆断层滑动需要断层深部有高压流体存在。论文总结了断层弱化的几种机制,以及流体在断层弱化和断层愈合中的作用。最后,运用高温高压实验手段模拟流体在断层弱化和愈合中的作用。流体对断层的弱化作用表现在物理作用和化学作用两方面。一方面,高压流体能够降低作用在断层上的有效应力,另一方面,流体会与断层中的矿物发生反应生成一些摩擦系数极低的层状矿物。流体与断层的反应是可逆的,地震后流体环境改变时,断层中的层状矿物可以转变成长石等强度较大的矿物,使裂隙愈合。因此,在地震循环的不同阶段,流体在断层中所起的作用不同。发生地震时,流体对断层的弱化作用主要表现在降低断层的有效应力;震后在流体作用下压溶、矿物相变等导致断层愈合。在间震期,流体不断积累,流体压力增大,同时流体的化学作用导致断层弱化。论文首先对高温高压设备进行了系统的轴压标定,对实验中影响轴压摩擦力的主要因素进行总结。轴压摩擦力可以概括为静摩擦力、软介质挤出摩擦力和轴压杆滑动摩擦力。大量实验数据表明,围压、温度、位移速率、装样方式(盐的类型)等实验条件都对轴压摩擦力有影响。其中,围压与静摩擦力和滑动摩擦力正相关;轴压位移速率与静摩擦力正相关,但影响较小,对滑动摩擦力没有影响;温度与静摩擦力和滑动摩擦力负相关,而且影响最显著。静摩擦力和挤出摩擦力对轴向应力的影响很小,影响应力精度的主要是滑动摩擦力,因此,轴压摩擦力标定主要是确定在给定的实验条件下,确定滑动摩擦力随位移的变化规律。在滑动摩擦力标定中,首先要确定摩擦力随轴向位移变化曲线的斜率k;其次,确定轴压杆与样品的压力接触点。在对实验曲线校正中,不仅要利用斜率k对压力接触点后面的应力曲线进行摩擦力校正,而且要对塑性变形段进行样品横截面的面积校正。盐套类型对轴压摩擦力的影响较大,特别是在实验条件接近盐的熔点时,摩擦力会显著降低,当样品周围的盐套处于熔融状态时,摩擦力最小。因此,使用低熔点的盐套,在较低的应变速率和较高的温度下,轴压摩擦力小,得到的实验曲线也更准确。在开展变形实验时,针对每种特定的实验条件和装样方式,在进入实际样品变形前,都要进行轴压摩擦力标定。如果实验样品强度比较低,要选择熔点低的盐套,而且要在盐套熔点温度之上进行变形实验,只有在样品的强度远大于盐套的强度时,才能够得到准确的样品强度。通过与气体设备相同条件下的实验比较,部分熔融盐装样方式的差应力误差约为+/-50MPa,熔融盐装样方式的误差为样品强度的5%-11%。部分熔融盐装样方式得到的样品强度比熔融盐得到的强度高约50MPa,并且都要高于相同条件下气体设备得到的强度,熔融盐实验的样品强度为气体设备强度的1.37倍。最后,采用花岗岩和角闪岩两种样品,在高温高压条件下模拟流体对断层破裂与愈合的影响。实验样品分别采用烘干处理样品和水中浸泡处理样品,代表无水和含水条件。通过实验力学数据和显微结构得出:干燥样品以贯通的破裂为主,含水样品以碎裂为主;在含水花岗岩样品中,长石微裂缝内出现暗色条带;在含水角闪岩样品中,高应变速率条件下形成的碎裂的角闪石,在低应变速率条件下边缘出现压溶作用;水和低应变速率能够促进反应或压溶发生,对断层的愈合有促进作用。更多还原

【Abstract】 The Wenchuan earthquake fault is a high-angle reverse fault, which dips at 50°-80°. High-angle reverse fault can hardly slip without presence of high-pressure fluid at depth. In this thesis, several mechanisms for fault weakening and fault healing are summarized. And finally the rock and fluid interaction is simulated under high temperature and high pressure.Fluid plays an important role in fault weakening, both in the physical and chemical aspects. On one hand, high pressure fluid can reduce the effective stress on the fault. On the other hand, the minerals may alter to some layered minerals which have very low friction coefficients with the participation of the fluid. And the reaction is reversible, while fluid environment changes after the earthquake, the layered minerals can also change to the strong minerals, which is helpful to heal the fault. Thus, fluid plays different roles during the earthquake cycle. When an earthquake occurs, the high pressure fluid makes the effective stress on the fault very low. After the earthquake, fault healing occurs with the fluid-assistant pressure solution and mineral phase change. During the interseismic period, fluid pressure increases with the fluid accumulation, and the fault weakens by fluid chemical effect.Before the simulating experiments, firstly, a series of experiments are carried out to calibrate the axial pressure. The calibrating experiments use the axial load cycle method based on the calibrated result of confining pressure and temperature by HAN (2009). The difference between the axial loads shown on the machine and the true axial pressure pressed on the sample is called total friction. It contains two kinds of forces: the frictional contact force and the squeezing force. The squeezing force makes the hit-point unclear,but does not affect the sample strength. While the contact force increases with piston in,and it makes the sample strength inaccurate. A series of experiments were carried out to determine the factors affecting the axial load. The confining pressure,temperature and strain rate are thought to be the main factors. The result shows that low confining pressure, high temperature and low strain rate give rise to the low axial friction. Using the molten salt cell assembly,the contact friction can be accurately determined. While in the solid salt assembly,it is difficult to determine the contact friction from the total friction. When the temperature is 200°C above the melting temperature of the confining pressure medium,the experiment results are most reliable. While when the temperature is below the melting temperature of salt,axial velocity highly affects the axial friction.The purposes of axial friction calibration are to determine the axial contact friction with axial displacement, and to determine the hit-point between the axial piston and the sample. The slope k of the stress with the displacement can be used to represent the friction force. The starting point where the stress goes up sharply is the hit-point, which means the Al2O3 piston begins to be connected with the sample. If the salt is not sufficiently molten, the hit-point is usually unclear. So it is important to make the experiment condition higher than the molting point of the salt, if a more accurate result is wanted.Finally, a few experiments are carried out to simulate the fault or crack’s opening and healing. The samples are granite and amphibolite. Some of the samples are dried to remove the free surface water and part of the structure water, and some samples are dipped into water in order to increase water content. Both the mechanical data and the microstructure show that the dry samples contain main fracture through the whole sample, and it slips during the experiments, while the main deformation mechanism in the wet sample is cataclastic flow. The dark bands in the microcracks are found in the wet granite sample. The suture edges of the wet amphibolite grains show that pressure solution occurred during the slow rate loading. So it can be concluded that water can promote the water and rock reaction, or pressure solution, under a low strain rate.更多还原