实验研究流体在地震断裂带愈合过程中的作用

Fluids in fault zones are intimately related to seismogenic and faulting processes. During post- and inter-seismic periods, fluids infiltration and their interaction with fault rocks can seal the cracks of fault zones. Healing and sealing of faults have been proved to be closely related to earthquake cycles by both field observations and laboratory experiments. The formation of impermeable fault seals between earthquake events is a general feature of many models of earthquake generation. Study the sealing processes of fault zones can provide important constraint on strength recovery and earthquake recurrence. Precipitation and cementation of minerals from hydrothermal fluids is a main sealing mechanism for the shallow brittle fault zones. In this project, we designed systematic fluid percolation and crack sealing experiments to investigate the sealing effects of hydrothermal fluids on seismic faults. Experiments will be conducted on a newly-developed fluid percolation and crack sealing apparatus, which can provide confining pressure up to 25 MPa and pore pressure to 2 MPa. Natural fault rocks collected from the ruptures of the 2008 Wenchuan earthquake and artificial powder samples will be used for experiments. CaCO3 saturated solutions with CO2 are chosen as pore fluid to flow through the samples, and therefore calcite is expected to precipitate and seal cracks. Other fluids (such as distilled water and CaCl2 solution) are also considered for comparative studies. Experiments will last for a long period to satisfy the slow sealing rate. During experiments, we measure the permeabilities of samples at regular intervals, and collect the pore fluids that flow through the samples. Permeabilities and their variations with duration time of experiment will be taken as a main indicator to infer the degree of sealing process. The collected pore fluids will be used for chemical analysis to investigate the mass transfer and element enrichment behaviors. Additionally, we will perform systematic analyses on the post-experiment samples, including microstructural, mineralogical and geochemical analyses. These results will be used to identify the sealing structure, new mineral phases, chemical changes and mineral dissolution-precipitation processes. Results from different kinds of fault rocks and different experimental conditions will be compared. Finally, we will combine these results to infer the sealing rates and sealing mechanisms, and the temporal evolution of permeability and pore structure induced by mineral dissolution-precipitation in CO2-H2O-rock system will also be discussed. Furthermore, permeability is a main parameter that determine the sealing degree of samples. For clay-rich fault rocks, permeability results measured from different pore fluids (gas and liquid) are largely different. Besides the recognized gas slippage effect, water swelling and adsorption effects of clay minerals should also be considered. To this end, we also designed detail quantitative and comparative experiments to clarify the role of clay minerals on affecting the water permeability of fault rocks.

震后与间震期内, 流体及其与岩石的相互作用会使断层逐渐愈合. 研究断层愈合过程对判断断层强度恢复, 约束地震周期及其复发过程具有重要意义. 热液流体中的矿物质沉淀胶结是浅部断层带裂隙愈合的主要方式. 本课题以震后热液流体对断层的愈合作用为切入点, 以采自汶川地震断裂带的天然断层岩样品为主要研究对象, 系统开展饱和流体对断层岩愈合作用的实验研究. 通过建立渗透率随时间变化的指标判断愈合程度与愈合速率. 实验中同时按一定时间间隔取流体样品进行化学分析, 结合对实验前后样品的结构观察与成分分析结果, 分析样品的愈合方式与机制, 推断矿物溶解-沉淀过程所导致的渗透率时空演化规律. 最后将实验结果与野外等其他方法获得的结果对比, 讨论对震后断层愈合过程的约束与启示. 另外, 渗透率是反映愈合程度的重要参数, 因此本课题还将针对实验室内断层岩气体和液体渗透率存在巨大差异的问题开展定量实验研究.

研究断层愈合过程对认识震后断层强度恢复,约束地震周期及其复发过程具有重要意义.本项目以研究震后断裂带流体渗流与愈合作用为主要目标,在以下三方面取得了新的认识与进展:(1)对含粘土矿物断层岩的气体和液体渗透率及遇水后的演化机制研究结果显示采用水测得的渗透率均比气体结果偏低,并且随着粘土矿物含量的增加差异逐渐变大.样品饱水后的渗透率变化可以由样品的孔隙度演化来解释.主要原因是粘土矿物表面吸附水及层间吸水膨胀作用导致样品的有效孔隙度减小,进而降低渗透性.根据实验结果提出了含粘土矿物岩石遇水后的渗透性与微观结构演化模型.(2)汶川地震断裂带流体传输性质与愈合作用研究结果显示跨断层的渗透性总体显示主滑动带低,两侧破碎带偏高的通道/障碍体二元模式,同震机械破碎和震后与间震期水-岩相互作用共同控制并改变断层带的渗透性演化与愈合过程.基于流体传输性质实验数据对汶川地震同震热压作用的模拟计算表明,流体的同震热压效应可以导致显著弱化.(3)以饱和CaCO3流体模拟断裂带浅部岩石渗流愈合过程的实验结果显示随着CaCO3溶液的渗透作用,所有样品的渗透率均逐渐下降.导致断层愈合及渗透率逐渐下降的主要原因是流体中的矿物质沉淀和压实蠕变作用.将实验结果进一步外推至野外实际结果显示断层主滑动带在震后几年内即可快速愈合,破碎带和围岩愈合则相对缓慢.