汶川地震同震破碎带井水位特征及其震后愈合研究

Fault zone hydrogeology plays a critical role in understanding the earthquake process and fault related deformation mechanism. The post-seismic fault zone water-level variation is recognized ultimately related to the co-seismic aquifer damage, post-seismic stress redistribution and healing inside the fault zone. The water level in the two distinct surface rupture zones of the 2008 Wenchuan earthquake display totally different pattern and also recorded perfect earth tide. In this application, we choose the water level data from Hole-1 and Hole-3 of the Wenchuan earthquake Fault Scientific Drilling project to conduct the fault zone hydrogeology research. The two boreholes penetrated the principal slip zone of the two ruptured faults, Yingxiu-Beichuan fault and Anxian-Guanxian fault respectively. We plan to analyze the water level response to earth tide and post-seismic borehole water level recession, to calculate the large scale aquifer conductivity and its long term evolution pattern from the beginning of the earthquake to present. The results will shed light on the damage and healing signal inside the two co-seismic fault zone, and by comparing with other researches from micro structure, frictional heat and mineral analysis, we are going to clarify the different deformation mechanisms of theYingxiu-Beichuan and Anxian-Guanxian faults from a hydrogeological perspective.

断裂带水文地质学研究有助于理解地震断裂作用过程和断裂变形机制。震后断裂带的水位变化更是与震时造成的破坏、震后应力调整以及震后愈合紧密相关，是了解地震破坏机制和复发周期的良好途径。本申请以分别贯穿映秀-北川和安县-灌县断裂带的汶川科钻一号孔(WFSD-1)和三号孔(WFSD-3)的井水位为研究对象，对井水位出现截然不同趋势、具有良好固体潮形态的断裂带水位进行长期监测，获取两口钻井连续的水文数据。采用震后水位衰减和水位对固体潮汐响应的方法，模拟求取震后断裂带内部大尺度含水层的导水系数，中等尺度上该参数随时间的变化规律，揭示汶川地震过程中两条发震断裂内部岩体受损状态以及震后愈合的情况。在此基础上综合考虑汶川科钻在断裂带显微构造、摩擦热和矿物组合上取得的成果，从水文地质学的角度探讨汶川地震过程中映秀-北川断裂和安县灌县断裂不同变形机制。

本项目以汶川地震同时形成的两条断裂带：映秀-北川断裂带和安县-灌县断裂带水位为研究对象，利用固体潮理论、水位Slug-test和水化学分析技术，结合钻孔岩心、录井数据，对两条断裂带的水文地质参数及其愈合过程进行研究。项目已经取得了一系列的重要认识，目前标注本项目资助的学术论文2篇，其中1篇为Journal of Asian Earth Sciences, 另外一篇为地球物理学报，另有多篇文章正在审稿或撰写过程中。主要的成果介绍如下：1）映秀-北川断裂为一条快速愈合的断裂，愈合过程受两种机制影响：一种为受地震影响而短时回弹，由裂隙孔喉细颗粒物质被疏通导致；另一种为断裂带裂隙随碳酸钙的不断沉降结晶而出现系统不可逆性愈合，模拟结果显示这一愈合速率为0.24mm/y，映秀-北川断裂带的愈合周期为20-30年；2）安县-灌县断裂带自始至终表现为低渗透率，含水层更多地体现为一个封闭的系统，井水位无法对理论固体潮产生有效的响应，而仅对大气变化有响应，其含水层导水率为0.0017~0.017m-2s-1，小于映秀-北川断裂带的0.024 m-2s-1；3）映秀-北川断裂带和安县-灌县断裂不同的愈合现象和水文地质参数实际上是两条断裂不同破裂模式和结构的体现，映秀-北川断裂是典型的粘滑型断裂，地震产生的应变主要集中在断裂带核部，同时在碎裂带形成大量可导裂隙，而安县-灌县断裂为长期蠕滑的断裂，蠕滑断裂应变的释放速度较缓并且变形的范围更广，往往形成类似韧性变形的构造。项目取得的进展为理解粘滑断裂和蠕滑断裂不同破坏机制及其断裂带结构提供了一种新思路，为这一地区的地震危险性评价提供了重要参考。