摘要
汶川MS8.0级地震位于紫坪铺水库库尾附近。2002年紫坪铺水库开始截流,2005年10月正式开始蓄水后,库区先后发生不少小震群。2006—2008年迅速增多的水磨震群与主震相距不足5km。因此,紫坪铺水库蓄水在汶川地震发生中的作用成为一个被广泛关注的热点问题,从断层的库仑应力变化出发有过很多讨论。文中以汶川MS8.0级地震前的紫坪铺水库地震台网数据为基础,通过分析龙门山断裂带以及库区小震的时空演化,研究水库附近流体压力扩散过程,得到以下认识:(1)紫坪铺水库所在区域存在一系列北东向展布的断裂,水库位于碳酸盐岩地层上,存在渗漏的地质背景。伴随水位波动升高,回水逐渐向西南淹没,地震分布范围也逐渐由大坝向南西扩展,二者关系密切。根据地震随时间的分布,分析了水压力的扩散过程,拟合得到水压力的宏观扩散系数为0.7m2/s。该扩散系数较好地勾画了紫坪铺水库蓄水和地震活动过程。扩散过程既与河流纵断面形态关系密切,也与地下流体孔隙压力扩散、水位变化有关。(2)紫坪铺水库蓄水前龙门山断裂带汶川段已处于高应力状态,库区正好位于汶川段西南区的闭锁部位。蓄水后龙门山断裂带有过三次地震增强和地震活动范围沿断裂扩展的过程,均发生在水位升高后。已有实验结果表明,在亚失稳应力状态下断层在失稳前的活动存在协同化过程。其作用是降低断层上弱部位的强度和扩大弱部位的总长度以提高断层协同化程度,从而增加断层强部位的应力。研究认为,3次高水位后地震活动的增强正反映了在强区域构造应力背景下库水渗透到部分闭锁断层段并使之弱化引起的连锁反应。汶川地震发生在高区域应力和水体的共同作用下,多次高水位起了弱化断层闭锁段的作用,使断层上协同化程度达到临界水平,在强部位的应力作用下造成了较长断层段的快速失稳错动。此项研究为进一步分析紫坪铺水库蓄水在汶川地震发生中的作用提供了新的事实和论据。
The 2008 Wenchuan Ms 8. 0 earthquake occurred near the upper reservoir of the Zipingpu, where the river closure was finished in 2002. After the first water filling of the reservoir in October 2005, some earthquake swarms were recorded, of which the most remarkable ones took place at Shuimo, less than 5krn away from the Wenchuan main shock, between 2006 and 2008. What is the role of the reservoir in triggering the Wenchuan earthquake has become a focused topic. In the earlier work, the study of the changes of Coulomb failure stress （ACFS） in the Longmenshan fault zone （LMSF） has pervaded discussions. To improve our understanding of this devastating earthquake, we made a detailed analysis of the local seismicity recorded by the Zipingpu local seismic network during the period from 31 July 2004 to 11 May 2008, which was constructed with a primary purpose for monitoring possible reservoir-triggered earthquakes. It is noted that a series of NE trending faults running through the reservoir, lying on carbonate beds, which forms the geological background for water percolation. With the water-level fluctuation and rising, the up-reservoir was involved in the backwater areas. The spatiabtemporal distribution of hypocenters demonstrated a clear migration pattern that indicated the pressure diffusion from the dam to uFreservoir, showing a hydraulic diffusivity （D） of 0. 7 m2/s. The D is closely related with the stream profile, which is the comprehensive reflection of the pore- pressure and reservoir-filling process. The Wenchuan segment of the LMSF had been locked with a high stress level before the reservoir filling. The range of seismicity extended for three times along the fault immediately after every rising of the water level. Previous experiments showed the existence of the synergism process of the fault under a meta-instahility state before fault sliding. It enhanced the stress on the stronger portion of the fault and the synergism degree by reducing strength of the weak portions and by increasing the total length of weak portions. We suggest that the three times of increasing seismicity after high water-level, in a high tectonic stress context, just reflected the water entering the partial locked fault, leading to weakening of the fault. The generation of the Wenchuan earthquake was the result of joint action of high regional tectonic stress and reservoir water. Many times of high water level have weakened the locked fault segment, on which the synergism degree reached a critical level and rapid instable slip occurred over a longer section due to the stress of the stronger portion. This study provides new facts and arguments for the further discussion on the role of the Zipingpu reservoir in the generation of the Wenchuan earthquake.
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2014年第1期150-160,共11页
Earth Science Frontiers
基金
国家自然科学基金项目(41172180)
地震动力学国家重点实验室开放基金项目(LED2010B01)
关键词
汶川地震
紫坪铺水库
流体扩散系数
亚失稳阶段
Wenchuan earthquake
Zipingpu reservoir
fluid diffusion coefficient
meta-instability stage
