探讨水头增量及其变化率对水库诱发地震活动的影响——以紫坪铺水库为例

摘要

分析水库蓄水在地震发生中的作用时,首先需要了解水在其中的扩散过程及其作用机理.文中通过紫坪铺水库蓄水后,汶川地震前的小震活动时空演化,探讨水库蓄水诱发地震活动的发生、发展过程及其机理.从比较地震时空分布和水体范围变化开始,进而探讨影响建模计算的各个因素,最后给出计算结果.得出以下几点认识:1)紫坪铺水库蓄水后到汶川地震发生前,水库周围的小震活动与水库蓄水密切相关,并且小震分布在汶川地震的地震断层上.2)受地形和介质的影响,水库影响范围实际上是有限的,并且是各向异性的.因此在建模计算前,半定量地获得了孔隙压力扩散系数,沿断裂走向和倾向分别为0.7m2/s和0.35m2/s;定性地设定了地表约束.3)计算结果表明紫坪铺水库附近地震活动的范围与孔隙压力增量有关.当应力值达到触发地震活动的阈值之后,不论水头增量是否处于高值,地震活动的兴衰皆受控于增量变化率.这表明孔隙压力对震群活动的触发是动态触发.%For analyzing the role that reservoir impounding plays in triggering earthquake,the process of diffusion of pore pressure and its mechanism of action should be understood firstly.The temporal distribution of seismicity,which occurred before the MS8.0 Wenchuan earthquake,following the impoundment of Zipingpu reservoir is studied in this paper.Then the mechanisms of the occurrence and development of reservoir triggered seismicity are discussed.A comparative analysis of the temporal distribution of seismicity and the submerged area by reservoir impounding is carried out firstly.Then the influence of various factors on modeling is analyzed in detail.After calculating,the pore pressure change by the Zipingpu reservoir impoundment is obtained.The following observations are made: (1)Conspicuous swarms of earthquakes,of which the sources are located on the same fault of the MS8.0 Wenchuan earthquake,occurred orderly with the impoundment of Zipingpu reservoir.(2)Because of the influences of the terrain and the medium,the range of effect of pore pressure change by the impoundmemt is limited and anisotropic.Hydraulic diffusivities(D)of 0.7 and 0.35m2/s along the fault strike and the fault dip are reached respectively by a semi-quantitative assessment.Of course,the qualitative pressure constraints on the surface are also applied for the modeling.(3)The calculation results show that the temporal distribution of seismicity near the Zipingpu reservoir is related with the pore pressure change.After the pore pressure reached the threshold of triggering earthquake,whether the pressure head change is high or not,the change rate of pressure head change plays a key role in the decrease or increase of seismicity.It means that the triggered seismicity by pore pressure is a dynamic triggering process.