溪洛渡库首区蓄水后震源机制分析

ANALYSIS OF CHARACTERISTICS OF FOCAL MECHANISM IN RESERVOIR HEAD REGION OF XILUODU RESERVOIR AFTER IMPOUNDMENT

目前世界第三大水库溪洛渡的库首区遍布碳酸盐岩、灰岩,存在溶洞,蓄水后水位升高140m左右,随之发生6 000多次地震,但绝大多数是1级以下微震,最大震级仅仅ML3;集中在库岸两侧10km、深度5km、从大坝向上游沿库盆40km长度的范围内。微小地震对于水库、大坝的安全没有产生影响。使用较为密集的水库数字地震台网资料反演得到蓄水前后700多次地震的震源机制,并进一步反演了全区和分区的应力场,发现:震源机制空间取向复杂、破裂类型多样,应力状态不均匀、不稳定。与他人使用大量天然地震资料反演的区域应力场不一致,表明水库诱发地震没有受到区域应力场的严格控制。分析认为库水涌入溶洞、渗透到裂隙、节理,原来存在的小断层面或者间断面的孔隙压力增加,摩擦强度、岩石破裂强度降低以及库水载荷加大造成弹性变形等共同作用是地震的成因。并且首先主要释放的是已经积累的区域应力、局部应力,然后才表现出以库水载荷产生的附加应力为主。库首区没有较大的活动断层,2014年库水位将再度升高数十m,伴随裂隙的贯通,应力场的调整,回流的库水淹没库盆上游,还会发生中小地震乃至中等强度地震。溪洛渡水库大坝抗震设防水准较高,不会造成安全问题。建议在诱发地震继续活动期间,尽早实施详细的水文地质、地球物理探测,提供精细的科学资料,为确定诱发地震的成因、寻找控制诱发地震的技术条件,以减轻突发事件的影响,为其他类似水库发挥示范作用。

There are carbonate rock,limestone and caves in the reservoir head area of Xiluodu Reservoir,which is the third largest reservoir in the world.After the impoundment,the water level has risen to about 140 meters,and consequently,more than 6 000 micro-earthquakes occurred on the reservoir bead region,with magnitude of the vast majority being less than 1 and the maximum magnitude ML3.These micro-earthquakes concentrated within an area of 10km in width from the reservoir banks,5km in depth,and 40km in length along the reservoir basin.These earthquakes did not affect the safety of the reservoir and dam.We inverted 700 focal mechanisms by using the waveforms recorded by the reservoir’s digital seismic network before and after the impoundment,and further inverted the stress field of the whole reservoir head region and the sub-regions.The results show a complex orientation of focal mechanism,different rupture types,and uneven and unstable stress state,which is not in consistency with other regional stress fields obtained by a lot of natural earthquakes,indicating the reservoir induced seismicity is not strictly controlled by the regional stress field.According to the analysis,the reservoir water flows into caves,penetrating into cracks and joints,leading to increase of pore pressure,reducing the friction and fracture strength of rocks,and generating elastic deformation caused by the increased load of reservoir water.The joint actions of these may be the cause of the earthquakes.The accumulated regional stress and local stress were released first,then,the additional stress produced by the reservoir water loading was dominating.There are no major active faults in the reservoir head area.Reservoir water level will rise again by tens of meters in 2014.With the penetration of cracks,the adjustment of stress field,and the backflow of water which will inundate the upstream region of the reservoir basin,the possibility of occurrence of moderate earthquakes cannot be ruled out.The seismic fortification criteria are high for the dam of Xiluodu Reservoir,so these earthquakes will not cause safety problems.We suggest carrying out detailed hydro-geological,geophysical explorations during the continuous active period of the reservoir-induced seismicity to obtain accurate scientific data for determining the causes of induced seismicity and searching for the technical approaches for controlling the induced seismicity.These measurements will mitigate the impact of emergencies and play an exemplary role for the other similar reservoirs.