龙滩水库蓄水后库区中小地震震源机制

Focal mechanisms of moderate and small earthquakes occurred after reservoir recharge in the Longtan reservoir region

本文在对龙滩库区2006年9月30日至2013年5月26日发生的3682次地震进行精定位的基础上,利用FOCMEC方法和改进的格点尝试法反演了ML2.0以上地震震源机制及区域构造应力场,并综合龙滩库区地层岩性、断裂构造和渗透条件等资料,探讨了地震活动类型与库区蓄水过程的关系,获得以下认识:(1)龙滩水库蓄水后的地震活动主要丛集在罗妥、八茂、拉浪、坝首和布柳河5个深水区,地震类型以逆断为主,正断和走滑也占有一定比例.蓄水初期,库区地震类型呈现多样性,蓄水约4年3个月后,地震主要发生在浅部地层中,并大多为逆断型地震.(2)5个地震丛构造应力场最大主应力方向以NW—SE为主,倾角均较小,中等和最小主应力分布较凌乱,在此构造应力环境中,龙滩库区主要断裂整体上呈现走滑运动的性质,局部伴生不同程度的逆倾滑或正倾滑运动分量.(3)库区深、浅部地震活动水平和地震性质之所以会随蓄水过程发生变化,可能与深、浅部构造应力环境、岩体力学性质和渗透性能的差异有关.

Based on the precise relocation results of 3682 earthquakes occurred in the Longtan reservoir from September 30,2006 to May 26,2013,focal mechanisms of events with magnitude greater than or equal to ML2.0and regional tectonic stress field have been inverted using the FOCMEC method and modified grid search method.Considering stratum lithology,fault and penetrative conditions,the relationship between seismic mechanism types and reservoir storage processing were discussed,and the following conclusions could be reached：（1）Earthquakes after the Longtan reservoir recharged were mainly clustered in the 5deep water level regions which are Luotuo,Bamao,Lalang,Bashou and Buliuhe.The main types of focal mechanism solutions are thrust faults,and normal and strike-slip faults are also present in a certain proportion.Focalmechanism types are multiplex in the early water storage stage of the Longtan reservoir.About 4years and 3 months later,earthquakes mainly occurred in the shallow subsurface,where exist most thrust faults.（2）Dominant direction of the maximum principal stress of regional tectonic stress field in 5earthquake clusters is in NW—SE,and its dip angle is very small.The parameter distributions of the middle and the minimum principal stress are irregular.In such an in-situ stress environment,most of the main fractures in the Longtan reservoir area exhibits a nature of strike-slip movement,and a few of them accompanied with thrust or normal movement components to some extent.（3）The depth of hypocenter and focal mechanism of earthquake change with reservoir storage process,which may be related to the differences such as the tectonic stress environment,mechanical properties of rock mass and penetrability in the shallow and deep subsurface.