库车坳陷的地震孕震环境初探

PRIMARY STUDY ON SEISMOGENIC CONDITIONS OF THE KUQA DEPRESSION, XINJIANG

库车坳陷中强地震前的小震活动不明显 ,而跨断层和流动重力等地球物理观测资料所反映的异常却相对清晰 ,笔者认为这是由其孕震环境所决定的。文中从库车坳陷的地质构造、地壳结构、地震活动、地球物理观测资料等方面对地震孕震环境进行了初步探讨。研究表明 ,库车坳陷是天山南缘和塔里木北缘的挤压破碎带 ,是具有前古生代基底的中、新生代沉积区 ;地壳结构不均一 ,地震震源分布在高低速过渡带附近 ;受塔里木地块周边 7级大震的启动 ,库车坳陷的地震活动具有后发性、成丛性、持续性和准周期性的特点。

Previous studies show that in the Kuqa Depression no significant activity of small earthquakes is observed before the occurrence of moderate strong earthquake, but the results of deformation measurement across fault and mobile gravity measurement display distinct anomalies. We believe that this fact may reflect the seismogenic condition in the Kuqa Depression. The Kuqa Depression is a compressive crushed zone located at the southern edge of Tianshan Mountains and the northern margin of Tarim Basin. It comprises 4 lines of active thrust and anticline belts, among which the Qiulitage belt is the most active earthquake controlling and generating structure. The Kuqa Depression is a Mesozoic Neozoic sedimentary area with Precambrian basement. The upper part of the depression consists of relatively loose fluviolacustrine sediments of more than 10,000m thickness, while the lower part consists of Precambrian basement rocks. The crustal structure of the Kuqa Depression displays both vertical and horizontal inhomogeneities. Seismic tomography reveals that the depth of the depression may reach up to 15km. Low velocity layer occurs at a depth of 20～30km, while the velocity of the layers from the lower crust to Moho is still relatively low. Magnetotelluric sounding also reveals that there is a high conductivity layer at the corresponding depth of the crust. The hypocentral distribution section across the Tianshan Mountains shows that most of the hypocenters are located at a depth range of 20～40km, indicating that the southern marginal fault of the Tianshan Mountains has cut the Moho. The upper crust in central Tianshan has higher velocity and appears as uplift, but in the Kuqa Depression it has relatively low velocity and appears as depression. A gradient zone occurs at a depth of 25～35km between the depression and uplift. Most earthquakes occurred at this gradient zone nearby the mountain front, and few earthquakes occurred at the other positions. The hypocenter distribution section in the Kuqa Depression (EW direction) shows that most earthquakes are distributed unevenly at a depth range of 18～40km. The seismic sources locate in the transition zone from high velocity to low velocity belts. It seems that the focal depth bears a close relation to the buried depth of the low velocity layer, and most earthquakes are distributed at the transition zone between the low and high velocity layers. Earthquakes of magnitude above 7 occurred around the Tarim Basin possess a quasi periodicity of 11 years. The activity of moderate strong earthquakes in the Kuqa Depression has the same quasi periodicity, but lags behind for 18 months. The earthquake of magnitude above 7 in Tarim may cause the occurrence of moderate strong earthquake in the Kuqa Depression. The seismic activity in the Kuqa Depression is characterized by earthquake cluster, and 3 earthquake clusters have occurred in the past 30 years. The result of deformation measurement on the middle segment of the Kuqa Depression shows that several years before the occurrence of moderate strong earthquake or during the process of earthquake occurrence, the deformation across the fault has increased. This anomaly has been observed for 3 times in the past 28 years. The anomaly appears as the rapid drop of observation value and the distortion of annual variation. It indicates the intensification of the thrusting of the upper wall over the lower wall of the fault. The result of gravity measurement also indicates that before or during the occurrence of earthquake, the gravity displays a positive anomaly. This anomaly may last 4 years, and then turn into negative anomaly rapidly. The earthquake usually takes place by the end of positive anomaly or during the rapid drop.