西藏高原札木、当雄地区小震的基本特征

为了研究青藏高原隆起的原因,根据西藏地区的地震活动性和交通等方面的情况,于1976-1977年分别在西藏札木、当雄地区设立了区域地震台网。本文根据所观测的小震资料,用和达法测定了小震的时空参数。采用圆盘形剪切位错震源模式,考虑了传播介质的吸收和地震仪频率特性的影响,计算了小震的震源参数。结果表明: 1.小震具有局部密集和震源深度浅的特点,当雄地区小震震源面向南倾斜。 2.小震的平均应力降为几巴至几十巴。 3.对地震纵波而言,介质的品质因子较低,Q_P的平均值为150。 4.在厚度为二、三十公里的地壳上部,地震纵波的平均传播速度较低,V_P的平均值为5.65公里/秒。

SOME CHARACTERISTICS OF SMALL EARTHQUAKES IN THE ZHAMO AND DAMXUNG DISTRICTS OF THE XIZANG PLATEAU

In order to study the cause of rise of the Qinghai-Xizang plateau, and according to the earthquake activities and transportation conditions in that area, regional seismo-logical networks were installed in the Zhamo and Damxung districts in 1976 and 1977 respectively. On the basis of the data thus obtained, the spacio-temporal parameters of the small earthquakes which occurred there were obtained by use of the Wa-dati method. A disc-shaped dislocation model was adopted for the focus of a small earthquake. After considering the effects of absorption in the transmitting medium and the frequency characterestics of seismographs, focal parameters of the earthquakes in the time domain were determined.The results are as follows:1. Small earthquakes are characterized by intense local concentration and shallow focal depths. The focal planes of the small earthquakes dip towards the south in Dam-xung region.2. The mean stress drop of the earthquakes ranges from a few bars to tens of bars.3. The QP values for longitudinal waves in that region are relatively low, with an average value of about 150.4. At the depth of about 20 km in the upper crust, the average velocity of longitudinal waves is relatively low, with an average Vp value of 5.65 km/sec.